All posts tagged moons

UPDATE (2016 Mar 24): The paper is now available for free on astro-ph.

The Astrophysical Journal published today a paper by my colleagues and myself investigating in detail a way to look for moons around transiting exoplanets.

The discoveries of thousands of planets and planetary candidates over the last few decades has motivated a parallel effort to find exomoons. In addition to providing a base of operations for the Empire, exomoons might actually be a better place to find extrasolar life than exoplanets in some ways.

This technique for finding exomoons, called the Orbital Sampling Effect, was developed by René Heller and involves looking for the subtle signature of a moon’s shadow alongside the shadow of its transiting planet host, as depicted in the image below.

At epoch (1), a satellite’s transits just before the planet. At epoch (2), the planet's transit begins, inducing a large dip the measured stellar brightness. At epoch (3), the satellite modifies the planet's transit light curve slightly but measurably.

The dark cloud shown around the planet represents the exomoon’s shadow, averaged over several orbits. At epoch (1), a satellite transits just before the planet. At epoch (2), the planet’s transit begins, inducing a large dip in the measured stellar brightness. At epoch (3), the satellite modifies the planet’s transit light curve slightly but measurably.

This simple technique has advantages over alternative exomoon searches in that it doesn’t require significant computational resources to implement. It can also use data already available from the Kepler and K2 missions. However, on its own, the technique can’t provide a moon’s mass, only its size, and it requires many transits of the host planet to find the moon’s quite subtle transit signature.

No exomoon has been found yet in spite of tremendous efforts to find them, so the search continues.


At journal club today, we talked about a study from Heller and Pudritz that looks at the formation of moons around gas giant planets in extrasolar systems.

Heller and Pudritz modeled the conditions in circumplanetary disks around Jupiter-like planets to find where temperatures are right for icy moons like Jupiter’s to form. Like Goldilocks, moon formation requires conditions that are juuust right: the planet can’t be too close to its star or too small.

But given the right conditions, moons will happily accrete around a gas giant and the most massive circumplanetary disks around super-Jovian planets can form moons the size of Mars.

Heller and Pudritz point out that this means if we find an icy moon around one of the many gas giant exoplanets orbiting at about 1 AU from their host stars, we can infer the planet didn’t form there. Instead, it must have formed farther out and migrated in.

And at 1 AU around a Sun- like star, the discovery of such an exomoon would naturally make it a high priority target for habitability studies.

Attendees at today’s journal club included Nathan Grigsby, Jared Hand, Catherine Hartman, Emily Jensen, Liz Kandziolka, and Jacob Sabin.