Habitable Exomoons Should Be Detectable

by Paul Gilster on July 23, 2009

The hunt for exomoons — satellites of planets around other stars — gets more interesting all the time. This morning I received a note from David Kipping (University College London), who has been studying methods for finding such objects. Kipping and colleagues have a paper soon to be published by Monthly Notices of the Royal Astronomical Society that discusses how to detect habitable exomoons using Kepler-class instrumentation. And it turns out that finding such worlds is well within our present capabilities.

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A bit of background: Kipping’s method is to analyze two useful sets of signals. Transit timing variations (TTV) are variations in the time it takes a planet to transit its star. Kipping and team acquire these data and then weave the TTV information together with what is called transit duration variation (TDV). The latter is detectable because as the planet and its moon orbit their common center of mass, velocity changes can be observed over time. Put TTV and TDV together and exomoon detections become possible.

Image: A tropical moon around a gas giant in an imagined solar system. We should soon begin to learn whether such worlds are common. Credit: Dan Durda (SwRI).

The new paper extends exomoon studies to habitable zones, and its findings are exciting. Assuming the kind of photometry available to the Kepler mission (and, in short order, ground-based telescopes), exomoons in the habitable zone down to one-fifth of an Earth mass should be within range of our instruments. Indeed, habitable exomoons around M, K and lighter G-class stars up to 100-200 parsecs away should be detectable. Kipping notes that up to 25,000 stars within Kepler’s field of view could be surveyed for exomoons of up to one Earth mass, while an extended survey of the galactic plane could extend the number to two million.

The most likely planet for an exomoon detection? A world something like Saturn, as Dr. Kipping explained in his email message:

…we find that low-density planets, like Saturn, provide a much better chance of detecting an exomoon, as opposed to say a Jupiter-like planet. This is because the transit depth increases with planetary radius, allowing for a more precise transit time measurement. In contrast, a low-mass planet allows for large wobbles in the orbital motion and therefore a large exomoon signature.

An additional finding is that we should be able to find lower-mass exomoons around M-dwarfs because their habitable zones are much closer to the star, allowing a larger number of transits for the observing time available. Given that these studies show a sensitivity down to 0.2 Earth masses, it’s also interesting to see in the paper that 0.3 Earth masses is the minimum habitable mass for a planet as calculated by earlier theorists.

How common are Earth-like exomoons? We don’t know, but note this (from the paper):

Our results suggest it is easier to detect an Earth-like exoplanet than an Earth-like exomoon around a gas giant. However, we have no statistics to draw upon to estimate which of these scenarios is more common. If a roughly equal number of both are discovered, it would indicate that the latter is more common due to the detection bias.

And that’s where we are now, waiting for the tsunami of data that Kepler and later space-based missions should provide us. The prospect of finding habitable moons around distant planets boggles the mind, but Kipping makes a strong case for our ability to make such a detection within a few short years. In any case, equipment like Kepler should be able to find such worlds if they exist. All-sky surveys focusing on M-dwarf stars would be ideally suited for continuing the hunt.

The paper is Kipping et al., “On the detectability of habitable exomoons with Kepler-class photometry,” accepted by Monthly Notices of the Royal Astronomical Society and available online.

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amphiox July 23, 2009 at 20:13

How earthlike are we positing these exomoons? Tidal flexing could keep an exomoon warm enough for liquid water even if the host planet did not orbit in the habitable zone.

In our own solar system we have one confirmed habitable planet, but at least 3 or 4 potentially habitable moons, though granted we are being a little loose with the “potentially” label.

Adam July 24, 2009 at 3:59

Hi Paul

Just another thing to look forward to from “Kepler”…

James M. Essig July 24, 2009 at 14:37

Hi Folks;

Given that within our solar system there are about a dozen moons larger than the Earth’s moon, a few of which are almost as big as Mars, one wonders if there might be more life bearing moons in the universe than planets. Perhaps many of these moon are Earth like in mass, surface and atmospheric temperature and in atmospheric chemistry. I can imagine the possibility of ETI civilizations living on such moons.

Earth or Mars scale planetary moons might be usefull to terraform thus providing our species with lots of colony space perhaps within a radius of as little as 50 lightyears from Earth for which there exists about 2,000 stars, most of which are type G, K, and M stars.

By the way Paul, that artist rendition of a planetary rise is beautiful and inspiring. Art work has great potential for interstellar space flight advocacy and no doubt can be a great tool for the goals of us practicioners of Tau Zero.

andy July 24, 2009 at 16:26

James M Essig: don’t get too impressed by the size of the moons in our solar system. Bear in mind that the moons in question are ice moons: both Ganymede and Titan have only around twice the mass of our own moon. Or to take the usual comparison that gets made, while Ganymede is about 8% larger than Mercury in terms of diameter, it has less than half of Mercury’s mass.

I hope there are habitable moons out there, if just to provide some physical justification for all those space art pictures with gas giants hanging in the sky of a habitable planet. I don’t think this is a situation that is often realised however, especially with the maximum mass of moon systems suggested by theoretical considerations and the evidence of our own solar system. While Jupiter has a more massive satellite system than Saturn’s, the maximum satellite mass in both cases is very similar. Jupiter just formed a system containing more high-mass satellites rather than a single “super-Titan”.

James M. Essig July 24, 2009 at 17:39

Hi andy;

Thanks for offering your perspective.

I hope that exomoons will prove useful for something. In the event that many if not most exomoons have no life, or atleast no advanced life, perhaps they can be ethically mined for resources such as for nuclear fission and/or fusion fuel.

One thing I am curious about is the current thinking regarding whether humans will ever set foot on the moons of either Jupiter or Saturn. I am aware of the high ionizing radiation flux density near the vacinity of such moons.

I have often heard of reports regarding the unfeasibility of landing humans on these moons due to radiation hazards. Is there any models to suggest that exomoons in orbit around extra-solar gas giants that are located within the habitable zone would have greatly reduce cosmic ray fluxes at their surface? I wonder if a magnetoshere similar to that of Earth along with a similar upper atmospheric chemistry on any requisite exomoon would make for a suitably shielded habitat for we humans.

Perhaps we could develop arbitrary exomoons into subteranian habitats for our species in the event that the radiation flux at the surface would usually prove too great for out door work and dailly living.

Spaniard July 24, 2009 at 18:49

I’m not sure that a Earth like moon could exists over a gas gigant so big as Saturn or bigger. Because to have a rotation near a day, the moon must be so close to the planet that reach the roche limit.
One possibility is that the moon was enough far to have a rotation not synchronizated, but must be too far.
Perhaps, if the gas gigant was little, like Uranus, then the planet could rotate enough fast without be so close to the roche limit.
But Uranus is a little gas gigant, nearly a superearth.
More beautyful must be twin planets, enough close to rotate on a day, but both with the same face to the another (so, there is no sea tides), both earth like.

kurt9 July 24, 2009 at 18:52

I probably mentioned this before. Jupiter has nasty radiation belts around it. Saturn has them too, but weaker than those of Jupiter. This suggests that even if a Jupiter is in the habitable zone, that its radiation belts will fry any large moons that orbit it such that the only life on them would be in the oceans (that would shield it from the radiation flux.

philw1776 July 25, 2009 at 10:31

An Earth sized exomoon IF it had a decent magnetic field could deflect the rad belts. Problem being that a Earth with it’s rotation stoped such that it’s synchronous with its orbit might not have a magnetic dynamo.

andy July 26, 2009 at 6:27

Despite its slow rotation and low mass, Ganymede has its own magnetic field, so magnetic fields on a more Earthlike moon are not necessarily impossible. Tough luck when/if the moon undergoes a magnetic reversal though.

Adam July 26, 2009 at 20:53

Hi Guys

15 MeV protons, which make Jupiter’s rad-belt so nasty, don’t penetrate an atmosphere very far – consider how high up the aurora are in Earth’s atmosphere for example. Earth’s belts have a similar energy range to Jupiter’s, just a lower particle count. Even cosmic rays, which are on average ~1000 times more energetic than rad-belt particles, blow up into muon showers before they get close to the ground.

The real issue is just how much atmosphere erosion a rad-belt would cause. I’m not so sure it would be very effective as the atmosphere loss of Earth via the polar wind needs the free-flowing solar wind to carry it away. In the rad belts an ion torus would form and most of the stuff would stay trapped, probably returning to the moon.

As I have said before we’ll only know what really obtains Out There if we actually go and look.

kurt9 July 28, 2009 at 13:18

Earth’s belts have a similar energy range to Jupiter’s, just a lower particle count. Yeah, a lot higher particle count. My understanding is that the first probes (Pioneer) were nearly fried and they use radiation hardened mil-spec electronics. Atmospheric erosion is an issue. The higher particle count would have greater ablation effect. Perhaps the gravitational field of the gas giant would reduce the effects of the solar wind.

The surface of the moon ought to be relatively radiation free, but the neighborhood space would be hostile to the extreme. The auroras on such a habitable moon ought to be quite spectacular.

The other issue that I think we talked about is that the largest Jovian moons tend to be about 1/1000 the mass of the gas giant, which means that a gas giant would have to be 13 times the mass of Jupiter in order to have Earth sized moons. Such a massive planet takes you into brown dwarf territory.

The only way to know is to go look.

andy July 28, 2009 at 16:28

The other issue that I think we talked about is that the largest Jovian moons tend to be about 1/1000 the mass of the gas giant, which means that a gas giant would have to be 13 times the mass of Jupiter in order to have Earth sized moons. Such a massive planet takes you into brown dwarf territory.

I can’t help but wonder what kind of worlds are in orbit around the recently-discovered object HD 16760b, one of several discoveries that indicates that using deuterium fusion as the planet/brown dwarf dividing line does not necessarily give the best picture of what is going on around other stars. HD 16760b has a mass at least 14.6 times that of Jupiter, but a near-circular orbit, which suggests it may well be the product of accretion. Satellite systems may provide one way of discriminating between brown dwarfs and planets: a brown dwarf that forms as an independent star will have very different conditions in its circumstellar disc than a circumplanetary disc that gets fed by material from the circumstellar disc in which it is embedded.

george scaglione July 30, 2009 at 17:44

i’d like to ad one thought of which i am sure we are all aware.these exomoons are plenty far away! i have been thinking and talking of late as anyone who keeps a good eye on this site would know – about going to some of the moons of our solar system.part of my reasoning is that this would help to boost some of the skills we will need to build starships.that is if we go about it the right way! but guys,lol, for exomoons we need starships in the first place! no kidding.but still,a cool project.glad we had the chance to say afew words.thank you very much your friend george

george scaglione July 31, 2009 at 9:24

let me just ad please that it was the augustine panel appointed by president obama that helped put “a bug in my ear” about moons.suggesting that we visit the moons of mars. very interesting comments.i hope we all see and discuss the full report,which as i understand it will be available at the end of august! (cool segway)…my gosh this is already the end of july!!!!! august first is ALREADY tommorrow! once again …the year just flies by!! thank you once again my friends,george

ljk September 9, 2009 at 13:02

Massive Satellites of Close-In Gas Giant Exoplanets

Authors: Timothy A. Cassidy, Rolando Mendez, Phil Arras, Robert E. Johnson, Michael F. Skrutskie

(Submitted on 3 Sep 2009)

Abstract: We study the orbits, tidal heating and mass loss from satellites around close-in gas giant exoplanets. The focus is on large satellites which are potentially observable by their transit signature.

We argue that even Earth-size satellites around hot Jupiters may be immune to destruction by orbital decay; detection of such a massive satellite would strongly constrain theories of tidal dissipation in gas giants, in a manner complementary to orbital circularization.

The star’s gravity induces significant periodic eccentricity in the satellite’s orbit. The resulting tidal heating rates, per unit mass, are far in excess of Io’s and dominate radioactive heating out to planet orbital periods of months for reasonable satellite tidal $Q$. Inside planet orbital periods of about a week, tidal heating can completely melt the satellite.

Lastly, we compute an upper limit to the satellite mass loss rate due to thermal evaporation from the surface, valid if the satellite’s atmosphere is thin and vapor pressure is negligible.

Using this upper limit, we find that although rocky satellites around hot Jupiters with orbital periods less than a few days can be significantly evaporated in their lifetimes, detectable satellites suffer negligible mass loss at longer orbital periods.

Comments: Accepted to ApJ

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Cite as: arXiv:0909.0770v1 [astro-ph.EP]

Submission history

From: Phil Arras [view email]

[v1] Thu, 3 Sep 2009 22:23:41 GMT (64kb)

http://arxiv.org/abs/0909.0770

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