The colors of other worlds

Carl Sagan famously called Earth the “pale blue dot”. Viewed from a large distance, that is what our complex, vibrant, living planet looks like. In the search for life around other stars, we should be looking for other pale blue dots, right?

Maybe not. There is some reason to think that not all habitable planets will be blue. We might be better off searching for pale red dots or pale green dots or even pale infrared dots. In fact, we already know of one blue planet (called HD 189733 b) that is certainly not livable, as its daytime temperature exceeds 700 degrees Celsius (and it’s a gas giant with no solid surface)!


The colors of planets, as measured by the amount of green vs. blue light reflected. Note the blue color of the exoplanet HD 189733 b. Credit: NASA, ESA, and A. Feild (STScI)

The color of an object represents how much each color of light bounces off of an object. Pop quiz: if a ball absorbs blue and green light but reflects red light and I shine a white light on it, what color will it appear? The answer is: red. The red light bounces off and reaches our eyes, while the blue and green is absorbed.

Two key factors determine a planet’s color: its source of illumination and what colors of light the planet absorbs. The illumination comes from its star. The peak of the Sun’s emission is in the yellow-green part of the spectrum. More massive stars are brighter and bluer, meaning they emit more blue light, and lower-mass stars are fainter and redder. Puny stars – called M stars or “red dwarfs” – emit very little visible light and instead are brightest at infrared wavelengths of light that our eyes cannot detect. What visible light red dwarfs do emit is very red. These stars are important because they are the most abundant type of stars in the Universe.

A planet’s reflectance is affected by a number of factors. Let’s take stock of the colors of the Solar System’s planets. Mars is the red planet; its color comes its surface rocks, which are heavily oxidized (rusted, essentially). We can see down to its surface because its atmosphere is very thin. Venus is mostly a featureless bright planet; its clouds are so thick and reflective that we cannot directly see its surface. Neptune and Uranus appear blue because the methane in their upper atmospheres absorb red light but reflect blue. Saturn’s moon Titan’s orange color is thought to be due to complex organic molecules that form a layer of haze in its atmosphere.


Earth as a pale blue dot. This image shows Earth as viewed from the Voyager 1 spacecraft when it was beyond the orbit of Neptune. Credit: NASA/JPL/Voyager (details of the how the image was taken here).

It turns out surfaces of other life-bearing worlds need not resemble the Earth. In fact, the Earth itself did not always look like a pale blue dot. As the Earth’s atmosphere and surface evolved, so too did its appearance. A study from 2007 used models of Earth’s evolution to show just how different Earth used to look. The most important changes in Earth’s appearance came from changes in the composition of Earth’s atmosphere, which was dominated by carbon dioxide until roughly two billion years ago.

Life on Earth is mainly green. Leaves and grass are the most visible forms of life, and their color comes from a single molecule: chlorophyll, the driver of photosynthesis. Photosynthesis is Earth’s primary source of organic matter. Yet photosynthetic plants need not appear green. A different study from 2007 used a broad range of photosynthetic organisms to understand the fundamental rules that govern how photosynthesis is adapted to take advantage of the energy of the parent star. They found that there is a systematic trend: photosynthetic plants around different types of stars should have different colors. Plant life on planets orbiting stars roughly similar to the Sun (so-called FGK stars) should be similar. But given their very low visible light emissions, red dwarfs systematically push photosynthesis on habitable planets to use the infrared and create plants that appear reddish instead of green.


What a familiar landscape might look like if Earth orbited a red dwarf star rather than the Sun. Credit: Tim Pyle, Caltech.

On a more general note, a recent study created a catalogue of the colors of 137 microorganisms that contain pigments. These included photosynthesizers as well as organisms from extreme environments such as deep-sea vents that do not depend on the Sun. There were some common characteristics (e.g., the importance of specific wavelengths associated with water absorption) but their colors were all over the map. This means that planets whose surfaces are dominated by different types of organisms may have very different colors.

It is the oceans that make Earth a pale blue dot. Yet there are other key surface constituents that are imprinted on Earth’s spectrum; desert sands, conifer forests, snow and ice are among the most important. A planet’s color is a mix of all of these and more, and clouds can hide some or all of these features. It is probably overly optimistic to expect that we could infer the presence or absence of life from a distant planet’s color.

And we should beware of over-interpreting other pale blue dots. Are they blue because, like Earth, they have global water oceans? Or, like Neptune, do they have thick methane atmospheres? Or, like HD 189733 b, do they have high clouds containing small glass-like silicate particles? The context – for example, a planet’s atmospheric chemistry depends on how much energy it receives from its star, which can be estimated from its orbit – can answer some of these questions. Yet given the zoo of possibilities, we should not rush to judgment.

The search for other Earths continues. Should we be looking for other pale blue dots, or green or red dots? Given the enormity of the task of finding and measuring the colors of other worlds, maybe we should just be happy with any dots that we get.


Real-life Sci-Fi world 8: the free-floating Earth

Welcome to Real-life Sci-fi worlds.  I use science to explore life-bearing worlds that are the settings for science fiction stories.

This post is a pared-down summary of an article I wrote for Aeon in April 2015 — see the original article here.

The Sun is pretty key for us here on Earth.  The number one goal of exoplanet-hunters is to find planets like Earth orbiting stars like the Sun.  And we’re getting closer: planets have been discovered with orbits at similar temperatures to Earth but around much fainter stars than the Sun.  There is an interesting alternative: worlds with no Sun.  Free-floating planets that roams among the stars.  Gravitationally bound to the Galaxy but not to any star.  Rogue planets.

A handful of free-floating gas giant planets have been discovered.  A recent study found that Saturn- to Jupiter-mass free-floating planets are extremely abundant, and may be more common than stars in the Galaxy!  While that study is somewhat controversial, it is clear that free-floaters are very common.  There are good reasons to think that rogue Earths are probably even more abundant than rogue giant planets.  So the question becomes: what would a free-floating Earth look like?  And could it host life?


What you might imagine that a free floating planet would look like. Spoiler: it doesn’t look like this!

The answer is: yes, rogue Earths could conceivably host life.  The key difficulty is that the planet must maintain a warm enough temperature for water to be liquid somewhere on the planet.  The only available heat source comes from the planet’s internal heat from the decay of radioactive elements such as uranium.  What the planet needs is to hold on to its heat as efficiently as possible.

There are two solutions. First, the planet could have a thick layer of ice covering an ocean.  If the ice is at least about 10 km thick, then the planet can maintain a global subsurface ocean of liquid water.  This is just like Jupiter’s moons Europa and Ganymede, as well as several other minor planets in the Solar System.  I call these icy rogue planets.


An icy rogue planet. Here, Jupiter’s moon Europa is standing in for the icy rogue. Another spoiler: it wouldn’t look like this!

Second, the planet could have a thick hydrogen atmosphere. Hydrogen is a very efficient thermal blanket.  Plus, it does not condense but rather remains in gaseous form even at ridiculously low temperatures.  A free-floating Earth with a thick hydrogen atmosphere can keep its surface temperature above the freezing temperature of water.  The planet could have lakes and oceans (and possibly life) on its surface.  The atmosphere must be thick: at least ten times thicker than Earth’s and probably more like a hundred times thicker.  I call these rogue blanketed planets.


How you might imagine a rogue blanketed planet would look. Here, Neptune is standing in for the rogue blanketed planet. Spoiler: once again, you would be wrong.

So: what would a free-floating Earth actually look like?  The answer is simple: a shadow.  Both types of planets — icy rogue planets and rogue blanketed planets — are exceedingly cold in their outer regions.  They emit virtually no visible light.  They would simply look like a black circle blocking the distant stars.


What a free-floating Earth would actually look like. The upper atmosphere (or ice layer for that matter) is so cold that it does not emit any visible light.

Let’s get to the money question: what kind of life could exist on these planets?  Almost all life on Earth’s surface relies on the Sun.  Specifically, on photosynthetic organisms that use the Sun to generate organic matter (aka food).  But there are organisms on the bottom of the ocean — at hydrothermal vents — that produce organic matter by taking advantage of chemical gradients (differences in chemical potential) that are themselves generated by Earth’s internal heat.  These creatures are called chemoautotrophs.

On rogue Earths, chemoautotrophs could produce the food for more complex organisms.  However, chemoautotrophs are not particularly efficient: they only harvest available energy about one one-thousandth as efficiently as photosynthetic organisms.  So you would need a lot of them to produce food for a whole planet.  Still, hydrothermal vents host entire food chains including weird creatures like giant worms, snails, and strange eels.


The biological community surrounding a hydrothermal vent along the Juan de Fuca Ridge. Credit: University of Victoria.

Free-floating Earths are in principle capable of hosting life, although it may be a while before we can test this idea.  But who knows how close a rogue Earth is lurking to the Solar System.  These planets are so faint that they could well be among our closest Galactic neighbors.

Again, here is the link to the original Aeon article that is a bit more poetic and covers more ground.


Is the Solar System special?

There are at least three aspects of the Solar System that are weird or at least unusual:

  1. No hot super-Earths.  About half of stars contain a planet larger than Earth interior to Venus’ orbit.  We don’t have one.  That puts us in the minority.
  2. Jupiter.  Only about 15% of Sun-lilke stars appear to have gas giants like Jupiter, and most of these are on very stretched out, “eccentric” orbits.  This makes us unusual at the ~5% level.
  3. Life.  Earth is the only planet in the whole freaking Universe that we are sure harbors life.  That is pretty awesome, and it makes us extra-special.

Is there a connection between these things?  I think so!  I just wrote a blog post about it here.  Let me summarize the main ideas.

The key question is: how do “hot super-Earths” form?  I think that these planets — or maybe just their building blocks — formed farther away from their stars and then were driven inward by the gaseous protoplanetary disk.  That process is called orbital migration.  It looks something like this:


If one of those migrating planets gets big enough, it can gravitationally attract gas from the disk and become a gas giant planet like Jupiter.  Gas giant planets are so massive (Jupiter is more than 300 times more massive than Earth) that they carve ring-shaped gaps in the disk.  Any more distant super-Earths that try to migrate inward are blocked by the giant planet:


What this means is that we owe a lot to Jupiter!  With no Jupiter, Uranus, Neptune and Saturn’s core may have migrated inward and invaded the inner Solar System!  Jupiter is our protector!

A nice feature of this model is that it is testable.  We expect an anti-correlation between the presence of Jupiters and hot super-Earths around other stars.  This anti-correlation shouldn’t be perfect but it should be there.  We’ll find out in a few years!

FYI, here is the more eloquent blog post I wrote for  And here is the technical scientific paper.


Real-life sci-ford world 7: Eyeball planets, both icy and hot!

Welcome to Real-life Sci-fi worlds.  I use science to explore life-bearing worlds that are the settings for science fiction stories.

We have already taken a look at hot Eyeball planets.  Today, I have a post on Nautilus’ blog about Eyeball worlds, both icy and hot.  Check it out here.  The article is a bit shorter and more simplified than some of the other ones, but it explains why the first potentially life-bearing planets we find are likely to be eyeballs.

Comments welcome!

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Real-life sci-fi world #6: Pandora (from the movie Avatar), the habitable moon of a gas giant planet

Welcome to Real-life Sci-fi worlds.  I use science to explore life-bearing worlds that are the settings for science fiction stories.  Up today: can the moon of a gas giant planet — like Pandora from the movie Avatar — really be habitable?

Pandora is one of the coolest-ever settings for a science fiction story.  The life-bearing moon of a gas giant planet.  Orbiting one of the nearest stars to the Sun (Alpha Centauri A).  Bonus: giant floating islands!  In my mind, the setting is what made Avatar such a spectacular movie.

Pandora orbiting its gas giant host.  Pandora is supposedly located in the Alpha Centauri system.  Credit: Avatar/James Cameron.

Pandora orbiting its gas giant host. Pandora is supposedly located in the Alpha Centauri system. Credit: Avatar/James Cameron.

Here is what we know about Pandora:

  • Pandora is the fifth moon of the gas giant Polyphemus (which has 14 habitable moons but no rings)
  • Polyphemus is the second of three gas giants and the fourth planet overall from the star Alpha Centauri A, just 4.4 light years away from the Sun.  Polyphemus is slightly smaller than Jupiter so it is probably roughly Saturn-mass.  Polyphemus actually has two “planetoids” that share the same orbit in its stable Lagrange points (L4 and L5). [Remember, this was a ninja move when we built the ultimate solar system].
  • Pandora has 80% of Earth’s gravity.  It is about 45% as massive and 75% as large as the Earth.
  • Pandora’s atmosphere is about 20% denser than Earth’s.  It contains some familiar compounds — like nitrogen, oxygen, carbon dioxide — as well as some exotic ones like xenon and hydrogen sulfide.  The high carbon dioxide content (about 20%) and hydrogen sulfide make the atmosphere highly poisonous for humans.

Let’s get down to business. We are going to tackle the following questions: Can Alpha Centauri A really host a planetary system like the one in the movie?  Can habitable moons like Pandora really form and survive around gas giant planets?  What would the conditions on Pandora really be like?

I won’t write about the floating islands or the actual lifeforms on Pandora.  I think they are spectacular and best left intact in our imagination.  The goal of this post is just to explore the setting.

Binary stars are a mixed bag (details here).  In some cases they can bad for life by destabilizing the orbits of planets in the habitable zone.  In other cases they are neutral and don’t have much effect.  Let’s take a look at the Alpha Centauri system.  Could Pandora and its host planet Polyphemus really exist in orbit around Alpha Centauri A? 

The Alpha Centauri system contains three stars: Alpha Centauri A and B, and Proxima Centauri.  Alpha Centauri A and B are both similar to the Sun in temperature and brightness.  They follow an elliptical orbit around each other.  Their closest approach is about 8.5 AU (1 Astronomical Unit or AU is the Earth-Sun distance) and their average separation is about 17.5 AU.  It takes about 80 years to complete a full orbit.  Here is what it looks like:

The eccentric orbit of Alpha Centauri A and B, with the orbits of the Solar System's planets for scale.  Credit: PHL@UPR Arecibo.

The eccentric orbit of Alpha Centauri A and B, with the orbits of the Solar System’s planets for scale. Credit: PHL@UPR Arecibo.

Proxima Centauri is a red dwarf star about 15,000 AU away from the others.  It is probably on a long orbit around the two more massive stars.  But given that one orbit would take millions of years, we are not 100% sure.  In any case, Proxima is very faint, at only about 0.2% of the Sun’s brightness.  Its gravity might play a role in this story but its light does not.

There are two key things we need to know:

  1. What is the location of Alpha Centauri A’s habitable zone (the “Goldilocks” range of orbital distances where a planet could potentially have life)? and
  2. Given the stretched-out orbit of the binary stars, could a planet stably orbit in the habitable zone?

Here is one estimate of the size of the habitable zones of Alpha Centauri A and B.  Remember, a planet orbiting too close to its star will get fried and a planet orbiting too far away will freeze over.  The habitable zone (in green) is where it’s at for life (even though a planet also needs to have the right kind of atmosphere and surface etc).

Extent of the (optimistic) habitable zones around Alpha Centauri A and B.  From this paper by Heller & Armstrong (2014).

Extent of the (optimistic) habitable zones around Alpha Centauri A and B. From this paper by Heller & Armstrong (2014).

Alpha Centauri A is brighter than Alpha Centauri B so its habitable zone is more distant.  Alpha Centauri B’s habitable zone actually wobbles a little bit because of the extra light it receives from Alpha Centauri A.  But Alpha Centauri B is too faint to affect Alpha Centauri A’s habitable zone.

Now, would a planet’s orbit be stable in the habitable zone?  At closest approach Alpha Centauri A and B come closer to each other than Saturn and the Sun (see image of orbit above).  These stars exert a strong gravitational pull.  Each star can knock the other’s planets out of their orbits!  Wider orbits are the ones in danger because the other star comes closer.

Studies show that a planet orbiting Alpha Centauri A can be stable out to about 3 AU (about 2.5 AU for Alpha Centauri B).  That’s good news!  The habitable zones of Alpha Centauri A and B are both stable!  [Note: this is for a configuration in which the planet’s orbital plane is the same as the orbital plane of the binary.]

Would the planetary system from Avatar be stable?  I can’t find much information on the configuration of the system, but we can improvise a little.  Pandora orbits the gas giant Polyphemus, which is the middle of three gas giant planets in the system.  So, there must be space for at least one more stable orbit farther out.  Let’s see… the habitable zone starts at about 1 AU and a more distant planet would be stable at least about 1.5 times farther out (see this post for details).  Since orbits are stable out to about 3 AU, this is not a problem. The whole 4-planet system envisioned in the story can be stable.

Wait! Astronomers have been searching for planets in the Alpha Centauri system for years.  Recently, an Earth-sized planet was found in a very tight orbit around Alpha Centauri B.  But they have found no signal from gas giant planets around either star!  In fact, there is so much data of the system that astronomers have ruled out the presence of a planet more massive than 4-8 Earth masses in the habitable zone of Alpha Centauri B (gory details here).

For Alpha Centauri A, there is no planet more massive than 10-12 Earth masses in the habitable zone.  The only possible way that a gas giant like Polyphemus could be hiding around Alpha Centauri A is if its orbit is oriented just right.  To avoid detection the planets’ orbits must be almost perfectly face-on.  Specifically, within about 5 degrees of perfect face-on alignment.  This fortuitous setup is very very unlikely — although not impossible of course.

Sorry, Avatar fans. Pandora is (almost certainly) not there.  Polyphemus does not orbit around Alpha Centauri A (unless its orbit is inclined “just so”).  Big frowny face.

Still, there is good reason to think that Pandora-like moons should exist nearby.  Let’s look at the numbers.

About 10-20% of stars like the Sun have gas giant planets.  A lot of those planets are located in or near the habitable zone.  If just one in every twenty Sun-like stars has a gas giant in the habitable zone, then there should be one within 30 light years of the Sun.  That’s not as close as Alpha Centauri (just 4 light years away) but it’s not too shabby!

Motivation restored.  Frown turned upside down. Let’s keep thinking about Pandora.

Can habitable moons like Pandora really form and survive around gas giant planets? 

Planets form in disks of gas and dust orbiting young stars.  Giant planets are mostly made of gas.  As they grow, a gas giant gathers its own little disk of gas and dust.  Moons form within this disk.

Artist's view of a giant planet growing in a gaseous disk around a young star.  The inset shows the disk of gas and dust around the planet itself -- it is in that disk that moons like Pandora are thought to form.  Credit: P. Marenfeld & NOAO/AURA/NSF.  (see also here)

Artist’s view of a giant planet growing in a gaseous disk around a young star. The inset shows the disk of gas and dust around the planet itself — it is in that disk that moons like Pandora are thought to form. Credit: P. Marenfeld & NOAO/AURA/NSF. (see also here)

The disk around a young gas giant planet basically forms a mini-Solar System!  The giant planet is the star and the moons are the planets. We don’t understand all the details of how moons form (or planets for that matter).  But all four giant planets in the Solar System have large moons.  Jupiter even has four of them (the Galilean satellites).  So we know that moons form efficiently.

This is good news: it makes sense for Pandora to have formed around its gas giant host Polyphemus (or around another giant planet).

But there are threats to life on moons like Pandora.  Many threats are indirectly tied to the gas giant host.

The first danger is for the moon to get too hot and to get fried.  What I mean by fried is that all of the moon’s water is vaporized — not good for life!

Compared with Earth-like planets, moons have two extra sources of energy.  The first is tides.  Tidal forces stretch a moon out and generate heat in the interior.  This can generate massive volcanism like on Io, Jupiter’s innermost big moon.  Io is cool to look at but persistent global volcanism (and the extra heat it brings) — not good for life.  We don’t want Pandora to look like this:

Io, Jupiter's volcanic moon.  The insets zoom in on the huge volcano.  Source: NASA/Jet Propulsion Laboratory

Io, Jupiter’s volcanic moon. The insets zoom in on the huge volcano. Source: NASA/Jet Propulsion Laboratory

The second extra source of energy on moons is energy radiated from their parent gas giants.  As a gas giant planet slowly shrinks under its own weight, it loses energy.  This energy is released as heat (infrared light).  For a moon orbiting very close to the gas giant this can be a big heat source and can push the moon toward getting fried.

Moons are more heated the closer they are to their gas giant host.  This is true for both tides and planetary light.  To be safe a moon’s orbit must be at least a few to ten times larger than the gas giant’s actual size (details here).  For scale, Jupiter’s two innermost moons — Io and Europa — have orbital distances of about 6 and 10 Jupiter.

In short: it’s better for a moon to be farther from the gas giant.


There is another threat to life on a moon.  It comes from the bombardment of charged particles.  These particles come mainly from the star, as part of the stellar “wind”.  These particles can act to erode a planet/moon’s atmosphere, change the atmosphere’s chemistry, and do other things that are generally thought to be bad for life.

Earth is protected by its magnetic field.  It creates a little bubble — called a magnetosphere — that shields the planet from the solar wind.

A representation of Earth's magnetosphere.  The planet's magnetic field deflects particles from the solar wind and keeps them from crashing unimpeded into Earth.  Credit: NASA.

A representation of Earth’s magnetosphere. The planet’s magnetic field deflects particles from the solar wind and keeps them from crashing unimpeded into Earth. Credit: NASA.

Small moons probably cannot maintain their own magnetic fields.  But they can be protected from the stellar wind if they stay within their giant planet host’s magnetosphere.  Small moons need to stay close to someone tough (their gas giant host) to avoid getting beat up by bullies (the solar wind).  The closer the moon’s orbit, the deeper within this protective shield the moon is.

But it can be dangerous to order that deep within a gas giant’s magnetosphere.  Jupiter’s moons are protected from the solar wind but can still be bombarded by energetic particles trapped as radiation belts.

The good news: Pandora is large enough that it can probably generate its own magnetic field.  It doesn’t have to worry about this.  The bad news: while it is claimed on the fan site that Pandora has a whopping 13 other habitable moons, it seems unlikely that they can all remain protected from these dangers.  [In fact, it is improbable to have that many large moons orbiting a gas giant and remaining stable — see here.]

Punchline: it is dangerous for moons to be too close to their gas giant hosts because they can overheat due to both receiving light from the gas giant and from tides.  But being close-in can offer some protection from bombardment by the stellar wind.  Since Pandora probably has its own magnetic field, it’s better for it to be farther from Polyphemus.  

What would conditions on Pandora really be like?

What can we reasonably infer about Pandora using astrophysics?  We don’t know all the details of what Pandora looks like (but see this fan-generated globe:)

Comparison of Earth and Pandora.  Note: Pandora is located in the Alpha Centuari system (not "alpha century").  Credit: Okiir.

Comparison of Earth and Pandora. Note: Pandora is located in the Alpha Centauri system (not “Alpha Century”). Credit: Okiir.

First of all, tides shape Pandora’s orbit around Polyphemus.  Pandora always shows the same face to its giant planet: one side of Pandora always sees Polyphemus looming in the sky and the other side never does.

A moon that is tidally "locked" to its planet.  As the moon orbits the planet, it always shows the same face.  People on the planet can never see the green side of the moon.  From Wikipedia Commons (

A moon that is tidally “locked” to its planet, like Pandora must be around Polyphemus. As the moon orbits the planet, it always shows the same face. People on the planet can never see the green side of the moon. From Wikipedia Commons

Our understanding of tides tells us that Pandora’s spin axis must be perpendicular to its orbit around Polyphemus (its obliquity must be very small).  Pandora’s orbit must be near-circular (but not perfectly circular due to gravitational kicks from the other moons).

How long is Pandora’s day?  This depends on Polyphemus’ mass and on the size of Pandora’s orbit.  Since Pandora’s spin is locked to its orbit around Polyphemus, the time to complete an orbit is the length of its day.

If Polyphemus is as massive as Saturn and Pandora’s orbit is similar to that of Saturn’s large moon Titan, then Pandora’s orbit around Polyphemus would take 15 days.  But if Polyphemus is as massive as Jupiter (or a little more massive) and Pandora’s orbit is closer to that of Jupiter’s moon Europa, then Pandora’s day would be just 2-4 days.  In either case, Pandora’s day is longer than Earth’s, possibly quite a bit longer.

Imagine you are standing on Pandora. You are at the right place such that Polyphemus is straight overhead. Let’s see what happens in the sky over the course of a day.  The Sun rises and sets.  So do the stars.  But Polyphemus stays in the same place.  Polyphemus goes from a tiny crescent (just before and after noon) to a fully illuminated circle (at midnight).  As noon approaches, the Sun inches across the sky toward the crescent Polyphemus.  Then the Sun passes behind Polyphemus.  This when Pandora passes through Polyphemus’ shadow.  It’s the only time all day (at this location) to see the stars without a huge, ultra-bright giant planet getting in the way.  It only lasts a few hours but this lull with no heat from the star affects the planetary climate.

A crescent Io orbiting Jupiter.  Image taken by NASA's New Horizons mission.  Credit: NASA/JHU/APL

A crescent Io orbiting Jupiter. Image taken by NASA’s New Horizons mission. Credit: NASA/JHU/APL

There is another big bright object in Pandora’s sky: Alpha Centauri B.  At its closest approach Alpha Centauri B would appear about 3000 times brighter than the full moon!  At its most distant point it is still about 250 times brighter than the full moon. When Alpha Centauri B is in Pandora’s night sky it would be hard to see any other stars!  Polyphemus’ other moons would also appear very bright in the sky.

When Alpha Centauri B shares Pandora’s daytime sky with Alpha Centauri A, it brightens things up a little. Depending on Polyphemus’ orbit, this can represent an increase of as much as 2% in the energy received by Pandora.  This is not bright enough to directly affect Pandora’s climate.  But since Alpha Centauri A is brighter, a planet in the habitable zone of Alpha Centauri B is directly affected by illumination from Alpha Centauri A.

Pandora’s official fan site lists some of its additional characteristics.  It is claimed that Pandora has an axial tilt of 29 degrees and a stretched-out, eccentric orbit.  We know that tides will have driven Pandora to have a low axial and a nearly-circular orbit around Polyphemus.  For this to work it is Polyphemus that must have an axial tilt of 29 degrees (which is similar to Saturn’s).  Since Pandora must orbit Polyphemus in the plane of the gas giant’s equator, this would give Pandora the same effective axial tilt.  The same goes for the stretched-out orbit.  If Polyphemus’ orbit around Alpha Centauri A is non-circular then so is Pandora’s.  The orbit need only be modestly stretched-out, with an eccentricity of a few percent.  This is completely reasonable for the type of planetary system we are expecting.

So there you have it: Pandora as a habitable world.  Although it is extremely unlikely that Pandora actually exists around Alpha Centauri A, it could be habitable.

A final note: there are candidate habitable moons in our own Solar System.  Jupiter’s large moon Europa may have a global ocean lurking under an icy crust.  Heat generated by tides keeps the ocean from freezing over.  It has been proposed that this might be a good home for life.

An artist's view of Europa's subsurface ocean.  Credit: NASA/JPL.

An artist’s view of Europa’s subsurface ocean. Credit: NASA/JPL.

There are several other small Solar System bodies that are thought to have subsurface oceans.  Like Jupiter’s other big moons Ganymede and Callisto.  And the largest asteroid, Ceres.

It has even been suggested that life may exist on Saturn’s large moon Titan.  This life might use hydrocarbons as a solvent instead of water.  Weird but totally possible!

To conclude: I think that the coolest thing about Pandora is that it opens up a whole new door.  Earth 2.0 need not be a planet.  The same goes for Earth 3.0 and 4.0. They could be moons. How cool is that?

In an upcoming post I will address this question in a more general setting.  Specifically, what conditions are required for a moon to be potentially habitable?


Real-life sci-fi world #5: a Dune planet (Arrakis)

Welcome to Real-life Sci-fi worlds.  I use science to explore life-bearing worlds that are the settings for science fiction stories.  Up today: a desert planet like Arrakis from the classic Dune books (and the movie and miniseries).  A tribute for author Frank Herbert‘s birthday (a couple days late).


Watch out for sandworms!

Dune is one of the all-time classic science fiction stories.  It is set on Arrakis, a desert-covered planet.  Arrakis is the only planet in the Dune Universe with melange (aka “the spice”), a life-extending drug that makes it possible for people to travel between the stars.  It is also infested with gigantic super-dangerous sandworms that live underground and are a prodigious source of nightmares.

This post will be a little different than the early real-life sci-fi worlds.  I’m going to use Arrakis as an example of a desert planet. But we will mix it up.  I will discuss the specifics of Arrakis but also some of the more general characteristics of desert planets.

Here are the questions I’ll address in this post.  What would Arrakis’ orbit be like?  Can desert planets like Arrakis be habitable?  How do desert planets like Arrakis lose their oceans?  How common are real-life desert planets orbiting other stars?  And what is likely in store for Arrakis in the future?

Here we go!


What do we know about Arrakis?  Let’s see…. (full details here or here)

  • It is the third planet orbiting the star Canopus, the second brightest star in the sky.  It has two moons.
  • It is mostly covered in sand dunes (hence the name) with some rocky outcrops.  There is no surface water but there are some canals used for irrigation (called qanats).  Some people (the Fremen) collect water in underground reservoirs with the long-term goal of terraforming Arrakis.
  • There is abundant water in the planet’s interior.
  • Salt flats indicate that Arrakis used to have lakes and oceans.
  • The atmosphere is primarily of nitrogen and oxygen in amounts similar to Earth.  The source of oxygen is sandworm metabolism instead of oxygenic photosynthesis.
  • There is some native vegetation similar to that found in deserts on Earth.  There are some grasses, cactii and bushes, as well as a few small animals.

A more general desert planet is simply one that doesn’t have much surface water.  It can have some ponds or lakes but no oceans.  Its surface is dominated by land.  Its atmosphere is dry — there isn’t enough water to provide much humidity on a global scale.

What is Arrakis’ orbit?

Let’s check out Arrakis’ host star Canopus.  Canopus is an “F-type supergiant” star located about 310 light years from the Sun.  It is about 15,000 times more luminous than the Sun!  To receive the same flux from the star, Earth’s orbit would be 123 times wider and it would take 430 years to complete an orbit!  [In other words, Earth’s “year” would be 430 times longer.]

Arrakis’ orbit must be pretty wide.  It could reasonably be hotter than Earth and on a closer orbit.  But it can’t be so close as to preclude habitability.  If we assume that Arrakis receives the same of energy from the star as Venus (which is pushing it a little), its orbital distance would be about 88 times larger than Earth’s around the Sun (it would be 88 “Astronomical Units”, or AU).  Arrakis would take 263 years to orbit Canopus.  That is about how long it takes Pluto to orbit the Sun.

I couldn’t find information on Arrakis’ orbital distance or eccentricity, its obliquity or spin rate.  Given that seasons are not mentioned, Arrakis’ orbit must be pretty circular and its equator is probably close to aligned with its orbit (in other words, it must have a low obliquity and eccentricity).

Can desert planets like Arrakis really be habitable?

Short answer: yes!

It turns out that desert planets like Arrakis might even be more habitable than planets like Earth.  What I mean is, the habitable zones of desert planets may be wider than for planets with oceans.  This was described in a very cool paper in 2011 (paper here, article here).

Desert planets with little surface water can be habitable closer to their stars than “ocean-dominated planets” like Earth with lots of water.  The hottest places on desert planets cool off more efficiently than on Earth-like planets because the humidity is lower; radiation from hot places is a key way that planets (and giraffes) keep cool.  The low humidity in the atmospheres of desert planets also reduces the greenhouse heating from water vapor. Finally, planets tend to lose their water by hydrogen escape from the upper atmosphere.  Since desert planets’ atmospheres are so dry they lose far less water than Earth-like planets.

Arrakis from the video game Dune 2000.  Copyright Westwood Studies/Electronic Arts.

Arrakis from the video game Dune 2000. Copyright Westwood Studies/Electronic Arts.

Desert planets are also better than Earth-like planets at avoid freezing over. When a planet gets cold, water turns to ice.  Ice is very reflective, so the planet absorbs less energy from the star.  This makes it colder, which makes more water turn to ice.  And so on.  This is called ice-albedo feedback (this was important for the oscillating Earth).  Since they have less water, there is a much less ice on a desert planet’s surface when it gets cold.  This effectively stops the ice-albedo feedback from getting carried away.

To summarize: compared with a planet like Earth, a desert planet like Arrakis can remain habitable both closer-in and farther-out from its star.  What is surprising is that it is water itself that reduces the habitability — the ability of a planet to have liquid water — of Earth-like or ocean-covered planets.  Of course, this is a hot topic so there is plenty of debate (see here for a recent perspective).

How do desert planets like Arrakis lose their oceans?

In the Dune universe Arrakis started off as a planet with oceans.  It was biology itself that dried up Arrakis and turned it into a desert planet.  The entire surface water budget of the planet was basically gulped up by sandtrout, the leathery precursors to sandworms.

The sandtrout … was introduced here from some other place. This was a wet planet then. They proliferated beyond the capability of existing ecosystems to deal with them. Sandtrout encysted the available free water, made this a desert planet … — from the book Children of Dune.

Arrakis’ water hasn’t left the planet, it’s just stuck inside of sandtrout, buried in the sand!  This is a case of a very destructive invasive species!  (Like the snakes that were brought to Guam that killed off most native bird species).

In a more general context, where do desert planets come from?  Maybe they just form without much water.  That can indeed happen.  Planets get their water by collisions of objects that condensed in the colder outer reaches of the system.  Planets that form closer to the star can sometimes just not get any water.  The trick is that to be potentially habitable, desert planets still need some water.  They can’t be bone-dry or there would be no liquid water — the requisite for habitability — anyway!

But can Earth-like planets or even ocean-covered planets turn into desert planets?  Well, it depends how efficiently they lose their water.  Water loss is a complicated process that requires a wet upper atmosphere and a lot of very energetic light from the star (X-rays and ultraviolet).  In simple terms, the planets that can lose their water are relatively close to hot stars.  It’s very tricky to calculate exactly but the punchline is: YES, many planets probably can and do lose their water.  These planets start off as ocean-dominated planets and can transition to being desert planets with wider habitable zones rather than just losing all their water too fast.  So in some situations — like very close to the star — an ocean planet must lose its water and become a desert planet in order to become habitable.

It’s easier to make a desert planet out of a rocky planet with some water (a super-Earth) than a gaseous planet (a mini-Neptune).  It turns out that both kinds of planets are very common.  But only small planets are rocky.  Planets smaller than about one and a half to two times as big as Earth.

Bulk densities (y axis) of small extra-solar planets of different sizes (x axis).  Mars, Earth and Neptune are included for scale.  Credit: Lauren Weiss and Geoff Marcy.

Bulk densities (y axis) of small extra-solar planets of different sizes (x axis). Mars, Earth and Neptune are included for scale. Credit: Lauren Weiss and Geoff Marcy (see this paper).  Discussed in detail here.

Small planets are typically rocky whereas larger ones are gas-dominated.  The small ones are the best candidates for being or becoming desert planets.  I could not find any information on Arrakis’ size or mass but it would make sense for it to be similar to Earth.

How common are habitable desert worlds like Arrakis?

Planets exactly like Arrakis must be very rare because stars like Canopusyellow supergiants — are very rare.  Only the most massive stars ever become supergiant stars.  And the yellow supergiant phase itself is just a few thousand year-long stepping stone toward the longer-lasting red supergiant phase.  In time, Canopus will go supernova (see below)!

In short, since only a few hundred yellow supergiants are known, the odds of finding a true Arrakis analog are slim to none.  Plus, since Arrakis takes so long (200+ years) to orbit Canopus and supergiant stars tend to be very far from the Sun, it would be extra-tricky to find a planet orbiting one.

But generic desert planets should be extremely common.  Their habitable zones are wide so there is a lot of real estate in which to find them.  The only requirement is for the planets to be less than about 1.5 times Earth’s size.  At least about 20% of all stars have planets of that size, and at least 10% of stars probably have planets of that size in a broadly-defined habitable zone.  There are almost certainly Earth-sized planets in the habitable zone in our immediate Galactic neighborhood.

But what fraction of these planets is a desert planet?  We don’t have any concrete way of knowing.  In the Solar System there are 2 planets in a broadly-defined habitable zone: one ocean-dominated planet (Earth) and one desert planet (Mars).  It has also been speculated that Venus was a habitable desert planet as recently as 1 billion years ago.

Optimistic estimate: Let’s assume that half of all small planets are desert planets.  There is likely a desert planet around one of the, say, 20 closest stars to the Sun.

Pessimistic estimate: Let’s say that ocean-dominated planets are much more common than desert planets.  Only 1 in 100 small planets is a desert planet.  We must sample the closest several hundred to 1000 stars to find one.  A good place to look may be in a star’s “Venus zone“.  The closest desert planet must still be located within about 50 light years.


Arrakis’ unpleasant future

Like I mentioned, Canopus is a yellow supergiant star.  It started off as a very bright, blue OB star.  These stars are super bright but burn up extremely quickly.  Within 10-30 million years, Canopus used up all of its hydrogen fuel and expanded into a yellow supergiant. [Nit-picky note: this means that Arrakis itself is younger than ~30 million years so the speculation in the Dune Encyclopedia that it lost its water 50 million years ago is off.]

During this expansion Canopus changed color but remained at about constant brightness.  So in Arrakis’ sky, Canopus became drastically larger and changed colors but the energy from the star remained roughly constant.  [Of course, we can speculate that this may have helped Arrakis lose some of its water if the sandtrout were not efficient enough….]

Within a hundred thousand years or so, Canopus will expand further to become a red supergiant (like Betelgeuse).  10-100 million years after that it willl go supernova then collapse into a neutron star or a black hole.

What will happen to Arrakis when Canopus goes supernova?

Best-case scenario: it is flung into interstellar space and survives as a free-floating planet. Arrakis will lose its source of heat but will be well-served by its underground habitats.

Worst-case scenario: it is completely vaporized by the ridiculously intense emissions from the supernova.  Not a happy ending!

There you have it: Arrakis from Dune as a real-life sci-fi world.  I am of course no Dune expert, so if I missed anything please let me know in the comments.  I also only skimmed the surface of some of these issues, so feel free to ask questions in the comments.

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Real-life sci-fi world #4: Earth around a brown dwarf

Welcome to Real-life Sci-fi worlds.  We are using science to explore life-bearing worlds that are the settings for science fiction stories.  Up today: an Earth-like planet orbiting a brown dwarf.


Planets have been found orbiting all kinds of stars.  Stars like the Sun.  Stars brighter and fainter than the Sun.  Giant stars.  Planets have even been found around brown dwarfs!  [You can poke around in the known extra-solar planets here or here.]

Brown dwarfs are wannabe-stars.  They are just too small.  They can’t generate the huge internal pressure needed to trigger hydrogen fusion in their cores.  So brown dwarfs don’t generate their own internal energy (they do fuse deuterium, but not for very long and that is a pretty wimpy energy source anyway).

With no internal energy source, brown dwarfs spend their time cooling off.  But it takes a while. Billions of years.

Brown dwarfs are between about 10 and 80 times more massive than Jupiter.  But they are about the same size as Jupiter!  Compared with Jupiter, brown dwarfs are just more “squished” under their own gravity.  They are much denser than Jupiter but not much bigger.

Relative sizes of brown dwarfs compared with the Sun, a red dwarf star, and Jupiter.  Credit: MPIA/V. Joergens.

Relative sizes of brown dwarfs compared with the Sun, a red dwarf star, and Jupiter. Credit: MPIA/V. Joergens.

Is there any chance for life around a brown dwarf?  We are confident that planets form around brown dwarfs.  Young brown dwarfs show all the telltale signs of forming planets.  A couple of planets have  been discovered around brown dwarfs, but those planets are too big and too cold for life.  Still, it’s encouraging.

Brown dwarfs are cooler than the Sun so their habitable zones are located much closer-in.  Typically at just a few percent of the Earth-Sun distance (which is defined as 1 “Astronomical Unit” or AU).

Plus, brown dwarfs spend their time cooling off, contracting and getting fainter.  That means that the habitable zone moves inward.  It looks like this:


How the brown dwarf habitable zone changes in time. The green shaded area bounds the habitable zone. At small orbital distances it is too hot and farther out is too cold. The brown dwarf itself shrinks as it evolves — its size is shown in red.  Any planet that comes closer than the Roche limit is torn apart.  This example is for a brown dwarf 4% as massive as the Sun. Adapted from this paper by Emeline Bolmont and collaborators (including me!).

A planet on a fixed orbit cools off along with the brown dwarf.  This corresponds to a horizontal line in the image above.  A planet with an orbital distance of, say, 0.01 AU starts off too hot.  The habitable zone sweeps inward and after about 100 million years it catches up to the planet.  The planet remains in the habitable zone for almost a billion years until it exits past the cold edge.  The frozen planet then cools off indefinitely along with the brown dwarf.

Any planet’s time in the habitable zone is limited, but many planets can spend upwards of a billion years with the right conditions for life.  Another concern is that any planet that enters the habitable zone spent some time too close to the star, in the Venus (runaway greenhouse) zone.  Could such planets retain their water?  [I think so, contrary to this paper.  It depends on the UV and X-ray emissions of brown dwarfs, which are not well known, but we just submitted a paper showing that most planets will keep their water.]

A planet close to a brown dwarf feels very strong tides (from the brown dwarf, not any kind of moon — large moons are not stable for planets close to brown dwarfs).  Tides affect the planet’s orbit and spin.  The planet’s obliquity is quickly driven to zero, such that the equator is lined up with the orbital plane.  The planet’s orbit becomes perfectly circular.  And the planet tidally “locks” and always shows the same face to the star (like for the hot Eyeball planet).  [In this setting tides also cause planets’ orbits to slowly expand.  It doesn’t really affect the story, but see here if you are interested in how it works.]

What would it be like to live on an Earth-like planet around a brown dwarf?  Let’s assume the planet is located in the habitable zone when the system is a billion years old.  For the example from above, that would put the orbital distance at about 0.005 AU.   If Earth’s orbit was the same size, that would put us on the surface of the Sun!  It is a ridiculously small orbit.

So what would be different?  First, as we already discussed, the brown dwarf would always appear in the same place in the sky.  And it would be HUGE!  Our Sun spans about half a degree in the sky.  This planet’s orbit is 200 times closer than Earth’s to an object about one tenth the size of the Sun.  That makes the brown dwarf appear about 20 times larger than the Sun (about 10 degrees across)!  It is as big as a softball held at arm’s reach!  And that “softball” is just hanging there in the sky all the time, never moving…. [cue creepy music]

Earth’s sky is blue because the atmosphere scatters blue light more strongly than red light (this is called Rayleigh scattering).  A brown dwarf emits no blue light.  It barely emits any visible light at all!  Its energy is mainly radiated at infrared wavelengths of light.  There is pretty much no scattering of the light from a brown dwarf.  This means that, to our eyes, the atmosphere would be basically transparent.

The brown dwarf would appear reddish-brown (and gigantic) in the sky.  But look just to the side and you can see the stars! It would basically look like nighttime except in the direction of the brown dwarf!  Clouds would simply be black patches blocking the stars.

The view from a planet orbiting a brown dwarf might look something like this (without clouds):

Author's impression of a brown dwarf 55 times more massive than Jupiter.  Credit: NASA/JPL-Caltech

Author’s impression of a brown dwarf 55 times more massive than Jupiter.  This is basically how the sky would look from a planet orbiting a brown dwarf. Credit: NASA/JPL-Caltech.

Of course, this assumes that inhabitants of these had eyes like ours.  More likely, their eyes would be sensitive in the infrared.  Earth’s atmosphere is almost completely opaque in the infrared except for a few transparent “windows“.  I can imagine these aliens’ eyes being sensitive at several different infrared wavelengths, to gather different types of information.  Maybe these wavelengths would appear as different “colors”.  OK, I’ll stop speculating (for now).


The surface of a habitable planet orbiting a brown dwarf is always illuminated the same.  So the climate would probably be very consistent.  This is not quite as simple as it seems — the planet is also spinning!  To always keep the same face pointed toward the star the planet needs to spin once per orbit.  And a planet in the habitable zone orbits its brown dwarf host in as little as 8 hours!  On average it takes more like a day.  So even though the planet is always facing its star, it spins at about the same rate as Earth!  No one has modeled the climate of this kind of planet so we don’t know exactly what it would be like.

The entire globe could be habitable.  With an atmosphere like Earth’s it would probably be cold on the night side and warm on the day side.  But if the planet’s atmosphere were thicker than Earth’s, then the temperature would be relatively constant everywhere.  Likewise, if the planet’s atmosphere is very thin then the difference in temperature between the day- and night sides would be much larger.


There are a couple of planets known to orbit brown dwarfs.  But they are Jupiter-like planets, not what we’re looking for.  There are all sorts of indirect hints that planets form readily around brown dwarfs.  But we haven’t found them yet.  The main reason is because brown dwarfs are so much fainter than stars, so it’s hard to use them as indirect

There is an interesting strategy in searching for habitable planets orbiting brown dwarfs.  It is based on the transit method.  It works like this.

Take a telescope and stare at some stars (or brown dwarfs).  Carefully measure their brightness as often as you can.  The signal you are looking for is a little dip in the star’s brightness.  The dip repeats once every orbit.  In the best-case scenario it looks something like this:

The dip in a star's brightness when a planet transits its star.   Credit H. Deeg

The dip in a star’s brightness when a planet transits its star. Credit H. Deeg

It’s hard to find Earth by the transit method: the dip in brightness is very small, it only repeats itself once a year, and the odds of Earth’s orbit being lined up right are small (about 1 in 200).

It might be easier to find planets in the habitable zone of brown dwarfs than a planet like Earth orbiting a star like the Sun.  Brown dwarfs are smaller than stars so a transiting planet creates a much deeper dip in a brown dwarf’s brightness.  The transit repeats itself every day instead of every year.  And the chances of the planet’s orbit lining up right are about 20 times higher!  The only problem is that, since brown dwarfs are faint, it is hard to measure their brightness accurately.  Still, this is a promising way to find planets around brown dwarfs (see this paper for the gory details).

Some ballpark numbers.  Within 30 light years of the Sun there are more than 400 known stars (details here).  The vast majority are low-mass (M) stars, also called red dwarfs.  20 are Sun-like (G) stars.  At least 48 are brown dwarfs.  I say “at least” because more are being discovered all the time.

Our Sun's closest neighbors.  Three brown dwarfs (including two in orbit around each other) have only just been discovered (with WISE).  Credit: Penn State University.

Our Sun’s closest neighbors. Three brown dwarfs (including two in orbit around each other) have only just been discovered (with WISE). Credit: Penn State University.

What fraction of brown dwarfs have a potentially habitable planet?  We don’t know.  About half of all red dwarf stars have a planet in the habitable zone.  This fraction is at least 5-10% for Sun-like stars.  Let’s be pessimistic and assume the frequency for brown dwarfs is even lower, just 1%.  There should still be an Earth in the habitable zone of a brown dwarf within 40 light years of the Sun.  Not too shabby!


A group of scientists is discussing the chances for life on other planets.  Except, these scientists are not on Earth.  They are on an Earth-like orbiting in a brown dwarf’s habitable zone.  They sit on old wooden chairs in a dusty room tiled with bookshelves.  Out the window is a starry sky and the giant, looming brown dwarf.

The scientists agree: life is unlikely to exist on planets orbiting stars.  Especially bright stars like the Sun.  The habitable zone is so far away that such planets would only feel very weak tides.  These planets could spin any which way!  The illumination on their surfaces would be changing constantly as the planets spun!  The climate could never be stable on such planets!  Not to mention the variations in illumination over the course of the year caused by the planet’s tilted spin axis (obliquity).  Without tides the planets’ orbits could even be eccentric!  No planet could be habitable with even a modestly-eccentric orbit!

Plus, stars emit ultraviolet light!  That should fry any life on the surfaces of planets orbiting stars. [Note: thank you ozone layer!]

Of course, how could a planet ever develop life in the first place without a hot early phase?  The millions of years before the habitable zone swept inward to include the planet — during which the planet was too close to the brown dwarf — must be responsible for the complex chemistry of life.  Without a hot phase, planets in the habitable zones of stars could never have complex life!

The story ends with a spaceship from an advanced alien civilization — that originated on a planet orbiting a star — approaching the system with evil intentions.  Bad news for narrow-minded thinking!

[Note that this list of criteria that appear vital for life is similar to the Rare Earth hypothesis.  There are 10-20 Rare Earth “factors” that are invoked as being necessary for complex life to develop and survive in a planetary system.  In my mind each of these factors is somewhat arbitrary and most are easily disproved. ]

There you have it: Earth around a brown dwarf.  A Real-life Sci-Fi world.  Special thanks to Franck Selsis and Philip von Paris for some of the ideas in this post.

Questions?  Comments?  Words of Wisdom?

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Real-life sci-fi worlds #3: the oscillating Earth

Welcome to Real-life Sci-fi worlds.  We are using science to explore life-bearing worlds that are the settings for science fiction stories.  Up today: the oscillating Earth.


Earth’s orbit is not fixed.  Gravitational kicks from the other planets change the shape of Earth’s orbit.  Earth’s orbit oscillates between being perfectly circular (having an “eccentricity” of zero) and being 6% from circular (having an eccentricity of 6%).  The tilt of Earth’s spin axis (its “obliquity”) bounces between about 22 and 24.5 degrees. The timescale for this to happen is about 20,000 years.  These changes are pretty minor and slow.  No big deal, right?

Well, it turns out that they are a big deal!  The very small changes in how Earth is illuminated by the Sun are responsible for the ice ages!  The ice ages are driven by Milankovitch cycles, the oscillations in Earth’s orbit and spin.

The three main variations in Earth's orbit linked to Milankovitch cycles.  The eccentricity is the shape of Earth's orbit; it oscillates over 100,000 years (or 100 k.y.).  The obliquity is the tilt of Earth's spin axis, and the precession is the alignment of the spin axis.  From XXX

The three main variations in Earth’s orbit linked to Milankovitch cycles. The eccentricity is the shape of Earth’s orbit; it oscillates over 100,000 years (or 100 k.y.). The obliquity is the tilt of Earth’s spin axis, and the precession is the alignment of the spin axis. Credit: Scott Rutherford.

Earth had much more dramatic ice ages in its past.  There were multiple times during its history (notably 650 million years ago, during the Neoproterozoic) when Earth is thought have been entirely covered in ice.  These are called “snowball Earth” episodes.  They lasted for millions of years.

A snowball Earth is a deep hole for a planet to fall into.  As a planet gets colder, parts of the planet start to freeze.  Ice is white.  Ice-covered rocks reflect more light than plain rocks.  So the planet reflects more light (energy) than it used to.  This makes it colder, and makes it freeze even more.  Which makes it reflect more light.  And so on.  This is called the ice-albedo feedback.

On a planet like Earth, the poles freeze over first.  If ice creeps down too close to the equator, then the planet can completely freeze over.  The global temperature can remain stable at ridiculously low values, with the equator at the same temperature as today’s Antarctica!  The bad thing is: since the planet is so reflective it has a hard time absorbing heat and melting out of the snowball.  The snowball Earth is disturbingly stable.  It can last a long long time (millions of years).

Earth escaped its snowball state thanks to geology.  The greenhouse effect in Earth’s atmosphere got stronger and stronger from years of accumulated volcanic gas.  [Thank you, carbonate-silicate cycle!]

But there is another way.  A Milankovitch-style solution. The more eccentric a planet’s orbit, the more total energy it receives from its star (for a given average orbital distance).  As a planet’s orbital shape oscillates, so too does the energy it receives from its star.  The planet is colder when its orbit is circular (or just closer to circular) and warmer when its orbit is more elliptical.

So if a snowball planet’s orbit gets eccentric enough then it can heat up and melt! This animation shows a climate model of an oscillating Earth.  When the planet’s orbit is near-circular — when its eccentricity e is low — it gets cold and freezes over into a snowball.  But when the planet’s orbit gets eccentric, it heats up enough to melt through and escape the snowball phase.  Of course, when the planet’s orbit gets more circular it just freezes over again…  It’s a repeating snowball planet!

An oscillating Earth.  Colors represent temperature: only green is above freezing.  Blue white are super cold and red is warmer but still freezing.  The planet's orbital shape oscillates following the bottom curve.  Credit: Dave Spiegel (from this paper).

Earth with an oscillating orbit.  The colors represent the temperature across the planet: only green is above freezing. Blue and white are super cold and red is warmer but still freezing. The planet’s orbital shape oscillates following the bottom curve (eccentricity e=0 means the orbit is circular). Credit: Dave Spiegel (from this paper).  More details here.

The periodic “snowball” is just one possible consequence of an Earth-like planet with an oscillating orbit.  In general terms a planet bounces between being an “eccentric Earth” and a more “normal” Earth.

The “eccentric Earth” is a pretty cool sci-fi world.  Stretching out Earth’s orbit into an ellipse (while keeping the same average orbital size) makes the climate much more extreme.  Winters are much longer and colder.  Summers are much shorter but intense.  The latitude of the planet where the star is directly overhead at the hottest part of the summer is “branded” during the passage close to the star.  This can can be good or bad for life, depending on the circumstances.  If the planet is generally at a nice temperature (like Earth), the extra heat from branding is generally a bad thing.  But for a much colder planet then extra heat is usually welcome.

How fast do the oscillations occur?  How fast can a planet’s orbit change?  Well, this depends on the other planets in the same system, the planets that are providing the gravitational kicks.  In particular, the mass and location of the biggest bully.  Let’s imagine a simple system where our planet is being pushed around by a single other planet.  The more massive the planet, the faster the oscillations.  And the closer the planet to the star, the faster the oscillations.  The time for a full cycle — for the orbit to go from near-circular to eccentric back to near-circular — can take between a few hundred years and hundreds of thousands or even millions of years.

Everything I’ve mentioned so far has only really looked at how the shape of the orbit can oscillate.  It turns out that the changes in the tilt of a planet’s spin axis — its obliquity — also have a big impact on the climate.  A recent study showed that planets with oscillating obliquities may have wider habitable zones.  This kind of planet remains protected against freezing over on much colder orbits than a planet with a fixed spin axis.  This kind of oscillating Earth may be good for life!


Anyplace on an oscillating Earth could be a good habitat.  Since it takes a long time for a planet’s orbit to change, the planet has time to adapt.  Of course, during an eccentric Earth period it may be good or bad to be close to the “branding spot“.  But there is nothing else to favor a given place on the planet.


All Earth-like planets are oscillating Earths.  Planets don’t form in isolation but in systems with many planets.  All planets receive gravitational kicks from nearby planets.  Oscillations in the planets’ orbits are inevitable in any system.  What can vary from one planet to another is how big the oscillations are.

Earth-like planets can be put onto wildly-oscillating orbits by giant planets.  Especially when giant planets go unstable and scatter each other.  When giants go unstable, the terrestrial planets have about a 50-50 chance or surviving (rather than being thrown into the host star or ejected into interstellar space).  The planets that survive tend to be on oscillating orbits.  Sometimes very strongly oscillating ones.

Some ballpark numbers.

The frequency of Earth-sized planets in the habitable zone is probably between 5% and 50% (depending on the type of star).  All of these planets’ orbits must oscillate simply because of the other planets in the system.  Weakly-oscillating Earths are ubiquitous.

But only a small number of planets have orbits that oscillate very strongly.  About 10% of stars have giant planets.  About three quarters of these systems have been unstable in their past.  Terrestrial planets should survive about half the time.  And a third to a half of the surviving terrestrials should have strongly oscillating orbits.  That makes 1-2% of stars with oscillating Earths.  Let’s say 1%.  There are several hundred stars within 30 light years.  There should be some strongly-oscillating Earths very close by, within 10-30 light years.


Imagine a system with multiple oscillating Earths!  This is actually a natural outcome of the process of planet formation.  This image is from a simulation I ran a few years back.  Two planets formed in this system: one near the inner edge of the habitable zone and one near the outer edge.  The two planets’ gravitational dance makes their orbital shapes oscillate every couple thousand years.


The orbits of two planets at two different times. At time 1, the inner (red) planet’s orbit is nearly circular but the outer (blue) one’s is eccentric, taking it beyond the boundary of the habitable zone (shaded in gray). At time 2 — about 10,000 years later — the outer planet’s orbit is circular but the inner one’s is eccentric and takes the planet inward past the inner edge of the habitable zone. From a simulation of planet formation in this paper.

They gray shaded area is the habitable zone.  When the inner planet’s orbit is circular, the outer planet’s is eccentric and vice-versa.  But the two planets want different things.  The inner planet is on an orbit similar to Earth.  It doesn’t want to get too hot.  It’s probably better for life when its orbit is circular.

The outer planet is at the cold edge of the habitable zone.  It could use a little extra heat.  It’s probably better for life when its orbit is eccentric.

Both planets are in their preferred configuration for habitability at the same time (during time 1, the left panel in the image)!  The system oscillates between having two nice habitable planets at time 1, then one planet that may be too hot and another that may be too cold a few thousand years later.

What kind of story could take place in a system like this?  Here is one idea.

A civilization arises on the cold outer planet.  It develops space flight and colonizes the inner planet.  The inner planet is only inhabited part of the time, when its orbit is close to circular so the climate is not too hot.

The inner planet is used mainly as a giant farm to feed the growing population on the outer planet.  There are also some nice warm-weather (beachy) vacation spots on the inner planet.  The farmers revolt against the unjust leaders of the outer planet.

Unfortunately for them, the revolt takes place when the inner planet’s orbit is changing rapidly.  Over a generation the planet plunges into its hottest time (its eccentric Earth phase).  Amid an inter-planetary war, the farmers try to bio-engineer a new climate.  A perfect place to try out a Daisyworld!

The farmers cover the planet with the whitest, leafiest plants they can find.  Their goal is to make the planet reflective enough that it absorbs less energy from the star and cools off.  But this is dangerous: they have to manage feedbacks (like the ice-albedo feedback) that could turn the planet into a snowball Earth!

This is a good one: an inter-planetary war between two oscillating Earths, with attempted climate engineering thrown in!


There you have it: the oscillating Earth!  Are any sci-fi stories set on oscillating Earths?

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Real-life Sci-Fi World #2: the Hot Eyeball planet

Welcome to Real-life Sci-fi worlds.  We use science to explore life-bearing worlds that are good settings for science fiction.  Up today: the hot Eyeball planet.

Planets very close to their stars are too hot for life, right?  Well, not always!

Take the Earth and move it closer and closer to the Sun.  It gets hotter and hotter and … then it gets fried.  What I mean by fried is that the greenhouse effect in Earth’s atmosphere crosses a point of no return.  At this point, Earth gets so hot that the oceans evaporate.  I guess boiled might be a better description than fried.  Over time, Earth’s water is lost to space.  Earth eventually turns into a hellhole like Venus.  Not a happy story.

But it doesn’t have to be like that!  There exists another solution: the hot Eyeball planet.


Imagine an Earth-like planet orbiting close to its star.  It doesn’t matter exactly how close, let’s just say that it is close enough that it should be fried.

Before we talk about the planet’s climate, there is something important about this planet that is different than Earth: how the planet spins.  Earth spins spins pretty fast: once a day.  But its spin axis — its “obliquity” — is tilted.

Earth's spin axis is tilted with respect to its orbit.  This is called the obliquity.  Earth's obliquity of about 23 degrees is why we have seasons!  From

Earth’s spin axis is tilted with respect to its orbit. This is called the obliquity. Earth’s obliquity of about 23 degrees is why we have seasons! From Wikipedia.

A planet close to its star feels strong tides from its star.  Like the tides Earth feels from the Moon, but much stronger.  Strong tides change how a planet spins.  Tides drive the planet’s obliquity to zero, so the planet’s equator is perfectly lined up with its orbit.  Tides also force the planet to always show the same side to the star.  It looks like this, except with the star in the middle and the planet orbiting the star:

A moon that is tidally "locked" to its planet.  As the moon orbits the planet, it always shows the same face.  People on the planet can never see the green side of the moon.  From Wikipedia Commons (

A moon that is tidally “locked” to its planet. As the moon orbits the planet, it always shows the same face. People on the planet can never see the green side of the moon. From Wikipedia Commons

So our possibly-fried planet always shows the same face to the star.  The planet is hot on its permanent day side and cold on its permanent night side. We’re talking blazing hot on the day side and deathly cold on the night side.  Frying pan and deep freezer.

What happens to the planet’s water?  It is heated up and boiled on the day side, and frozen on the night side.  But winds transport the water vapor from the day side to the night side.  Water that boils away on the day side can end up as ice on the night side.  This can create a cold trap: all of the planet’s water can be locked up in a giant layer of ice on the permanent night side.  Dry day side, ice-covered night side.

But the story doesn’t end there.  When a layer of ice gets thick enough, its bottom layer melts.  This causes the ice to flow.  This is how glaciers behave.

So our planet’s thick night side ice cap should spread out and slowly flow toward the day side.  There may be a trickle of water that flows into starlight to be evaporated all over again.  There are characteristic wind patterns that pile clouds up in a specific region on the night side (to the East of the anti-stellar point). Here is a cartoon of what this looks like:

A hot Eyeball planet.  The star is off the image to the left.  The left side of the planet is in permanent daylight.  The right side is in permanent darkness and is covered with a giant ice cap.  Image by Jeremy Leconte, from this paper.  Note that characteristic wind patterns pile up clouds in a specific region on the night side (to the East of the right-most [anti-stellar] point).

The name Eyeball planet comes from the planet’s non-uniform appearance.  The night side is icy, the day side is rocky.  The sub-stellar point, the place on the planet where the star is always directly overhead, is really hot.  If the planet is close enough to the star, rocks could even melt at the sub-stellar point.  That would be the pupil.  Rivers that flow from the night side to eventually evaporate on the day side might even look like veins.

The hot part of the name is a hint that there is another kind of eyeball planet (the cold kind of course).  We’ll get to that one later.


Where on a hot Eyeball planet would you want to live?  It’s a classic Goldilocks story.  The day side is roasting and dry.  The night side is frigid and icy.  In between, it’s just right!  The sweet spot — what I call the “ring of life” — is at the “terminator” (not the movie), the boundary between night and day.

Here is a nice artist’s view of a hot Eyeball planet:

Artist's conception of a hot Eyeball planet.  The permanent day side is sun-baked and dry.  The permanent night side is covered with ice.  In between lies a thin ring of

Artist’s conception of a hot Eyeball planet. The permanent day side is sun-baked and dry. The permanent night side is covered with ice. In between lies a thin habitat: the ring of life.  Image credit: Beau.TheConsortium (see this article).

The ring of life is bounded by deserts on one side and ice on the other.  There is a constant flow of water from the night to the day side.  In other words, a series of rivers, all flowing in the same direction. The Sun is fixed in the sky right at the horizon, and the area is in permanent light.  Conditions are pretty much the same across the ring of life, from the equator to the poles.

To speculate, I can imagine vegetation following the rivers onto the day side until they dry up.  Different ecosystems interspersed along the way.  I also wonder if there would be mountains at the edge of the ice sheets, since the ice-covered continents would be heavily weighed down (this is called isostasy).


How many hot Eyeball planets exist in our Galaxy?  Let’s see. About half of all stars like the Sun have a planet that might fit the bill!  These planets are usually called hot super-Earths.  These  planets — at least the ones that have been found so far — tend to be a little big larger than Earth.

But not all hot super-Earths are likely to be hot Eyeball planets.  The hot Eyeball can only exist for a limited range of planetary conditions.  A planet with too thick of an atmosphere has too strong a greenhouse effect, melts its ice and gets fried.  There are a couple of other conditions that need to be met to be able to properly hot-Eyeball-it up (see here for the gory details).

I can’t realistically estimate what fraction of hot super-Earths have the right conditions to be hot Eyeballs.  Still, there are a few hundred billion hot super-Earths in our Galaxy.  Let’s be pessimistic and say that only 1 in 1000 of these has the right conditions.  There would still be a couple hundred million hot Eyeball planets in the Milky Way!  Not too shabby.  Statistically-speaking, there should be one in our immediate Galactic neighborhood (within about 100 light years).  People are actively looking for Hot Eyeball planets as we speak (see here), so stay tuned.


Life abounds in the “ring of life” on a hot Eyeball planet.  This includes an intelligent species (say, like us).  The story centers around the rituals that adolescents on this planet must experience.  The rites of passage.

There are rivers flowing from the night side of the planet, through the ring of life, onto the day side.  Each river flows across the day side until it becomes so hot that it evaporates.  Vegetation grows on the banks of each river, narrow green fingers threaded across the barren rocky landscape.

The first rite of passage is to take a trip down the river and make it back to the ring of life.  The rivers only flow in one direction, so the way back has to be by foot.  And you have to stay close to the river to have a chance to bear the heat.

The second rite involves an excursion onto the icy night side of the hot Eyeball.  The teenagers must find a sacred thermal spring in the vast icy plain and return with a sample of its mineral-infused water.  They must cross mountains and navigate the ice, all in the dark.

The final rite of passage is less dark (pun intended).  The ring of life provides an easy path for an “around the world” trip.  That’s a much more fun ritual!

There you have it: the hot Eyeball planet!  I have seen a couple of posts about hot Eyeball planets on sci-fi websites (see here and here), so I imagine there are sci-fi stories set on hot Eyeballs.  I would love to hear about any stories you know of.

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Real-life sci-fi worlds #1: the eccentric Earth

Welcome to Real-life Sci-fi worlds.  We are using science to explore life-bearing worlds that are good settings for science fiction.

Let’s take the Earth and change just one small characteristic: the shape of its orbit.


Earth’s orbit is nearly a perfect circle.  Earth is always the same distance from the Sun (to within a few percent).  So Earth receives the same amount of energy from the Sun throughout the year.  [It’s the tilt of Earth’s spin axis — not changes in the Earth-Sun distance — that causes the seasons, of course.]

Many of the known extra-solar planets have orbits that are pretty elliptical (we talked about this in a previous post). The average “eccentricity” is about 25% or 0.25.  An eccentricity of zero means a perfect circle and 1 is infinitely stretched out.  The higher the eccentricity, the farther the Sun is from the center of the ellipse.

Orbits with the same average distance ("semimajor axis") but different eccentricities e.  The higher the eccentricity, the more stretched out the orbit.  Credit: NASA Earth Observatory.

Orbits with the same average distance but different eccentricities e. The higher the eccentricity, the more stretched out the orbit. Credit: NASA Earth Observatory.

The more eccentric an orbit is, the closer it passes to the star at its closest approach.  At the same time, the planet passes by this closest approach very fast.  The planet spends most of its time far away from the star.  The total amount of energy received by the planet is higher for eccentric orbits, but only by a very small amount unless the orbit is extremely elliptical (with e larger than about 0.5-0.75.

Motion of a planet on an elliptical orbit.  From Wikipedia Commons.

Motion of a planet on an elliptical orbit. Rp and Ra are the planet’s closest and farther approaches from the star.  From Wikipedia Commons.  [FYI, this website has a nice interactive orbit simulator.]

On a planet with a stretched-out eccentric orbit, everything is more extreme than on a circular orbit.  The hot is hotter and the cold is colder.

For a very eccentric orbit, the planet is basically branded during its very short closest approach to the star.  The planet then spends the rest of its orbit — especially the long cold winter far from the star — cooling off.  Like running from the hot tub into the snow and then back (although if you’re like me, you spend a lot more time in the tub than in the snow, but planets in orbit do the opposite!).


An eccentric Earth is not uniformly heated.  There are much larger temperature swings than on Earth.  Still, the entire globe of an eccentric Earth with the same average orbital distance as Earth is habitable.  What matters is the total energy received over an orbit, not the instantaneous heat from the star.

There is a particularly interesting place on an eccentric Earth.  It is the location where the star appears directly overhead during the planet’s closest passage to the star.  This is where the planet is branded.  This location receives a short burst of heat that is stronger than anywhere else on the planet.  [In reality, it is of course spread over some area.]  Let’s call it the branding spot.  But the planets were are talking about are spinning and they spend many days near closest approach.  So the branding “spot” is really a ring at constant latitude.  Imagine the tropics but shifted up or down to any latitude.

Is it good or bad to be sitting on the branding spot?  Well, it depends.  For a planet with the same average orbital distance as Earth, there is a danger from overheating.  If the orbit is eccentric enough then the branding spot gets fried.  Not the place you want to be!

But if the planet is much colder then it could be nice to be near the branding spot.  To be close to the once-a-year burst of heat in an otherwise icy world.  This image shows a pretty cool example taken from a real climate model.

    Temperature map of a hypothetical planet over a full year. White means frozen and other colors indicate habitable temperatures. The planet is a desert world with an eccentric orbit. The only place that is heated enough to melt the ice is at the South pole once per year. Taken from this scientific paper.

Temperature map of the climate of eccentric Earth over a full year.  The temperature at a given latitude on the planet can be read by the change in color along the horizontal line at that latitude.  White means frozen and other colors indicate habitable temperatures. The planet is a desert world with an eccentric orbit. The only place that is heated enough to melt the ice is at the South pole once per year. Taken from this scientific paper by Courtney Dressing and collaborators (including me!).

The branding spot of this planet is at the South pole (it’s actually a spot, not a ring, since it’s located at the pole).  The entire planet is covered in ice.  But once per year the South pole is heated sufficiently that it melts and produces conditions friendly to life.  This only lasts for about a month before freezing over, but it is the only time on the entire planet that it happens.


The typical giant extra-solar planet has an eccentricity of about 0.2-0.3.  Unfortunately, it’s very hard to measure the orbital shape for small planets.  There is some evidence that smaller planets tend to have less eccentric orbits, but it’s pretty tentative.  Simulations also show that small planets probably have more circular orbits than giant planets.  But those same simulations also show that some Earth-sized planets should actually have very elliptical orbits.  So even though eccentric Earths are likely to be less common than eccentric Jupiters, they should still exist. In fact, it is planet-planet scattering among Jupiters that probably stretches out the orbits of the Earths anyway!

Some ballpark figures.  About 10% of stars have a giant planet like Jupiter.  80% of those underwent planet-planet scattering.  Earth-like planets survive half of the time.  10% of these have orbits with eccentricities larger than 0.2.  That makes a few eccentric Earths per thousand stars.  There are more than 1000 stars within 50 light years of the Sun.  So there are likely to be a few eccentric Earths close by.  I wouldn’t be surprised if an eccentric Earth turns up in the coming years (or is already lurking among the known planets).


What kind of story could take place on an eccentric Earth?  What is different about an eccentric Earth is how the climate changes in such an extreme way during the year.  And the most interesting location is the branding “spot” or ring.

Here are two ideas for story lines.

Story 1 takes place on the icy world shown in the climate simulation above.  The planet completely covered in ice except for one month a year at the South Pole.  It is freezing and pretty Hoth-like.  A cold cold place.

There is a network of tribes that survive on the icy world.  Given the harsh conditions the population density is low.

Each tribe is adapted to its local ecosystem.  Some tribes specialize in fishing through holes in ice-covered lakes.  Others follow and hunt the animal populations.  Still others subsist on small amounts of greenery that manage to survive in localized settings.

Every year many of the tribes migrate to the South Pole to enjoy the month-long burst of warmth.  This month is accompanied by a massive burst in biological production, with new vegetation and insects feeding animals.  Those animals are hunted by the tribes.  It is feast time!

But during this month the population density goes from very low to very high.  There are skirmishes between rival tribes and the occasional battle.  This is also the time for inter-breeding between the tribes.  This is of vital importance to maintain genetic diversity in each isolated tribe.  The tribes have an agreement about how things go during

I don’t want to make this post too long so I’ll lay out a few more specific ideas:

  • Star-crossed lovers from different tribes do a Romeo-and-Juliet impression.
  • A giant man-eating beast hibernates all year long at the South Pole except for the month during which his prey flocks to him.
  • A tribe discovers an alternate source of heat.  It starts to create its own warm spot on the planet, triggering massive changes in the planet’s climate and a battle for control of the planet….

Story 2 takes place on a hot eccentric Earth.  The climate is Earth-like when the planet is far from the star but dangerously hot during close approach.  The branding spot is deadly hot.  It is a ring located at about 70 degrees North (a little North of the Arctic circle on Earth).  Along the branding ring the temperature gets up to 100 degrees Celsius (the boiling point of water; 212 degrees Fahrenheit).  This only lasts a couple of weeks during summer (it’s shorter than in the icy world described above because the planet’s orbit is closer to its star).

The planet is covered with life.  But the Northern-most part of the planet is abandoned during the summer.  The population flees to the South to avoid the heat.

The story follows a small band of over-ambitious hunters caught too far North.  Summer  approached too quickly and they were trapped North of the branding line.  Their only hope: to go North.  Since the maximum heat is right at 70 degrees latitude, they hope to get to the Pole where it is (a little bit) cooler.  It’s a story of survival (or not) in extreme and changing conditions.  All sorts of cool things to imagine….


There you have it, the first real-life sci-fi world: eccentric Earths.  Are there any science fiction stories set on eccentric Earths?



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