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?

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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.

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:

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).

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.

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:

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.

BUT WAIT……

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.

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:)

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.

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.

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.

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!

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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?