Giraffes and planets

Giraffes are covered in patches.  The patches are brown and the space in between the patches is white.  Everyone knows this.

Here is something you probably didn’t know: you can see those patches in infrared light.  In simple terms, infrared light measures heat (at least at the temperatures we are used to in everyday life).  A hot potato is brighter in infrared light than a cool potato.  You get the idea: bright = hot.

A giraffe in visible light (left) and in infrared light (right). The color bar shows how the brightness corresponds to temperature (in Celsius). Credit: Inside Nature’s Giants, Series 1, Episode 4: The Giraffe (Channel 4, UK)

Giraffes use their patches to cool off.  They have a system of blood vessels under each patch.  They use their patches as “thermal windows” to get rid of extra heat.  A giraffe only heats up a fraction of its body (the patches) but it cools off efficiently.  Why does this work?  Because the amount of energy that an object radiates depends very sensitively on its temperature.  If you double the temperature of a potato you make it 16 times brighter!  A potato can cool off twice as fast by doubling the temperature on just one sixteenth of its surface.  Likewise, a giraffe cools off faster by increasing the temperature by a few degrees under its patches.

Planets also use hot zones to cool off.  The hottest places on Earth — deserts — emit the most energy.  This image shows that the Sahara desert emits way more than its share of heat:

IR_emission_Earth_Gomezlealetal2012

A map of the energy emitted by the Earth in infrared light. Redder means The top panel is the average during July 2001 and the bottom panel is an average over the entire year of 2011. From Gomez-Leal et al 2012.

The Sahara desert helps Earth keep cool!  It’s not the only thing emitting infrared energy of course: how a planet cools is a very complex process.  But this is important to know.  A planet with completely uniform temperature is not very good at cooling off.  But a planet with a few hot spots (like deserts) can beam a lot of excess heat into space and keep cool.  This is exactly what happens (with a few more details like humidity and clouds included) in 3-dimensional climate simulations.

There you have it: giraffes and planets have something in common.  And deserts actually keep the planet cool!

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Something amazing will happen on June 15th but no one on Earth will see it

I discovered something spectacular completely by accident.  I was getting ready for the announcement of the discovery of the extra-solar planet Kepler-186 f.  You remember, the Earth-sized planet in the habitable zone?  It was all over the news (even in French) just a couple months ago.

I made an animation of the Kepler-186 system.  The planets went around and around on their orbits.  The brightness of the star dipped every time a planet passed between Earth and Kepler-186 (the star).

Here is what the movie looks like (for the month of June):

Kepler186_newmovie

Animation of the Kepler-186 system. Each colored circle is one of the planets going around on its orbit. The green planet is the famous Kepler-186 f. The bottom plot shows how the brightness of the star dips every time a planet blocks part of the star’s light from reaching Earth. This is a planetary transit.

Each blip in the star’s brightness is a transit.  That is when a planet passes between our telescope and the star, blocking a little bit of the star’s light.  See that weird blip around June 15th?  Let’s watch that part again in slow-motion:

Kepler186_triplezoom

That blip is deeper than the others. It’s got a weird shape too. It’s not from one planet blocking the light from the star, it’s from three!  As seen from Earth (or really, from the Kepler space telescope), three planets pass in front of the star at the same time!  A triple transit!  They are the first-, second- and fourth-closest planets to the star.  It would be nice if the habitable zone planet were one one of them.  But hey, this is still pretty awesome!

My friends that work on Kepler told me that a triple transit has already been seen (for example, in the Kepler-20 system).  What would really be spectacular is if two of planets passed in front of each other while they were blocking their star.  The shadow of one planet would fall on the other.  A planet-planet eclipse!

This got me really excited.  I went ahead and built a model for what will happen on June 15th. Here is what it might look like when the planets pass in front of the star:

frame_0024

The view of the Kepler-186 star on June 15th. Each line shows the orbit of a planet passing in front of the star. Each planet is labeled (as either “b”, “c”, or “e”, meaning the first, second, and fourth-closest planets to the star). The bottom points show how the observed brightness of the star evolves.

Planet c (second planet from the star) starts to transit first.  Next comes planet e (fourth) then planet b.  Planet b is closest to the star so it orbits fastest.  It catches up with planet e and the two planets briefly overlap.  This causes a short-lived increase in the brightness of the star.  Instead of two separate planets blocking the star, during the planet-planet eclipse there is effectively just one.

What’s so special about planet-planet eclipses anyway?  They are powerful tools for studying the orbits of the planets.  The shape of a planet-planet eclipse is one of the only ways to determine the inclination (the tilt) between two planets’ orbits.  Among all the Kepler data only one clear planet-planet eclipse has been found.  These are rare but extremely powerful.  They are basically the Bengal tigers of astronomy!

Now, we don’t know the exact path of each planet across the star.  The animation I showed is for a lucky geometry.  It turns out that there is about a 50% chance of a triple transit happening.  If planet c’s path across the star is too high-up or low-down (if its “impact parameter” is too large) then it is already done before planet b’s transit starts.

The planet-planet eclipse is a relatively low probability event.  This is because the planets are so small compared with the star!  You need a “lucky” setup for them to pass in front of each other.  A planet-planet eclipse has just a 5-10% chance of happening.  Well, there is never a super high probability of seeing a Bengal tiger except at the zoo.  And astronomy takes place in the wild (believe me)!  Still, a 5-10% chance of finding something historic seems worth a shot.

Here are three possible configurations of the planets during the June 15th transit:

Three possible transit geometries for the event on June 15th in the Kepler-186 system.  In the top panel there is a double eclipse but no triple eclipse and no planet-planet eclipse.  In the middle panel there is a triple transit but no planet-planet eclipse.  In the bottom panel there is both a triple transit (lowest point of the curve) and a planet-planet eclipse (the small central bump).

Three possible transit geometries for the event on June 15th in the Kepler-186 system. In the top panel there is a double eclipse but no triple eclipse and no planet-planet eclipse. In the middle panel there is a triple transit but no planet-planet eclipse. In the bottom panel there is both a triple transit (lowest point of the curve) and a planet-planet eclipse (the small central bump).

How could we observe this?  The whole thing lasts about 6 hours.  The triple transit — when all three planets are in front of the star — can last anywhere from not at all to an hour.  The planet-planet eclipse (if it happens) only lasts about 10 minutes.

The signal is small.  Each planet only blocks a few ten-thousandths of the star’s light.  We need to be able to detect a change in brightness of the star that is that small.  And we need to do it fast, since some of these events may only last ten minutes!  Plus, Kepler-186 (the star) is not very bright.  So this is a very challenging measurement.  There is only one telescope capable of making these observations: the Hubble Space Telescope.

I put together a team of experts.  People who know how to make this kind of observation happen.  Some spectacular people: Avi, Brice-Olivier, Philip, Darin, Elisa, Tom, Franck, Jason, Daniel, Franck, and Emeline.

There are a couple of issues that make the observation with Hubble tricky.  First, the telescope has very little on-board memory.  We want to carefully measure the brightness of Kepler-186 every minute or so.  But there isn’t enough memory to store all the images.  And downloading the images to Earth takes about 5 minutes, which would leave big holes in the signal.  The solution was not to simply point the telescope at the star but rather to slowly drift past it.  That way, the star’s light would be spread out across the camera (after being already passed through a “grism” to disperse it by color).  Different parts of the chip would represent different times.  The details of this were tricky but a couple of great observers (Avi and Brice) figured them out.

Another problem is that Hubble orbits very close to the Earth.  It is in low-Earth orbit.  It takes about 96 minutes to go around the Earth once. Hubble can only see our target star when it is not blocked by the Earth.  Unfortunately, the star spends almost half of each Hubble orbit out of view, behind the Earth. This leaves a 45-minute hole in the data.  A planet-planet eclipse or triple transit is shorter in duration than the length of the observing window.  So even if they happen, there is a 50/50 chance that Hubble would miss them.  This is a bummer but there is no way around it.  The bad thing is that it drops the chance of Hubble seeing a triple transit to about 25%.  And the chance of Hubble seeing a planet-planet eclipse to 3-5%.

I was really excited so we kept going.  We wrote a proposal to observe Kepler-186 on June 15th.  We couldn’t pass through the normal proposal process because this was happening so soon.  Normally you have to propose to observe something (a star, galaxy, planet, …) up to a year in advance.  I had only discovered the existence of this event in March!  So we applied for special, last-minute observing time (“director’s discretionary time”).  I sent in the proposal in early May.

Drumroll ……… and a big frowny face.  A week later someone at Hubble got back to me.  They appreciated the proposal but did not award us any observing time.  Bummer!

Why didn’t we get the time?  Well, I can understand their point of view.  A triple transit is awesome, but we wouldn’t learn any more about the planets than we would from three separate normal transits.  In some cases a precisely-timed triple transit could help figure out the planets’ masses (using the “transit timing variations” technique).  But, Daniel found that the triple transit wouldn’t help all that much.

Observing a planet-planet eclipse in the system would be spectacular.  As I mentioned above, only one has ever been found before.  And that was for bigger planets: a super-Earth and a Saturn-sized planet.  The possible planet-planet eclipse on June 15th is for two roughly Earth-sized planets.  Plus, it is in a very high-value system that includes a potentially habitable planet (and maybe another one).

A planet-planet eclipse would tell us the inclination (tilt) angle between the projected orbits of planets b and e.  This would be very interesting to know.  A small inclination would tell us that the planets are located in a thin disk.  But wait!  Don’t we already know the answer?  Well, sort of.  There is only a small chance of ever finding a system like Kepler-186 with five transiting planets unless the planets’ orbits are confined to a thin disk.  So, if Hubble saw a planet-planet eclipse it would almost certainly measure a very small inclination between the orbits of planets b and e.  I can see how the Hubble reviewers may have thought that we would not learn anything really new.

There are two strong counter-arguments to this line of thinking.  First, we shouldn’t place too much faith in models.  I mentioned that we think we already know the answer, that the planets’ orbits must be confined to a thin disk (like a Frisbee).  But what if we are wrong?  That’s not impossible.  And it would actually be much more interesting if our guess was wrong.  I think it’s worth testing.

I mean, the transit could look like this:

An inclined configuration of the Kepler-186 planets.  This scenario is not ruled out by observations.

An inclined configuration of the Kepler-186 planets including a planet-planet eclipse.

In this example, the orbits of the two planets that eclipse are inclined by almost 90 degrees with respect to each other!  Although I think it’s unlikely, we cannot rule out that this is the true configuration.

My second counter-argument is that planet-planet eclipses are just so so so rare!  Among the tens of thousands of transits seen by Kepler, only one planet-planet eclipse has been found.  Even a small chance (5%) of finding another seems worth going for.  Imagine this: by going through a big nasty dumpster you have a 1 in 20 chance of finding a diamond the size of an apple.  Would you do it?  The odds are not great, but the potential payoff is spectacular.  I would totally do it!

Finally, the next triple transit in the Kepler-186 system won’t happen until the year 2047!  I’ll be 70 and probably more interested in controlling things with my mind than in looking for transiting planets.  Plus, I’m impatient.  I don’t want to wait!

In the end, I don’t blame the people at Hubble who decided not to implement our proposal.  They have a very hard job.  They get asked for 10-20 times more observing time than they can give.  It’s not easy to decide who gets it.  I am bummed about it, but I understand their decision.

SUMMARY.  This was a spectacularly fun project.  I really enjoyed it.  I learned all sorts of new things about transits and observing (and making animated gifs).  I made some great contacts.  I really went for it with the Hubble proposal.  I did my best to make it happen.  I thought we had a good shot at getting the observing time.  But we didn’t get it.  I’m disappointed but I feel good that I didn’t hold back.

The main reason I am bummed is because I will never know if a triple transit or planet-planet eclipse happened on June 15.  And no one else will either.


UPDATE: After sharing this post, several colleagues told me that they thought that the triple transit might be detectable using a ground-based telescope (as opposed to space-based).  My good friend Stephen Kane was able to secure the night of June 15th on a 2-meter telescope at the Indian Astronomical Observatory.  This was the right longitude to be able to see the entire triple transit from a single location.

Another drumroll…. and another big frowny face!  Patchy weather.  Bad seeing.  No useful data.  Bummer!

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Building the ultimate Solar System part 5: putting the pieces together

We are building the ultimate Solar System.    In Part 1 we chose the right star.  In Part 2 we chose the right planets. In Part 3 we chose the right orbits for the planets.  In Part 4 we learned two ninja moves about how more than one planet/moon can share the same orbit.


Today’s job: putting it all together to build the ultimate Solar System.

Let’s look at our ingredients.

  1. A star a little smaller than the Sun (50-70% of the mass of the Sun).
  2. Worlds (planets or moons) between half and twice the size of Earth, with between one-tenth and ten times as much water as Earth.
  3. Any other accessories that we might need (e.g., gas giant planets).

We can arrange these ingredients in any way we choose.  Our goal is to pack as many worlds as we can into the habitable zone.  We can use our ninja tricks (co-orbital planets and moons).  But the system must remain stable for billions of years.  It won’t do us any good if there is a dynamical instability that sends a bunch of habitable worlds falling onto the star.

I am torn between two ultimate Solar Systems.  Both are awesome.  The first one only contains Earth-sized planets.  The second also includes gas giant planets with Earth-sized moons.

Let’s build these suckers!

ULTIMATE SOLAR SYSTEM 1.  Let’s only include Earth-sized worlds.  No gas giants.  As we saw, the number of planets we can cram into the habitable zone depends on how big they are.  Bigger, more massive planets need to be more widely spaced.  And for maximum orbital compactness we want planets that are all the same size.  I’m going with planets that are half the mass of Earth (about 80% as big as Earth).  Planets of this size definitely satisfy the criteria for habitability.  I guess I’m a little nervous that the very smallest planets in our chosen mass range might be borderline habitable.  And we don’t want to end up with a system full of Marses!

We can fit six stable orbits into the habitable zone.  Each orbit has two sets of binary Earths.  These are Earth-sized planets with Earth-sized moons.  Each binary planet is in a Trojan (co-orbital) configuration with another binary planet, separated by 60 degrees on their orbit around the star.

Six orbits.  Two binary planets per orbit.  Two planets per binary. That makes 24 habitable worlds in a single system!

Here is what the system looks like.

Our first ultimate Solar System. Each orbit around the star (thick gray line) contains two pairs of binary Earths in a co-orbital (Trojan) configuration.  The green shaded area represents the habitable zone.

Most of the worlds are Earth-like in composition with oceans and continents.  But I threw in a few water worlds and few desert worlds.  Good vacation spots!  I especially like the binary Earths that consist of a desert planet and a water world.

ULTIMATE SOLAR SYSTEM 2.  Now let’s include gas giant planets.  We can fit the orbits of four gas giants in the habitable zone (in 3:2 resonances).  Each of those can have up to five potentially habitable moons.  Plus, the orbit of each gas giant can also fit an Earth-sized planet both 60 degrees in front and 60 degrees behind the giant planet’s orbit (on Trojan orbits).  Or each could be a binary Earth!  What is nice about this setup is that the worlds can have any size in our chosen range.  It doesn’t matter for the stability.

Let’s add it up.  One gas giant per orbit.  Five large moons per gas giant.  Plus, two binary Earths per orbit.  That makes 9 habitable worlds per orbit.  We have four orbits in the habitable zone.  That makes 36 habitable worlds in this system!

Here is what it looks like.

Our second ultimate Solar System.  Each orbit around the star (thick gray line) harbors a gas giant planet orbited by five large moons.  There is also a binary Earth in both the leading and trailing Trojan points with the gas giant (60 degrees in front of and behind the giant planet in its orbit around the star).

Our second ultimate Solar System. Each orbit around the star (thick gray line) harbors a gas giant planet orbited by five large moons. There is also a binary Earth in both the leading and trailing Trojan points with the gas giant (60 degrees in front of and behind the giant planet in its orbit around the star).

As in ultimate system 1, most worlds are Earth-like with oceans and continents.  With the odd ocean planet and desert planet for vacations (or, why not, for prisons).  There is a lot more variety than in ultimate system 1 because the planets don’t have to be the same size.

The systems of large moons around giant planets would feel the effect of tides.  I made the moons a bit smaller than the planets within the binary Earths to avoid making tides too strong (since they can cause massive volcanism).  Realistically, the innermost moon of each gas giant might very well get roasted by volcanoes, like Io orbiting Jupiter.  That would still leave 32 habitable planets in the system.  Not too shabby!

Which should we choose as the most ultimate?  Ultimate Solar System 1 or 2?  Hmm……

Here comes a ninja master move!  We don’t have to choose between our two ultimate Solar Systems.  We can put them both in a binary star system!  Of course, binary stars do pose a threat to planetary systems.  This was actually the subject of my very first post on this blog. A binary star that is too close destabilizes orbits in the habitable zone.  A binary star that is too far away gets kicked around by very distant stars (really!) and ends up disrupting planetary systems.  But a binary separated by about 100 AU should not disturb the orbits of our ultimate systems.  Especially since the habitable zone is pretty close-in.

The ultimate Solar System. It consists of two of our chosen stars orbiting each other at a distance of about 100 Astronomical Units (= AU; 1 AU is the Earth-Sun distance). Each star hosts one of our ultimate Solar Systems.

This is my ultimate Solar System.  Two of our chosen stars on a wide orbit.  Each hosts one of the systems we just built.  That makes a total of 60 habitable worlds in a single system! 24 in ultimate system 1 and 36 in ultimate system 2.  That’s why this is the ultimate Solar System!

The ultimate Solar System makes a great setting for storytelling.  Just imagine!  Wars between moons orbiting the same gas giant planet.  Coalitions, alliances, trickery!  High-end worlds with their own dedicated beach moons.  Worlds launching long-range missiles at their Trojan counterparts.  Intelligent beings who learn orbital dynamics at a very young age.  Prison worlds that revolt!  Plus, if one species took over all of the worlds orbiting one of the stars, there could be a whole other series of battles with the worlds orbiting the other star.

There you have it.  The ultimate Solar System according to me.

[There is an awesome game called Super Planet Crash that lets you build your own planetary system and see how the orbits evolve.  It's super addictive so watch out!]

 


EPILOGUE: To finish things off, let’s get to the why.  Why did we go through this exercise?  What was the point?  This is a question I ask myself on a weekly basis.  With all sorts of problems affecting humanity — global warming, terrorism, obesity to name a few — why do I spend my days thinking about planets so far away that we will not reach them in our lifetimes? 

I don’t have an answer that will satisfy everyone.  I don’t even have an answer that always satisfies me.  I admit that I do get into funks during which I can’t justify what I do to myself.  I get seriously bummed out.  But whenever I talk with almost anyone about what I do, they are fascinated.  People love astronomy.  They love planets.  They want to hear about planets around other stars.  Planets that could have life.  And they want to know the answers to the questions that I’ve been asking.  Honestly, I think it is that interest from other people that gets me through those funks.  That reassures me that I’m doing something worthwhile.  Something that people care about and are interested in.  And that makes me want to spend time writing this blog.  To get people thinking about planets and life.  To keep people using their imaginations and asking questions.  And to show people that science is inherently fun.

So thank you to everyone who has ever been interested in what I do and given me a boost.  Thanks to several of my teachers.  Thanks to my friends (Andrew West, Ken Sherbenou, Jonah Shaver and Franck Selsis come to mind).  Thanks to my parents.  Thanks especially to my wife Marisa and my sons Owen and Zachary.  You make this all worthwhile.

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Building the ultimate Solar System part 4: two ninja moves — moons and co-orbital planets

We are building the ultimate Solar System.    In Part 1 we chose the right star.  In Part 2 we chose the right planets. In Part 3 we chose the right orbits for the planets.


Today’s job: Discovering two ninja moves that will allow us to pack way more worlds in the habitable zone.

The last post (part 3: choosing the right orbits) was pretty simple stuff.  You can cram more small planets into the habitable zone than big ones.  Nothing too shocking.  Well, learning the basics always comes before the ninja moves.

What makes these moves ninja (to use ninja as an adjective) is that they put more than one world on the same orbit.  This means that we can pack a lot more worlds into our star’s habitable zone.  It’s like the 6-pack: a way to cram more awesomeness into a limited space.

Let’s meet the ninjas.

NINJA MOVE 1: MOONS.  A planet’s moons orbit the star just like the planet does.  I used this to my advantage this when I built a better Solar System.

Here are the large moons in the Solar System:

Large moons of the Solar System, with Earth for scale.  The moons are ordered by which planet they orbit.  From Wikipedia (https://en.wikipedia.org/wiki/File:Moons_of_solar_system_v7.jpg).

Large moons of the Solar System, with Earth for scale. The moons are ordered by which planet they orbit. From Wikipedia.

The biggest Solar System moons orbit the biggest planets (Jupiter and Saturn).  Systems of moons form like mini-Solar Systems, in disks of gas and dust around gas giant planets. [In fact, large Solar System moons have some properties in common with extra-solar planets].  The moons are located very close to the gas giants.  The orbits of the most distant large moons are only about 30 times larger than the radius of their host planet.  In comparison, Earth’s orbit is about 200 times larger than the radius of the Sun.

We want worlds in our ultimate Solar System that are a little bigger than these large moons.  We want worlds about half to twice Earth’s size. Although there is some debate, I’m going to allow any gas giant that is Saturn-sized or larger to have large moons.

In the Solar System, Jupiter has the most (four).  Given how close-in the Solar System moons are located, large moons are likely to stay close.  But how many big moons could a gas giant have?  Well, at least as many as Jupiter (four).  But probably not that many more.  The orbits of planets and moons tend to be spaced logarithmically.  Think, 1, 10, 100, 1000 rather than 10, 20, 30, 40.  The farther from the star/planet, the bigger the spaces between planets/moons.  If the zone with large moons extends from 5 to 50 times the planet’s radius, this only gives us room for 5 large moons spaced like Jupiter’s.  We’ll stick with a maximum of 5 large moons per gas giant planet.

Could a planet like Earth have a moon large enough for life?  The jury is still out on how to form such a moon (probably by a giant impact between two big growing planets).  But there is no reason not to consider this possibility.  However, an Earth-sized planet probably could not have more than one large moon remain stable.   [Note that Earth may have had a second large moon that crashed into the Moon!]  In fact, if an Earth-sized planet had an Earth-sized moon, this would essentially be a binary planet.  Each planet would orbit the other, as the pair orbited the star. Pretty awesome concept!  Pluto and Charon are basically a binary (minor) planet. Charon is about half as big as Pluto and about 10% as massive.

A binary Earth would behave mostly like the Earth-Moon system does today.  But tides would be much stronger.  The two Earths always show each other the same face as their orbit their common center of gravity.

A binary Earth.  An Earth-sized planet with a similar sized moon orbit their common center of gravity.  Each planet keeps the same side pointing to the other.  Credit: Wikipedia http://en.wikipedia.org/wiki/Double_planet

A binary Earth. An Earth-sized planet with a similar sized moon orbit their common center of gravity. Each planet keeps the same side pointing to the other. Credit: Wikipedia

The planets each make one full rotation for each orbit around each other.  This means that a day should be about a month in length.  This may have some impact on the planets’ climate, but probably in a good way.  Slowly-rotating planets may remain habitable closer to their stars than fast-rotating planets.

SUMMARY: A gas giant could have up to 5 moons large enough to be habitable.  Planets in our chosen size range can also have large moons but probably only one.

NINJA MOVE 2: CO-ORBITAL PLANETS.  When you hear the word “Trojan”, you probably don’t think of asteroids.  But they are real!  What is interesting about the Trojan asteroids is that they share the same orbit as Jupiter.  And so do the “Greek” asteroids.  This image shows where these asteroids are located.

The inner Solar System.  The planets are labeled and the blue lines show their orbits.  The small dots are asteroids.  The main asteroid belt is shown in white.  The green dots -- called "Greeks" and "Trojans" -- are co-orbitals with Jupiter.  From Wikipedia. http://en.wikipedia.org/wiki/File:InnerSolarSystem-en.png

The inner Solar System. The planets are labeled and the blue lines show their orbits. The small dots are asteroids. The main asteroid belt is shown in white. The green dots — called “Greeks” and “Trojans” — are co-orbitals with Jupiter. From Wikipedia.

The Trojan and Greek asteroids are about 60 degrees in front of and behind Jupiter.  Normally, when an asteroid comes close to Jupiter, the planet’s strong gravity deflects the asteroid.  Eventually the asteroid’s orbit takes it close to Jupiter.  Jupiter launches the asteroid out of the Solar System.

The Trojan and Greek asteroids live on islands of stability.  It turns out that the positions 60 degrees ahead and behind Jupiter are protected from its strong gravity.  These are called the L4 and L5 points (the L is for Lagrange, who discovered that they are stable).  Since they share the same orbit, they are also called co-orbitals.

Lagrange points of a planet (blue) orbiting a star.  L4 and L5 are the place where co-orbital planets are most likely to be.  From Wikipedia  http://en.wikipedia.org/wiki/Co-orbital_configuration

Lagrange points of a planet (blue) orbiting a star. L4 and L5 are the place where co-orbital planets can survive.  The other points (L1, L2 and L3) and not stable.  Credit: Wikipedia

Asteroids that orbit at L4 or L5 are stable.  They can orbit happily at those points forever.  They don’t stay exactly at L4 or L5; rather, they trace little circles about those points.  That is why the Trojans and Greeks are clouds instead of all being found at a single point.

Co-orbital (aka Trojan) planets are like a person walking with a man-eating tiger but always staying behind it, just in its blind spot.  Perfectly safe, it turns out, but with mortal (gravitational) danger right nearby.

L4 or L5 would be stable islands for an Earth-sized planet.  Even one with a large moon.  In fact, two Earth-sized planets — one at L4 and one at L5 — could be stable. In some circumstances L4 or L5 could even be stable for another gas giant (but just one).

Now switch out Jupiter for Earth.  Earth also has L4 and L5 points.  Earth even has a Trojan asteroid.  Two Earth-sized planets can share an orbit in their mutual L4/L5 points.  Separated by 60 degrees, the two planets’ orbits are stable.

Systems of planets that include co-orbitals have to be a bit more widely-spaced.  Otherwise they become unstable.  That means that we can’t cram quite as many orbits into the habitable zone.

The orbits of planets packed into the habitable zone of our chosen star, with co-orbitals (Trojan planets).  Each orbit is occupied by two planets separated by 60 degrees.  The planets are either 0.1, 1 or 10 times Earth's mass.  The shaded area represents the habitable zone, which extends from about 0.2 to 0.4 Astronomical Units (AU; 1 AU is the Earth-Sun distance).  The number of pairs of co-orbital planets that can be packed into the habitable zone is 9, 6, and 2 for planets with 0.1, 1, or 10 times Earth's mass, respectively.

The orbits of planets packed into the habitable zone of our chosen star, with co-orbitals (Trojan planets). Each orbit is occupied by two planets separated by 60 degrees. The planets are either 0.1, 1 or 10 times Earth’s mass. The shaded area represents the habitable zone, which extends from about 0.2 to 0.4 Astronomical Units (AU; 1 AU is the Earth-Sun distance) for our chosen star. The number of pairs of co-orbital planets that can be packed into the habitable zone is 9, 6, and 2 for planets with 0.1, 1, or 10 times Earth’s mass, respectively.

Even though there are fewer orbits in the habitable zone, there are more planets.  With just one planet per orbit we were able to fit 14, 7, and 3 orbits of planets of 0.1, 1 or 10 times Earth’s mass.  Including co-orbitals we can only fit 9, 5 and 2 orbits.  But two planets per orbit makes it 18, 10 and 4 planets in the habitable zone.  Give them each a large moon and the numbers are doubled.  Boom!

As we saw previously, a system of gas giant planets tends to have different orbital spacing (in resonances).  The gravitational effects of Earth-sized Trojan planets don’t change anything in that case.  So we could still fit four gas giant planets in the habitable zone of our chosen star.  Of course, we can add in some ninja moves there too…

SUMMARY: Given one planet orbiting a star, stable islands exist on the same orbit: 60 degrees in front and 60 degrees behind the planet.  A gas giant planet can have an Earth-sized planet in each of these points with no effect on orbital stability.  Two (but not three) Earth-sized planets can share the same orbit, separated by 60 degrees.  These are called co-orbital or Trojan planets.  Wider orbital spacing is needed for a system of co-orbital planets.

OVERALL SUMMARY: We are becoming ninjas!  With moons and co-orbitals, many worlds can share the same orbit. This means we can pack more Earth-sized worlds into the habitable zone.


Up next: putting the pieces together to build our ultimate Solar System.

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Building the ultimate Solar System part 3: choosing the planets’ orbits

We are building the ultimate Solar System.    In Part 1 we chose the right star.  In Part 2 we chose the right planets.


Today’s job: choosing the right orbits for the planets.

Let’s get started.  Our goal is simple.  We want to pack as many planets into our star’s habitable zone as possible.  We have one key constraint: our system must remain stable for billions of years.  We don’t want any planets falling into the star.  Or crashing into each other.  Or getting thrown out into interstellar space.  No, we want their orbits to remain relatively constant and definitely not do those terrible things.

Orbits. Gravity. That should be pretty simple, shouldn’t it?  [Short answer: No.  Long answer: keep reading.]

How close together can we cram planets’ orbits without making the system unstable?  The habitable zone is only so wide.  The closer we can pack the planets, the more we can fit into the habitable zone.  And the awesomer our ultimate Solar System becomes.

This image shows systems of planets in our preferred size/mass range packed as tightly as possible:

The orbits of planets packed into the habitable zone of our chosen star.  Each black circle around the star is the orbit of a single planet with a mass of either 0.1, 1 or 10 times Earth's mass.  The shaded area represents the habitable zone, which extends from about 0.2 to 0.4 Astronomical Units (AU; 1 AU is the Earth-Sun distance).  The number of planets in a stable system that can be packed into the habitable zone for planets with 0.1, 1, or 10 times Earth's mass is 14, 7, and 3, respectively.

The orbits of planets packed into the habitable zone of our chosen star. Each black circle around the star is the orbit of a single planet with a mass of either 0.1, 1 or 10 times Earth’s mass. The shaded area represents the habitable zone, which extends from about 0.2 to 0.4 Astronomical Units (AU; 1 AU is the Earth-Sun distance). The number of planets in a stable system that can be packed into the habitable zone for planets with 0.1, 1, or 10 times Earth’s mass is 14, 7, and 3, respectively.  [Technical details: each pair of planets is spaced by 10 mutual Hill radii.  (see Smith & Lissauer 2009)]

What can destabilize a system of planets is the planets’ gravity.  The more massive (bigger) the planets, the stronger the gravity.  So we can pack low-mass (small) planets together more efficiently than high-mass (big) ones.  That means we can put a lot more small planets in the habitable zone than large ones. Systems can be packed most tightly if all the planets have the same mass.  If we mix planets with different masses in the same system, they need to be more widely-spaced (for the details, see this paper).

When planets become big enough, their orbital spacing changes.  Big planets are pushed around by the gas disks in which they form.  This pushing — called “orbital migration” —  puts planets in special configurations called resonances.  In resonance, the time for nearby planets to complete an orbit becomes a simple fraction, like 3/2.  Meaning, the outer planet completes 3 orbits for every 2 orbits of the inner planet.  It can be any simple fraction but the most common ones are simple, like 3/2 or 2/1 or 4/3.  Planets, even massive ones, are usually stable in these resonances.  So as planets get more massive they don’t need to be spaced farther and farther apart.  For example, in this image, four Jupiter-mass planets fit comfortably within the habitable zone of our chosen star.

Orbital layout of Jupiter-mass planets in the habitable zone.  Each pair of adjacent planets is in 3:2 resonance, meaning the outer planet completes 3 orbits for every 2 orbits of the inner planet.

Orbital layout of Jupiter-mass planets in the habitable zone. Each pair of adjacent planets is in 3:2 resonance, meaning the outer planet completes 3 orbits for every 2 orbits of the inner planet.

Why do we care about Jupiter-sized planets?  They are way too big and don’t have solid surfaces.  Well, wink wink, we’ll talk about this tomorrow.

There you have it.  Gravity and orbits show us how to squish as many planets as possible into a given area.  And the area we care about is, of course, the habitable zone!  Of course, it’s possible that different-sized planets could have different habitable zones.  But that’s a story for another day.

SUMMARY: The right orbits is the configuration that can squeeze the most planets into the habitable zone.  Small planets can be squished tighter than large ones.  We can fit 14 of our smallest planets in the habitable zone of our chosen star, or 7 Earth-sized planets, but only 3-4 of our largest (10 Earth mass) planets.

But not to fear: tomorrow’s ninja moves will add two big twists to this story.  And blow your mind.

 


Up next: Two ninja moves — moons and co-orbitals

 

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Building the ultimate Solar System part 2: choosing the right planets

We are building the ultimate Solar System.    In Part 1 we chose the right star.


Today’s job: choosing the right planets to put in our ultimate Solar System. 

Let’s stick to two defining characteristics: a planet’s size (or mass) and its composition. We want Goldilocks-ish planets.  They shouldn’t be too small, too big, too dry or too wet.  But how small/big/dry/wet is too small/big/dry/wet?  Let’s check it out.

But first, a quick reminder of why we care (aka exoplanet porn):

Four known roughly Earth-sized planets.  From left to right, and in increasing size order, the planets are Kepler-20e, Venus, Earth, and Kepler-20 f.  (The Kepler-20 planets are artists' impressions.)  Credit: NASA/Ames/JPL-Caltech

Four known roughly Earth-sized planets. From left to right, and in increasing size order, the planets are Kepler-20e, Venus, Earth, and Kepler-20 f. (The Kepler-20 planets are artists’ impressions.) Credit: NASA/Ames/JPL-Caltech

NOT TOO SMALL.  A planet that is too small has a couple of key weaknesses.  First, a planet less massive than about 10% of Earth’s mass doesn’t have strong enough gravity to hold onto an atmosphere for billions of years.  Second, a planet less than about 30% of Earth’s mass is unlikely to have enough internal heat (from Uranium-238) to maintain plate tectonics for billions of years.  Why do we care about plate tectonics?  Because it is Earth’s long-term thermostat.  Then again, in some cases tides can heat planets enough to help with plate tectonics.  In fact, tides should help out for planets in the habitable zones of our chosen, relatively cool stars.

Summary: Too small means smaller than about 10%-30% of Earth’s mass.  For a similar composition to Earth, that is about half to 70% the size.  Let’s let the little guys in and call it half.

NOT TOO BIG.  Why couldn’t we live on an Earth that was a hundred or a thousand times more massive?  Because we wouldn’t have a surface to stand on!  Planets smaller than 1.5 to 2 times the Earth’s size are “super-Earths”.  They are rocky and presumably similar to Earth.  But larger planets are dominated by gas instead of rocks.  No surface to stand on.  Not what we’re looking for.

Summary: Too big means bigger than about twice Earth’s size.

NOT TOO DRY.  This seems like an easy one.  Life on Earth requires water.  We want water on our planets.  But how little water is too little?  This is tricky.  All of the water on Earth’s surface is called one “ocean”.  Earth has about ten more oceans of water in its interior.  If Earth had half or one tenth its current water budget, how would it be divided between the interior and surface?  This is difficult to know.  It depends on how a planet’s water is divided between the planet’s surface and interior.  And this depends on how water cycles through the planet.  If a planet is too dry then all its water could be trapped in the interior with none on the surface.  But exactly how much is not known.

A small water content on planet’s surface can in some cases be beneficial.  So-called “Dune” planets with very little water may be protected against a runaway greenhouse effect and could be habitable closer to their stars than planets like Earth.  Still, you need some water.  Mars probably formed with less water than Earth, yet there is evidence that Mars had episodes of liquid water flowing on its surface.

Summary: Too dry means that there is no water on the planet’s surface because it is all trapped in the interior.  Although this number is not known, let’s throw out a guess and say that too dry means less than about 10% of Earth’s water content (proportional to its mass).

NOT TOO WET.  The question here is whether we want our planets to have continents.  Or can they be water worlds covered in deep global oceans?  A water world cannot have the same geological thermostat as the Earth, because Earth’s relies on burying atmospheric carbon in surface rocks (and later erosion of those rocks).  Still, it’s possible that other geological feedbacks could step in and provide a stable climate.  There hasn’t been enough work on this subject to be sure.

waterworld_movie

Is this really what we want in our ultimate Solar System?

Let’s start with planets with continents.  Continents have been pretty good to us, what with forests and beaches and mountains and all. Our ultimate Solar System should focus on planets with continents.  If we’re in the mood we can always sprinkle in some water worlds.  If nothing else, these could be good vacation spots and encourage civilizations to invent interplanetary travel.

There is a nice new paper that calculates the boundary between water worlds and planets with continents.  It showed that if Earth had about ten times more water it would be a water world.

Summary: Too wet means water worlds with more than about 10 times Earth’s water content (relative to its mass).

OVERALL SUMMARY: We want planets that are not too small, big, dry or wet.  We want planets with atmospheres, surfaces, oceans and continents.  This translates to worlds between about half and twice the size of Earth, with between 10 times less and 10 times more water than Earth.  With maybe a few water worlds thrown in for scuba-diving!

But remember that one way to pack more worlds in our ultimate Solar System is by making them moons of gas giant planets.  So we will be thinking about more than just the Goldilocks planets!

 


Up next: What orbits for the planets in our ultimate Solar System?

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Building the ultimate Solar System part 1: choosing the right star

We are building the ultimate Solar System.  Here is an introduction to the game. 


 

What kind of star will anchor our ultimate Solar System?

It comes down to two choices: stars like the Sun or cooler, redder stars sometimes called “cool stars” or “red dwarfs”.

Why not stars bigger than the Sun?  Because they don’t last that long.  The Sun will become a red giant in about 5 billion years, putting its total lifetime as a normal “main sequence” star at about 10 billion years.  Stars that are bigger than the Sun are also brighter and hotter and they burn out faster.  A star two times as massive as the Sun only lives for about 1 billion years.  That may seem like a long time, but it took about that long for life to appear on Earth!  In contrast, a star one third as massive as the Sun lives for 100 billion years!  That is seven times longer than the age of the Universe.  Small red stars basically live forever.  They are cool and relatively faint, but they are as constant as it gets.

Why not really really tiny “stars” that are even smaller?  Those are called brown dwarfs.  They are actually not bad candidates for having Earth-like planets in their habitable zones.  In fact, they might be relatively easy to detect in some cases.  BUT, brown dwarfs fade.  They don’t burn hydrogen in their cores like normal stars do.  So, they cool down and their habitable zones sweep inward in time.  Any planet has a fixed, relatively short, time in the habitable zone.  In some cases a planet can last upwards of a billion years in the habitable zone so I’m only calling this short when you compare it with cool stars that burn at the same brightness forever.

What to choose: stars like the Sun or cool red stars?  On one hand, the Sun has been good to us.  Earth is here and, let’s be honest, it kicks ass. But that doesn’t mean that Earth wouldn’t be even better off around a different kind of star.  Let’s face it, the Sun is going to fry us in the not-all-that-distant future (say, in a billion years or so).  And cool stars have a lot going for them.  For example, the Earth-sized planet Kepler-186 f was recently discovered in the habitable zone of a cool star.

Let’s have a head-to-head.  I’ll go through a few different factors and see who wins, cool stars or Sun-like stars.

STELLAR LIFETIME.  Cool stars live basically forever.  It’s hard to beat that.  Although the Sun’s 10 billion year lifetime is not too shabby.  This figure shows how long stars with different masses live.  Cool stars are smaller than about half of an Earth mass.

The lifetime of a star as a function of its mass.  The Sun, at 1 Solar Mass, has a lifetime of about 10 billion years!  Credit: http://astronomy.nmsu.edu/tharriso/ast105/Exoplanets.html.

The lifetime of a star as a function of its mass. The Sun, at 1 Solar Mass, has a lifetime of about 10 billion years! Credit: Tom Harrison.

Still, the Sun is slowly getting brighter and hotter.  The Sun today is about 30% brighter than it was in its infancy.  And in a billion years or so, the Sun will be so bright that Earth will probably get fried.  Cool stars don’t change.  They keep cranking away at the same brightness for eons. WINNER: Cool stars.

THE HABITABLE ZONE.  In this game the habitable zone is our real estate.  It’s where we want to build our ultimate planetary system.  It is in the habitable zone that a planet can have liquid water and therefore life (as we know it) on its surface.  [I have a series of posts about the habitable zone (written with Franck Selsis) coming soon.  So, I won't dwell on the details here.]

This diagram shows where the habitable zone is located for different types of stars.

 

The habitable zone.  The y axis is the stellar mass (the Sun = 1) and the the x axis is the orbital radius (Earth = 1).  The colored curves shows how estimates of the habitable zone change for different types of stars.  Credit: Chester Harman.

The habitable zone. The y axis is the stellar mass (the Sun = 1) and the the x axis is the orbital radius (Earth = 1). The colored curves shows how estimates of the habitable zone change for different types of stars. Some known planets are included (with artistic impressions).  Credit: Chester Harman.

The habitable zone is much closer-in for cool stars than for the Sun.  This is because cool stars are fainter.  To keep warm you need to stand a lot closer to a candle than to a bonfire!

The habitable zone is narrower for low-mass stars.  It’s almost a full AU wide for the Sun but only a few tenths or hundredths of an AU wide for low-mass stars.  Does this mean there is less space for planets?  No!  The orbits of a system of planets tend to be spaced in a logarithmic way (for example, at 1, 2, 4, 8, 16 rather than at 1, 2, 3, 4, 5) .  There is about the same amount of “dynamical space” for planets in orbit around cool stars and Sun-like stars.  WINNER: Tie.

RADIATION.  All stars give off light with a wide spectrum of different energies.  Our eyes only see at certain wavelengths, in what we call “visible” light.  High-energy light such as ultraviolet (UV) and X-rays can be damaging.  UV light causes sunburns.  Strong X-ray and UV irradiation can act to strip a planet’s atmosphere or dry it out.  This is a pretty complicated process: water is first broken into hydrogen and oxygen, then the hydrogen can be kicked off into space never to return.

Even though they are cooler and fainter, low-mass stars have proportionately larger high-energy light than Sun-like stars.  This comes from the outer part of the star that are magnetically active, called the chromosphere.  In a star like the Sun, the chromosophere is very active and produces a lot of X-rays and UV for just a small fraction of the star’s life.  Then it quiets down.  But for low-mass stars the chromosphere keep cranking out high-energy light for billions of years.

High-energy light is not all bad.  Some UV irradiation may have been needed to kick-start life. It has also been proposed that UV may be a requisite for photosynthesis.  In this game mutations from UV light sound good.  Because we want our aliens to be as crazy and diverse as possible!  But, too much UV and you’re fried.  How much is too much?  We don’t know.  Hmmm….   WINNER: Slight edge to Sun-like stars.  But cool-ish stars that get a little more but not too too much high-energy light might be okay.

TIDES.  When you think of tides, you probably think of the Moon making the ocean slosh around.  Tidal forces are simply differences in gravity.  When a planet is close to a star (or the Moon to the Earth), the side facing the star feels a stronger gravity than the opposite side.  This stretches the planet out.  The amount of stretching changes if the planet moves a little closer or farther from the star.  This causes a few things to happen.  The planet tries to always show the same face to the star (like the Moon does to the Earth).  This means the planet spins once every time it goes around the star.  In this image it’s a moon that is “locked” to a planet.

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 (http://en.wikipedia.org/wiki/File:Synchronous_rotation.svg)

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

Same idea for a planet locked to a star.  Only one side of the planet ever sees the Sun.  Permanent night on one side, permanent day on the other.  And permanent sunset in between!  [I'll mention in passing that tides also make a planet's orbit as circular as possible, but that's not too important right now.]

Put simply, tides do two things that matter in this context.   First, tides heat up the planet by dissipating energy inside it.  Second, tides makes the planet always show the same face to the star.

Are these things good or bad?  Some heat from tides could be helpful — it might stir things up in the mantle and help plate tectonics to occur.  Too much is bad.  Io, Jupiter’s closest big moon, is strongly heated by tides and is riddled with permanently active volcanos.  Same goes for having the same side of the planet face the star.  A planet with a thin atmosphere could end up getting fried on the dayside and frozen on the nightside.  But on a planet with an Earth-like atmosphere, clouds can pile up on the dayside, smooth out the temperature across the whole planet, and even widen the habitable zone.

Tides are stronger for planets that are closer to their stars.  We know that the habitable zone is closer-in for cooler stars.  So tides are stronger in the habitable zones of cool stars.

WINNER: Sun-like stars, by a hair.  There are enough “bads” to make me wary.  But weakish tides are okay, even beneficial.

THE FINAL VERDICT

I am pretty torn.  As I explained, we don’t know whether UV is good or bad for a planet or whether tides are good or bad.  The head-to-head is a wash.  Based on what we know right now there is no compelling argument in either direction.

So I’m going with my gut.  I’m choosing lowish-mass stars.  Stars about half the size/mass of the Sun.  Toward the hotter end of the cool stars.  These stars end up with most of the pluses of cool stars without the minuses.  These stars live a lot longer than the Sun with a more gradual change in brightness.  Their habitable zones are just close enough to their stars that tides are strong enough to possibly help with plate tectonics and widen the habitable zone but too weak for massive volcanism.  These stars emit high-energy light (UV etc) for longer than the Sun but not forever.  And the clincher is that we can find planets in the habitable zones of these stars!  The star Kepler-186 is half the size and half the mass of the Sun.  It is host to habitable zone Earth-sized planet Kepler-186f and possibly an additional yet-to-be-detected habitable zone planet.  We can’t find Earth-sized planets in the habitable zones of Sun-like stars (yet).


 

Up next: which kind of planets do we want in our ultimate Solar System?

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Building the ultimate Solar System

A while back I performed an experiment called build a better Solar System.  The game was to make better use of the Solar System’s habitable real estate.  In the game I was required to keep all of the Solar System’s planets (and large moons) and their orbital configurations.  Just by switching the orbits of different planets and moons I built a Solar System with seven potentially habitable worlds!  Here is what it looked like:

A re-imagined Solar System with seven potentially life-bearing planets!  The liquid water habitable zone is shaded in green.

A re-imagined Solar System with seven potentially life-bearing planets! The liquid water habitable zone is shaded in green.  For an explanation of how I came up with this, see this post.

This better system is a little different than the Solar System we have now.  Venus became one of Jupiter’s moons, Jupiter took Mars’ orbit, Mars took Venus’ orbit, and Earth’s moon was switched out for Titan (currently Saturn’s largest moon).  Good stuff.

Let’s take this game to the next level.  I want to build the ultimate Solar System.  I want to build a planetary system with the most possible habitable worlds. 

I hope you are not asking yourself why.  It’s all about imagination.  But, imagination constrained by science (and, to a lesser extent, by reality).  Imagine all the stories you could tell about a system with lots of habitable worlds!  Alliances, wars, vacations on other planets, even orbital trickery!

Let’s do this systematically.  I’ll discuss one piece of the puzzle at a time, then we’ll put them together.  I’m going to go nuts and do this all next week (19-23 May 2014).  Here is the layout (I’ll add links as these are posted).

1. What kind of star do we want? (Mon)

2. What kind of planets do we want? (Tues)

3. What types of orbits should planets have? (Weds)

4. Ninja moves: moons and co-orbitals (Thurs)

5. Putting it all together: some ultimate planetary systems.  (Fri)

 

Here we go!

 

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Another planet in Kepler-186?

That spanking new planet’s already a star.
K-186 f, you know who you are.
You’re making us wonder if we’re all alone.
The planet out there in the habitable zone.

I’ve been on the radio.  Been on TV.
Talking ‘bout the planet. Just what can we see?
Just what do we know about this special rock?
Are there little green men?  Just how do they talk?

We don’t know nearly as much as we’d like.
And five hundred light years is kind of a hike.
But if there might be life, I’ll clean off my bike.
Or rather my rocket.  I’ll set it to zoom.
Or maybe I’ll dust off my old wizard broom.

Wait, stop!  This is heading in the wrong direction.
The planet’s too far for a close-up inspection.
If we blast our fastest rocket high up into space
It’ll be 10 million years ‘til we reach that far place.

So we’ll just have to rely on our telescopes,
computers and brains.  That’s right, we’re no dopes.
To figure things out we need to stop blundering.
We need to chill out and do some good wondering.

Let’s take a close look at the entire system.
Are there more planets?  Might we have missed ‘em?
If there are more planets, they will need space.
Now is there anything that’s out of place?

The four inner planets are crammed like sardines.
No space for another to fit in between.
But then there’s a gap and it’s pretty wide.
You could easily fit another planet inside.

So I sat down and ran some simulations
on my computer.  And these calculations
show that a planet really can stick around.
Right in the gap.  The gap we just found.

The Kepler-186 planetary system, reimagined with a hypothetical sixth planet.  The top part shows a view of the planets' orbits.  The habitable zone is shaded.  The bottom part compares the amount of energy received by each planet with the energy received by the Solar System's planets.  The hypothetical planet -- at its most likely location --  receives just slightly less energy from its star than the Earth does from the Sun, placing it toward the inner edge of the habitable zone.

The Kepler-186 planetary system, reimagined with a hypothetical sixth planet. The top part shows a view of the planets’ orbits. The habitable zone is shaded. The hatched area is the “gap” where another planet could exist.  The bottom part compares the amount of energy received by each planet with the energy received by the Solar System’s planets. The hypothetical planet — at its most likely location — receives just slightly less energy from its star than the Earth does from the Sun, placing it toward the inner edge of the habitable zone.

Another planet?  Wouldn’t we find it?
Well, not if its orbit is out of alignment.
If it’s just tilted by one small degree
we probably would miss it.  It’s that hard to see.

But this extra planet should have the same girth
as the other planets.  About like the Earth.
Just like the others it’s a rocky place.
One more rocky planet floating out there in space.

This planet wouldn’t be boiling or freezing.
It should be quite warm.  It would be quite pleasing.
It could have oceans and great lakes and rains.
And rivers that wind their way across the plains.

It would be number two in the habitable zone.
Its big brother would not have to be all alone.
Two planets with water, one near and one far.
Two places for life around the same star.

Of course I am being a little bit careless.
It may be, in fact, that these planets are airless.
All we really know is planet f’s size.
And hints there might be this extra guy.

The trickiest part is, how can we find it?
The planet that’s sitting there out of alignment.
I think that it’s there but I have no proof.
And the Kepler satellite last year went poof.

Now Kepler’s back but it’s not as strong.
So it can’t go back to check if I’m wrong.
For now there’s no other ‘scope in the land
or even in space or New York or Japan
that can find this planet.  It’s just too faint.
So maybe it’s in there or maybe it ain’t.

I hope that we’ll find it, I’ll never say never.
But someone will have to do something real clever.
I hope it comes soon.  Hope it’s not too slow.
I think that it’s there.  But for now we don’t know.

Now…..
If you find a planet you don’t get to name it.
The planet’s not yours. You don’t get to claim it.
But this extra planet has not yet been found
so I don’t think that anyone will make a sound
if I call this planet a special thing.
Planet Marisa, that has a nice ring.

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An Earth-sized planet in the habitable zone of a cool star

In the spirit of last week’s poetic post, here are two more stanzas for your reading pleasure….

There is a new exoplanet in town.
This planet has only just now been found.
Why should you care?  It’s only one more.
Well this is one planet we’d love to explore.

This planet’s orbit is really just right.
It isn’t too cold.  You won’t freeze at night.
And it isn’t too hot.  You won’t get sun-baked.
In fact you could go take a dip in a lake.

OK, the rhyming stops there…. Let’s get to business. 

I want to introduce a newly-discovered planet called Kepler-186 f.  I was lucky enough to be part of the team that found this planet (it was led by scientists at NASA and the SETI Institute).  What is special is that the planet has the right temperature for water to be liquid on its surface.  All life on Earth depends on liquid water, so this is a pretty big deal!  Disclaimer: this planet needs to have the right kind of atmosphere and the right composition to actually have liquid water on its surface.  We don’t know anything about the planet’s surface or atmosphere.

Kepler-186 f is the most distant planet in the Kepler-186 system.  The system was discovered by the remarkable Kepler space telescope.  Each of the five planets in the system is slightly bigger than Earth, but none is more than 40% larger.  This means that the planets are probably rocky or at least have solid surfaces.  Only bigger planets — with sizes larger than about 1.5 to 2 times the size of Earth — are likely to be gaseous “mini-Neptunes”.

Below is an artist’s view of Kepler-186f.  You can see the four other planets closer to the star.  One is even about to transit in front of the star!

Artist's view of the potentially habitable planet Kepler-186f.  Credit: NASA/Ames/JPL-Caltech/T. Pyle.

Artist’s view of the potentially habitable planet Kepler-186f. Credit: NASA/Ames/JPL-Caltech/T. Pyle.

We don’t know anything about the planet apart from its size and orbit.  So this is just speculation.  Still, let’s let our imagination run wild.  To be consistent we need to take into account the fact that the planet’s host star (Kepler-186) is different than the Sun.  Kepler-186 f’s sky is not blue because there is not enough blue light reaching the planet.  There are also different kinds of clouds than on Earth.  As we’ll see below, this planet needs a decent amount of greenhouse heating to maintain liquid water on its surface.  This heating could very well come from a relatively dense atmosphere containing carbon dioxide (CO2).  If that is the case, then complex climate simulations tell us that Kepler-186 f should have multiple cloud layers.  A layer of water clouds should exist at a similar altitude as on Earth.  Another layer of CO2 ice clouds should be located much higher up and cover a significant fraction of the planet’s surface.  But watch out — the clouds can’t cover up the surface and make the image boring!  Everyone wants the planet to have vast oceans but some continents!

This image compares the orbital layout of the Kepler-186 system with the Solar System:

Comparison between the Kepler-186 system and the Solar System. The green areas represent each star’s habitable zone. Credit: NASA/Ames/JPL-Caltech/T. Pyle.

Kepler-186 f’s orbit is located in the outer parts of the habitable zone.  It receives one quarter to one third as much energy as the Earth does from the Sun (within the errors bars on the observations).  This is less than Mars.  The planet needs an atmosphere to heat it up.  This heating can come from greenhouse gases in its atmosphere.  The most common greenhouse gases in the Solar System are water vapor and CO2.  An atmosphere including CO2 (as on Mars and Venus) and a little Nitrogen (as on Earth) can heat the planet’s surface sufficiently for water to be liquid.  The exact amount of CO2 and Nitrogen that are needed depend on exactly how much energy the planet receives from the star.  It’s comparable to the density of Earth’s atmosphere.  Punchline: Kepler-186 f does indeed belong in the habitable zone.

Below is a comparison between the layout of Kepler-186 and three other planetary systems that contain a planet in the habitable zone.

Schematic view of the Kepler-186 planetary system.  The top part shows a top-down view of the planets' orbits.  The planets' sizes are to scale but not on the same scale as the orbits.  The bottom part shows a comparison between four different systems with small planets in the habitable zone (gray shaded areas).  Credit: Sean Raymond, also Bolmont et al (2014).

Schematic view of the Kepler-186 planetary system. The top part shows a top-down view of the planets’ orbits. The planets’ sizes are to scale but not on the same scale as the orbits. The bottom part shows a comparison between four different systems with small planets in the habitable zone (gray shaded areas). Credit: Sean Raymond, also Bolmont et al (2014).

Kepler-186 f is not the hottest planet out there.  It is indeed toward the outer parts of the habitable zone.  However, it does receive a little more energy from its star than the outermost planet in the GJ 581 system (GJ 581 d).  That planet has been studied like crazy.  It has been convincingly shown that it represents a potentially habitable planet, as a reasonable atmosphere (with CO2) can heat the planet enough for water to be liquid on its surface.

What makes this new planet special?  Well, it’s not the first planet discovered in the habitable zone.  And it’s not the first probably-rocky planet discovered in the habitable zone.  It is the smallest planet found in the habitable zone so far.  But there will certainly be even smaller ones found in the near future.  There is no reason to get caught up on labeling this planet the best or more interesting.  In my opinion, this is an important planet because it is a stepping stone on the path to finding true Earth analogs around other stars.

Also of interest: simulations of the formation of the Kepler-186 system systematically predict that an additional planet should exist between planets e and f.  Another planet could easily “hide” in there and not be detected by Kepler.  Such a planet could be in the inner parts of the star’s habitable zone.  Pretty exciting (but speculative).

More information: The paper presenting the discovery of Kepler-186 f (led by Elisa Quintana) comes out in Science April 18, 2o14 and can be accessed here.  The companion paper presenting a study of the system’s formation, tidal evolution and habitability (led by Emeline Bolmont) was submitted to the Astrophysical Journal and can be accessed here.

 

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