Astronomers Are Hunting for Exomoons with Kepler’s Help
The universe seems almost infinitely reductive: our galaxy rotates around a central hub, planets orbit their planet stars, moons orbit their parent planets, and the odd moonlet may even orbit a moon.
Almost from the moment astronomers began finding planets around distant stars, they thus began talking about the moons that might orbit those alien worlds. It wasn’t that they had any hope of discovering something as tiny as a moon: the smallest things they could find at the time were giant planets like Jupiter. But if a Jupiter happened to orbit in its star’s Goldilocks Zone, where temperatures were relatively balmy, and if that Jupiter happened to have a moon about the size of Earth — not impossible, surely — then that hypothetical moon might have a chance of harboring life. That’s a lot of ifs, which made talk of so-called exomoons seem like more of a marketing gimmick designed to gin up public interest in exoplanet science than a serious area of research.
Not any more, though. Thanks to the exquisite precision of the orbiting Kepler space telescope, the prospect of finding exomoons has finally come within reach and the search is now officially on. Speaking at this week’s American Astronomical Society meeting in Austin, Harvard astrophysicist David Kipping announced a new project called the Hunt for Exomoons with Kepler, or HEK. “We actually have no idea how common moons are in other planetary systems,” says Kipping — but we could be on the verge of finding out.
Kipping had started thinking about exomoons even before Kepler was launched in 2009. “It was my entire Ph.D., more or less, at University College, London,” he says. He was originally thinking about transits — the passage of a planet in front of its star, which is just what Kepler was designed to look for as a sign that the planet exists at all. In particular, Kipping was wondering about what might make a transit last longer or shorter than you might expect. “It dawned on me,” he recalls, “that if a planet had a moon, that could cause a speedup or a slowdown.”
The reason: if a moon happens to be leading the planet as it passes by, it will pull the planet across the face of the star a little faster than average. If it happens to be following, it will hold the planet back. Not only that; whether the moon is leading or trailing, the silhouette of the planet and moon will be wider than that of a planet alone — the planet-moon system will block more of the star’s light. If the moon is directly in between the planet and the gaze of Kepler, on the other hand, or if it’s between the planet and the star, more starlight will reach Kepler’s sensors — and the moon itself will not be visible.