The Hunt for Exomoons with Kepler, or HEK, is an astronomy project designed to search for observational evidence of exomoons (extrasolar moons). The “exo” part of the word simply means that the moon lies outside of our own solar system. Because the nearest star to us is several light years away, the stars which we look at in our hunt are in the range of 10’s to 1000’s of light years away. So far, no-one has ever found an exomoon but there has never been a systematic search for their existence before. HEK will therefore test the hypothesis that moons exist in other solar systems aside from our own. Our primary mission is to determine the occurrence rate of large moons around viable planets hosts, which we denote with the symbol η☾.
Why should we care about exomoons? Perhaps, the most fundamental reason is life. Science fiction writers and film makers have long toyed with the idea of moons teaming with life, such as the moon “Pandora” in the recent film Avatar (pictured). But this is not just science fiction- astrobiologists believe that Europa, Titan and Enceladus (the moons of Jupiter and Saturn) are potentially viable homes for some form of primitive biology. Sadly though, there are no moons in our own solar system which offer truly Earth-like conditions such as that depicted in Avatar.
But could there be a vast population of habitable exomoons out there just waiting for us to find them? If such habitable moons are possible, then there could even be more habitable moons than habitable planets. Planet-based life could even be a rarity in the Galaxy! HEK cannot tell us whether life inhabits exomoons or not, but the first step is to establish whether moons big enough to support a biosphere exist or not. HEK will hopefully answer this question.
Another important implication of moons is that aside from being habitable themselves, they also may affect the habitability of any planets they orbit. For example, the Moon (pictured) is thought to stabilize the axial tilt of the Earth which is beneficial to the climate and habitability of our planet. If the Moon wasn’t there, would our planet still have complex life (like us) on it? With just one known example, the Earth-Moon system, it is difficult to make this determination. But HEK will seek evidence of exomoons around habitable-zone planets in order to say whether planets in the habitable-zone of their host star frequently have large moons or not.
Finally, the third important reason to try and find exomoons is that they can teach us a lot about how planets, moons and solar systems form and evolve. The solar system is billions of years old and so we have only been observing it for a very short fraction of that time - therefore a major challenge in planetary astronomy is work out how our own solar system formed and evolved over all that time we weren’t looking. A significant piece of this puzzle is how moons form and evolve. Taking the example of the Earth-Moon system once again, the Moon is thought to have formed through a giant collision between the primordial Earth and a Mars-sized planet which drifted too close (pictured). This enormous collision broke up the smaller planet into a disc of vaporized rock which slowly coalesced into what we now call the Moon. This extraordinary tale is our best guess for how the Moon formed but we have no idea as to whether this happens all the time in other solar systems or whether the Earth-Moon system is somehow a freak in the Universe. Only by detecting a population of planet-moon systems can be hope to answer whether the Earth-Moon system is unique and so whether solar systems like our own reside in the cosmos.