Sep 08 2009


Published by under Astronomy
Comments: 6

There has been much talk in recent years about the ongoing discovery of exoplanets – planets around suns other than our own. The technology to detect these distant objects has been increasing recently. There are two primary methods for detecting exoplanets: One method is to look for the wobble in the parent star caused by the gravity of the planet. The planet and its star revolve around their center of gravity, which will not be in the dead center of the star itself, but off center, causing a wobble.

earth-saturn-sThe second method is called the transit method in which we look for alterations in the amount of light we can see from a star because of a planet that moves in front of it.  This requires that the view of this system from the earth is roughly in the plane the planet is orbiting – the planet has to move in front of (transit) the star from our perspective. The new Kepler telescope is designed to detect light from many stars with sufficient sensitivity that it can find small (earth-size) planetary transits.

With all the talk about exoplanets, there has been, until now, little talk of exomoons – moons around planets around other stars. Until Kepler it has simply not been possible to detect exomoons. David Kipping of University College London has now published a paper in which he says that this is feasible.

This is exciting. While the whole exoplanet exercise is fascinating and is giving us the ability to have some data about the constitution of other stellar systems, interest is highest in earth-like planets – planets around the same size and temperature as earth, specifically able to have liquid water. This, of course, is because we want to know how many planets out there might be capable of harboring life. This will enable us to get a better handle on at least one component of the famous Drake equation that can be used to calculate the likely number of alien civilizations out there.

It is likely, however, that earth-sized moons of larger gas giants could have conditions suitable for organic life. Even smaller moons could be candidates. In our own system, Jupiter’s moon Europa likely has a liquid water ocean under its surface ice crust – complete with alien creatures swimming around. In fact, life-bearing moons may vastly outnumber life-bearing planets.

Kipping thinks that Kepler is sensitive enough to detect the wobble in a large and light planet (like Saturn) caused by an earth-sized moon. We are just gearing up for the flood of new exoplanets Kepler will discover over the coming years, and now Kipping adds exomoons to the buffet (how’s that for mixing metaphors).

Kepler has already passed a test of its ability to detect exoplanets (by detecting planets we already knew were there). At present Kipping’s paper just deals with the mathematical plausibility of exomoons. We don’t know yet of any exomoons so this ability cannot be tested as it was for detecting exoplanets. It seems that we just have to wait and see – when Kepler detects new explanets will their transits wobble due to the presence of large moons. NASA could also point Kepler at known exoplanets of a size and mass that would make them good candidates for detecting their moons.

If this scheme works, then the number of possible candidates for exobodies that might contain life that we detect in the near future may significantly increase. Cool.

6 responses so far

6 thoughts on “Exomoons”

  1. EA says:

    Unfortunately, we don’t know what the odds of life developing are given Earth-like conditions. We’d need to know more about how life developed on Earth to know that. We do know that if things are exactly are as they were on Earth, life will develop. That doesn’t tell us what the odds of life developing were given highly similar conditions. So while finding planets and moons like Europa with conditions conducive to organic chemistry like on early earth are tantalizing, given what we know, the odds of life being there could be everything from highly likely to infinitesimal.

  2. glasstree says:

    Earth might be exceptional, but we will have to look before we know.

    We have evidence for cellular life within ~200 million years of the Earth’s crust solidifying.
    I think something like that might be used as a dirty estimate for the probability of life (as we would recognize it) arising (somewhere like Earth).

    [years before life with X geologic conditions] / [total years of X geologic conditions on earth] = [dirty estimate we could use to make predictions, tests, refinements.]


    There are 2 reasons moons are generally exciting for life.

    1) Earth-like moons. Not Europa-like, or Titan-like, but Earth-like moons with the possibility of Earthish life.

    2) Numbers. It might be more likely for Earth-like moons to form than Earth-like planets. But either way, existing at all raises the total odds for life. This is distinct from the excitement over extremophiles & Europa, which widen the range of “Earth-like”.

  3. glasstree says:


    A = [total years of X geologic conditions on earth]
    B = [years before life with X geologic conditions]

    Probability Estimate = 1 – [B/A]

  4. Michael Kingsford Gray says:

    The odds of life developing on an Earth-like planet stand at 100% so far.

  5. EA says:

    Micheal –

    That’s not correct. The key is in the term “earth-like.” Sure, if everything is exactly was as it was on earth when life developed – atom by atom, motion by motion – then the odds of life developing should be 100% give or take the quantum indeterminacy. But earth-like means similar to Earth in chemical composition, but not quite identical. We don’t really know what the odds of life developing are given those conditions. It could be everything from highly likely to so unbelievably unlikely as to be absurd. It’s all over the map. It’s a lottery where the odds of winning aren’t known. More lottery tickets could theoretically mean increasing the odds from infinitesimal to still infinitesimal.

  6. llewelly says:

    Unfortunately, we don’t know what the odds of life developing are given Earth-like conditions.

    Probes like Kepler will indicate what those odds are. Kepler will tell us where planets and large moons are. Kepler’s descendants (and possibly Kepler itself) will tell us which of those planets and large moons have life-indicators such has high amounts of O2 in their atmospheres. It is only through such surveys that we can learn how frequently life occurs on worlds with Earth-like conditions.

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