Writing at Discovery News, Markus Hammonds describes the search for potentially habitable moons of large extrasolar planets. It’s news to me that astronomers are actually capable of detecting sizable exomoons by virtue of the wobbles they induce on their planets’ motions, especially since that’s how many planets have been discovered. The degree of sensitivity required is exceptional.
Hammonds writes about a study, written by a team of astronomers led by one David Kipping, that studied Kepler-22b for signs. This planet orbits the yellow dwarf Kepler-22, slightly less massive and dimmer than our sun some 620 light years away. The planet seems to orbit within its star’s habitable zone, with global temperatures of a hypothetical Earth-like world plausibly being only a few degrees warmer than the global temperatures of the Earth.
Kepler-22b is a planet with a 95 percent probability of being in its parent star’s habitable zone. Around 620 light-years away from us, it has a radius about 2.4 times as large as Earth, and is about 10 percent as massive as Jupiter. With that size, it’s most likely to be a gas giant.
Unfortunately, no moon was found around Kepler-22b. If it has any moons at all, they must be smaller than half Earth’s mass. Nonetheless, this was far from a wasted exercise. Planet hunters now have a small arsenal of tools and techniques at their disposal — enough for Kipping and his colleagues to draw the conclusion that if any Earth-like moon is there to be found around similar planets, they will find it.
Planet Kepler-22b was chosen for this search for several reasons. As well as being comfortably in the habitable zone and having been confirmed by Kepler observations, this planet also had radial velocity data available for it, and the observations contain very low noise (take it from me, noise in observations is the bane of an astronomer’s life!).
While no Earth-like exomoons could be found around Kepler-22b, the fact that moons should be very easy to see if they’re there is heartening. What’s more, it’s worth bearing in mind that this does not mean that Kepler-22b has no moons at all. For example, Titan, Saturn’s giant moon, has only 2 percent the mass of Earth.
Based on their models, the authors give a 95% confidence than any hypothetical satellite system of Kepler-22b has a total mass less than or equal to 54% the mass of the Earth. In their conclusion, the authors do raise the possibility of a smaller exomoon, they allowing for the possibility of a moon twice the mass of Mars (about 20% the mass of Earth).
We find no evidence for an Earth-like exomoon around Kepler-22b and yet have shown that the present data can easily detect such an object via signal injection. Current observations therefore dictate that Kepler-22b does not possess an Earth-like habitable moon. Our results then, combined with the very robust measurement of the planet’s radius, mean that Kepler-22 does not possess an Earth analog. This does not mean that the system possesses no options for an inhabited world, with notable possiblities being a smaller, presently undetectable moon (e.g. MS∼0.2M⊕) or a possible ocean on Kepler-22b. However, it is now clear that this is not the location to find a second Earth.
The problem with any low-mass and hence low-gravity world in a habitable zone of a star, however, is that over time it’s likely to stop being habitable, most notably by losing lighter substances like water and air to space. In this specific case, taking a look at the quoted figures for Kepler-22′s luminosity and keeping in mind the mass-luminosity relationship, Kepler-22 might plausibly be a younger star than our own sun. All that means is that any hypothetical low-mass but habitable satellite of Kepler-22b would be less advanced in the process of dessication than Mars.