Detecting Habitable Exomoons

by Paul Gilster on December 21, 2009

What a welcome event the release of James Cameron’s new film Avatar must be for scientists working on the question of exomoons — satellites orbiting extrasolar planets. Imagine being a Lisa Kaltenegger (CfA) or David Kipping (University College London), hard at work exploring exomoon detection and possible habitability when a blockbuster film is released that posits a habitable moon around a gas giant. The film’s exomoon, called Pandora, fits a scenario that exomoon hunters tell us could exist, orbiting a giant planet in the habitable zone of its star, and it draws public attention as never before to exoplanet and exomoon detections.

super_earth_exomoon

Interesting exomoon scenarios beyond gas giants are also possible, as this image shows. We’re learning that we can detect exomoons using tools like transit time duration measurements, and there are other methods, too, like microlensing and distortions of the Rossiter-McLaughlin effect. A paper from Kipping that we examined here recently makes the case that a Kepler-like instrument could detect exomoons around gas giants in the habitable zone down to 0.2 Earth masses — that would yield 25,000 stars, for example, in Kepler’s field of view that could undergo screening for such moons.

Image: This artist’s conception shows a hypothetical ringed super-Earth as viewed from one of its moons. Both super-Earth and moon are habitable and contain liquid water. Credit: David A. Aguilar, CfA.

What Lisa Kaltenegger asks in her new paper is whether we could make any definitive statements about the habitability of such worlds. And it turns out the answer is yes, assuming we’re using the right equipment (a 6.5-meter instrument, like the James Webb Space Telescope, would be required). And assuming that the exomoon in question is close to maximum separation, in which case an exomoon transit across its star (as seen from Earth) is comparable to the transit of a planet of the same size, provided that the projected semimajor axis is larger than the stellar radius. The transits of planet and exomoon, in other words, need to be separate events so that we can make the necessary measurements to characterize the exomoon.

From the paper:

Habitable-zone exomoons may be detected in the near future with missions like Kepler and could be orbiting their planet at a distance that allows for spatially separate transit events. In that case transmission spectroscopy of Earth-like exomoons is a unique potential tool to screen them for habitability in the near future. Spatially separating the exomoons from their parent planet improves their detectability because their absorption signature are about two orders of magnitude lower than the absorption features of an EGP [extrasolar giant planet] spectra.

We learn here that low-mass M-class dwarfs are the easiest stars to work with because of the favorable contrast of star to planet. In fact, Kaltenegger’s work shows that in the best case scenario, atmospheric H20, CO2 and O3 features in the infrared could be detected in one year for transiting habitable exomoons around M5 to M9 stars at a distance of up to 10 parsecs. Note that nearby M-dwarfs are ideal for this investigation because their habitable zones are close to the star, increasing transit probability and transit frequency.

M-dwarf exomoons have another advantage in the issue of habitability. We’ve long debated whether a planet in the habitable zone of such a star would be capable of hosting life considering that it would probably be tidally locked to its star, one side constantly baked by sunlight, the other in constant darkness. Whatever the answer (and tidal lock may not be a show-stopper for life on such worlds), an exomoon around a gas giant in this scenario, says Kaltenegger, would be tidally locked not to the star but to the planet, and would therefore have regular night-day cycles, just like Earth.

Cameron’s film depicts a habitable exomoon in the Alpha Centauri system, specifically around Centauri A. If such a moon were to exist around Centauri A, a 6.5-meter instrument like the JWST would indeed be able to characterize it, and quickly. Kaltenegger notes:

“You would only need a handful of transits to find water, oxygen, carbon dioxide, and methane on an Earth-like moon such as Pandora.”

In fact, under idealized observing conditions, a single transit observation of an Earth-like body around Centauri A would potentially allow screening for habitability. We know from earlier work that gas giants are unlikely around Centauri A or B — we would have probably found them by now — but we can’t rule out Earth-class planets. The person to turn to on such matters is Debra Fischer (Yale), whose ongoing work examining the Centauri stars for planets may soon yield answers to such questions. Fischer was recently quoted in Popular Mechanics in an article discussing the accuracy of the Avatar movie. Here’s a snippet from the article discussing Fischer’s radial-velocity work at the Cerro Tololo Inter-American Observatory (CTIO) in Chile.

This “wobble technique” has already ruled out the presence of Jupiter- or Saturn-scale exoplanets (like Polyphemus) around the Alpha Centauri stars. But “there’s a very good chance,” Fischer says, that planets with masses near that of Earth’s could grace this star system. “There’s still so much we don’t understand about planet formation around single stars,” Fischer says — let alone triple-star systems like Alpha Centauri. So in terms of what may be out there, “it’s almost an open slate.”

An open slate indeed. And certainly the question of habitable exomoons around nearby M-dwarfs remains open as well. The paper is Kaltenegger, “Characterizing Habitable Exo-Moons,” available online. Also see David Kipping’s new paper “Pathways Towards Habitable Moons,” (available online), from the “Pathways towards Habitable Planets” Symposium in September, which includes this interesting statement that confirms Kaltenegger’s optimism on characterizing habitable exomoons:

For the case of a system within 10pc and a 0.2 M⊕ habitable exomoon, we predict that molecular species could be found using transmission spectroscopy with JWST after the binning of ∼30 transit events.

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Terraformer (a.k.a Tobias Holbrook) December 21, 2009 at 10:36

Hmmm. One of the issues about M-dwarf habitability is the solar flares they have a tendency to release, which erodes away the atmosphere. A gas giants magnetic field protecting it might be just what the terraformer ordered.

Other issues are the brightness spikes which sometimes occuraround such stars. Not sure how that would affect moons differently to planets.

Carl December 21, 2009 at 12:45

Now I know why the name ‘Pandora’ has been used often here.

Any civilization on an exomoon could have technical advantages over our own. One is obvious evidence of true celestial motions. Asimov had argued that astronomy and physics would have been advanced long ago if Venus were to have a moon visible from Earth; the notion that only our world could be a primary body wouldn’t be entrenched as it was. Intelligent beings indigenous to an exomoon… assuming clear skies… would see their world swing about the parent planet, as long as they were on the hemisphere facing it.

A presumed jovian world would more than likely possess satellites numbering over a dozen, making the local space of an inhabited exomoon a certain lure, once technology permitted space flight. Yes, the large gravity well would require a lot of energy/propellant to navigate (astrogate?), but flight times to such neighbors would be small compared to Earth-Mars transits.

kurt9 December 21, 2009 at 12:49

There is flaring of the M-star. There is also the fact that any planet in the Goldilocks orbit is probably going to be tidally locked (one face towards the sun). Lastly, if the primary planet has a gas giant like van allen belt, the radiation of such will cook any moons such that there is no life on them, at least land life. The oceans might be OK, but the immediate space neighborhood will be hazardous in the extreme.

Wedge December 21, 2009 at 16:53

@Carl, I’ve thought about that as well. If you look at some of the big struggles in the advancement of science and exploration on Earth you find that a lot of those problems are dramatically transformed on an exo-moon. Universal time keeping becomes much easier because of the likely presence of additional moons in the system. Geocentrism becomes untenable in the face of evidence available to the naked eye. Things like the longitude and latitude problems on Earth become almost trivial on an exo-moon (assuming it’s tidally locked), instead of accurate clocks and other instruments you need nothing more than a simple quadrant.

And, of course, once rocketry develops there is a much greater draw for manned spaceflight as there will be a plenitude of nearby targets of exploration and colonization, some of them likely much more interesting than our Earth’s Moon.

andy December 21, 2009 at 17:37

Gas giants aren’t particularly common around M dwarfs, those that exist tend to be located quite a way out from the star, beyond the ice line. Gliese 876, with its two gas giants located near the habitable zone, seems to be a bit of an oddball.

Whether Neptune-class planets are likely to host Earthlike moons, and whether such systems are likely to survive migration so close to the star is another matter.

bigdan201 December 21, 2009 at 20:03

They probably used the Centauri system in the movie because thats what people are most familiar with.
The exomoon possibility is intriguing. It’s entirely possible that there are exomoons around gas giants with almost earthlike gravity (gravity is much more important for habitability than size, though theyre very related).
The radiation belt of gas giants is a definite consideration.. however, im pretty sure some of jupiters moons get much less radiation than the others.. and if the moon had its own magnetosphere, that could make it even better (i believe ganymede does). Then again, theres the possibility of a big exomoon around a superearth.. who knows what else is out there, but theres certainly alot of possibilities.
I always wondered what the night sky would look like from a gas giants exomoon.

Athena Andreadis December 21, 2009 at 21:16

One of the earliest and best depictions of such a concept is Joanne Vinge’s Eyes of Amber which richly deserved its Hugo Award. Its aliens are real aliens, imaginatively, sensitively and consistently portrayed.

Which brings us to Cameron’s Avatar. My opinion of it: Jar Jar Binks Meets Pocahontas.

Dispatcher December 21, 2009 at 22:19

I’ve enjoyed lurking around this site on and off over the months; now I will join in. As to Avatar calling attention to the concept of habitable moons, that is true. Yet the concept is not ‘alien’ to science fiction. Recall that the gas giant Yavin of Star Wars lore possessed a habitable moon. In the Halo universe, the Halo structure orbited the moon Basis, which in turn orbited the gas giant Threshold. There are probably other examples as well. Yet the difference in the case of Avatar is, the star in question actually exists in Alpha Centauri A and its relative ‘proximity’ beckons to us.

I think one thing might be problematic with a moon orbiting a jovian world; radiation. Such a moon would at least need to orbit outside of a world’s radiation belts. I understand that jovian worlds have been largely ruled out at the Centauris. However, relatively benign saturnian (as to mass) or even neptunian worlds may make better hosts for habitable moons.

Thanks for the thought provoking articles and posts!

FrankH December 22, 2009 at 0:27

I think “habitable moons” is like trying to describe a Unicorn. You can get into all sorts of details about the fur, the horn, which way it twists as it grows.. and in the end you’re describing a mythical beast.

A gas giant probably can’t form a potentially habitable (at least 2x the mass of Mars) moon. An icy moon won’t survive for long in a star’s HZ even if the gas giant somehow accretes an Earth sized (or at least 2x the size of Mars) moon.

If the gas giant captures a habitable rocky planet in the star’s HZ (or drags one with it as it migrates inwards) then maybe it could support life… but that’s a big if.

It’s worth looking for big moons around any gas giants orbiting in a star’s HZ… but it’s more than likely that the star can be written off as a host to habitable worlds.

Adam December 22, 2009 at 5:55

Presumably Polyphemus is a super-Neptune class planet and Pandora was captured from a solar orbit, after forming in a Lagrange point that it wandered from due to perturbations from Alpha Centauri B. That’d be my best guess on how to make it fit.

Freederick December 22, 2009 at 9:22

The post talks about “easy detection of exomoons in the Centauri system” and even detailed atmosphere scans. But a careful reading shows that this describes a “best case scenario”, to wit, a transiting planet with exomoons. This would require that the Sol-Centauri line of sight lie fairly accurately in the plane of the Centauri ecliptic–a statistically improbable situation. So there still might be exomoons there, just not transiting and so not easily detectable.

andy December 22, 2009 at 12:41

Having seen the trailers of Avatar, I had my suspicions that it would be as Athena Andreadis described “Jar Jar Binks meets Pocahontas”… very visually impressive but plotwise quite derivative. Hopefully going to see it some point soon to make up my own mind about it.

Terraformer (given name = Tobias Holbrook, getting my digital paper trail up) December 22, 2009 at 12:42

Pandora? Probably the project of some Forerunner aiming for a PhD in Advanced Terraforming; i.e. building a planet/moon from scratch given the resources of the system. Stellar mining if you need a giant planet and there isn’t one, accretion of smaller bodies into an Iron rich moon… a Forerunner ship could have a lot of fun in an empty system. Polyphemus was probably a Neptune sized body, which was probably in the process of losing it’s atmosphere. If it migrated inwards before gaining a thick atmosphere, though, it would be turned into a water world.

But Habitable exomoons around M-dwarves are probably unlikely, given all the issues they have to contend with. Tidal flexing might allow a molten core, though, whcih has a lot of benefit

JJeans2.0 May 8, 2012 at 22:23

The thought quite captures the mind that a possible exomoon might orbit around it’s parent planet that might be possibly neptune-sized, saturn-sized, or maybe a super-Earth, tidally locked to it’s star. The hit movie” Avatar” by James Cameron, brought up the possibilities of these exomoons inhabiting some form of extraterrestrial life.

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