Astrobiology research: Life possible on extrasolar moons

Jan 10, 2013
Artist’s conception of two extrasolar moons orbiting a giant gaseous planet. Credit: R. Heller, AIP

(Phys.org)—In their search for habitable worlds, astronomers have started to consider exomoons, or those likely orbiting planets outside the solar system. In a new study, a pair of researchers has found that exomoons are just as likely to support life as exoplanets.

The research, conducted by René Heller of Germany's Leibniz Institute for Potsdam and Rory Barnes of the University of Washington and the , will appear in the January issue of Astrobiology.

About 850 —planets outside the solar system—are known, and most of them are sterile , similar to Jupiter. Only a few have a and orbit their host stars in the habitable zone, the circumstellar belt at the right distance to potentially allow liquid surface water and a benign environment.

Heller and Barnes tackled the theoretical question whether such planets could host habitable moons. No such exomoons have yet been discovered but there's no reason to assume they don't exist.

The climatic conditions expected on extrasolar moons will likely differ from those on extrasolar planets because moons are typically tidally locked to their planet. Thus, similar to the Earth's moon, one hemisphere permanently faces the planet. Beyond that moons have two sources of light—that from the star and the planet they orbit—and are subject to that could significantly alter their climates, reducing stellar illumination. "An observer standing on the surface of such an exomoon would experience day and night in a totally different way than we do on Earth." explained Heller. "For instance stellar eclipses could lead to sudden total darkness at noon."

Heller and Barnes also identified tidal heating as a criterion for exomoon habitability. This additional energy source is triggered by a moon's distance to its host planet; the closer the moon, the stronger tidal heating. Moons that orbit their planet too closely will undergo strong tidal heating and thus a catastrophic runaway greenhouse effect that would boil away surface water and leave them forever uninhabitable.

They also devised a theoretical model to estimate the minimum distance a moon could be from its host planet and still allow habitability, which they call the "habitable edge."This concept will allow future astronomers to evaluate the habitability of extrasolar moons. "There is a habitable zone for exomoons, it's just a little different than the for exoplanets," Barnes said.

The exquisite photometric precision of 's Kepler space telescope now makes the detection of a Mars- to Earth-sized extrasolar moon possible, indeed imminent. Launched in 2009, the telescope enabled scientists to reveal thousands of new extrasolar planet candidates. Since 2012 the first dedicated "Hunt for Exomoons with Kepler" is under way.

Heller and Barnes' paper, "Exomoon Habitability Constrained by Illumination and Tidal Heating," will be published in the January issue of the journal Astrobiology.

Explore further: Transiting exoplanet with longest known year

More information: R. Heller, R. Barnes: Exomoon habitability constrained by illumination and tidal heating. (Preprint) In: Astrobiology, issue 01/2013. arxiv.org/abs/1209.5323

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javjav
4 / 5 (4) Jan 10, 2013
It seems to me tat the habitable zone for these planets is not only possible but even bigger than it is for earth sized planets. This is because 3 reasons:

- First, the temperatures in both sizes of tidally locked moons may be very different, so there are more chances to find the right temperature for liquid water at some point in between.

- Secondly, both theoretical models and local examples predict several moons around big planets, so there is higher probability that one of them could have the appropriate size and atmosphere for a given orbit, in comparison with a single earth like planet that is a single chance. Look at Jupiter and Saturn moons, they are very different even when their orbit the same planet.

- Third, there is more energy available, either due to tidal heating, sudden temperature changes due to eclipses, and stronger winds, which also means more differentiated areas in a same moon between the poles and equator. Bigger ranges also mean more chances
evolution3
4 / 5 (2) Jan 11, 2013
It seems to me tat the habitable zone for these planets is not only possible but even bigger than it is for earth sized planets. This is because 3 reasons:

- First, the temperatures in both sizes of tidally locked moons may be very different, so there are more chances to find the right temperature for liquid water at some point in between.


Problem with that is, that tidally locked may cause big heat differences and thus producing strong winds all the time. May be a harsh environment.
RealScience
5 / 5 (1) Jan 11, 2013
@Evolution3 - That would be true for a PLANET tidally locked to its STAR (central heat source).
But for a MOON tidally locked to its PLANET, the tidally-lock moon's 'day' (light-to-dark) is its orbital period around its planet.

And moons move fast around large planets. If the moon had an orbit of only a few days (like some of Jupiter's large moons), then the side of the moon facing the star would change every few days.
Thus if it had an atmosphere a short-period moon would not have huge temperature differences even if tidally locked.
evolution3
5 / 5 (1) Jan 12, 2013
@Evolution3 - That would be true for a PLANET tidally locked to its STAR (central heat source).
But for a MOON tidally locked to its PLANET, the tidally-lock moon's 'day' (light-to-dark) is its orbital period around its planet.

And moons move fast around large planets. If the moon had an orbit of only a few days (like some of Jupiter's large moons), then the side of the moon facing the star would change every few days.
Thus if it had an atmosphere a short-period moon would not have huge temperature differences even if tidally locked.


My bad...you are right.
GSwift7
5 / 5 (1) Jan 14, 2013
I think the biggest question is whether they can hold their atmosphere.

Based on our solar system, it seems difficult for a small terrestrial planet to keep its atmospher when it orbits in the habitable zone. I wonder if being a moon would help or hurt the likelyhood of keeping an atmosphere? I guess the answer to that would depend on a LOT of variables, and we'll have to wait and take it on a case by case basis until we get a good volume of observations.

The largest moon in our solar system isn't big enough to retain an atmosphere in the habitable zone. It will be interesting to see if larger moons are common in other systems. I would say that if they aren't as big as Mars at the very least, then the chance of them being habitable is slim.
Modernmystic
not rated yet Jan 14, 2013
Finally an article which illustrates at least a FEW of the variables which might exclude habitability instead of looking at the orbit and what they estimate the surface temperature to be, and cry "HABITABLE!!!!"

Quite refreshing to see some intellectual honesty in this piece.
typicalguy
not rated yet Jan 19, 2013
I think the real question is, how do we get rid of those pesky Ewoks?