Our solar system is full of moons. Of the 8 major planets, 6 of them have at least one natural satellite in tow, and several of those moons are very interesting places. Icy moons in the outer solar system may even be secretly harboring life. But what about moons elsewhere in the galaxy?
The Hunt for Exomoons with Kepler (HEK) is an astronomy project intended to try and find exomoons. And not just any exomoons; the kind of moons that could be a haven for life. While the Kepler telescope has, sadly, been forced into retirement, the data it collected lives on. And there's a lot of data still to sift though.
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The idea of habitable moons is already well known to fans of science fiction. From Star Wars to Prometheus, the idea of a habitable world orbiting a gas giant is quite well ingrained on our collective subconscious. Perhaps this is what inspired the idea back in 2009 that we could look for exomoons with Kepler.
Since then, the idea has come forward in leaps and bounds, and we know of several gas giants within their parent stars' habitable zones. Some even expect that exomoons may even be the best place to start looking for extrasolar life. The latest development in this story saw a team of astronomers, lead by Harvard-Smithsonian‘s David Kipping, take a closer look at Kepler-22b to try and hone their techniques.
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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.
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Admittedly, we don't know much about moons elsewhere in our galaxy, nor do we know much about what might make those moons habitable. But it's a big galaxy out there. If we keep looking, with techniques this sensitive, we're bound to find something eventually. The important thing is that we're looking - and we know what we're looking for.
For anyone interested in the full details, the paper is available from arXiv.
Image: Artists impression of a habitable moon in an extrasolar gas giant system (created using NASA imagery. Credit: Markus Hammonds/supernovacondensate.net