Exomoons are drawing more interest all the time. It may seem fantastic that we should be able to find moons around planets circling other stars, but the methods are under active investigation and may well yield results soon. Now David Kipping (Harvard-Smithsonian Center for Astrophysics) and colleagues have formed a new project called HEK — the Hunt for Exomoons with Kepler. We thus move into fertile hunting ground, for there has never been a systematic search for exomoons despite the work of ground-breaking researchers like Kipping, Gaspar Bakos (Princeton) and Jean Schneider (Paris Observatory). It’s definitely time for HEK as Kepler’s exoplanet candidate list grows.
Kepler, of course, works with transit methods, noting the dip in starlight as an exoplanet passes in front of the star under observation. HEK will use Kepler photometry to look for perturbations in the motion of the host planet that could flag the presence of a moon. Variations in transit timing (TTV) and duration (TDV) should be the most observable effects, the former being variations in the time it takes the planet to transit its star, while transit duration variation is caused by velocity changes induced by the fact that the planet and moon orbit a common center of mass.
The team will also look for eclipse features, where the moon might occult the planet during a planet-star eclipse. Back in 2009, Kipping and team ran a feasibility study on Kepler’s ability to find the moon of a gas giant in the habitable zone of a star (see Habitable Moons and Kepler). Assuming moons on circular, coplanar orbits around the host planet, the results showed that Kepler could detect exomoons down to 0.2 Earth masses. This is a large moon indeed, for as Kipping’s new paper on this work points out, the most massive moon in our Solar System, Ganymede, is 0.025 Earth masses (our own Moon is 0.0123 Earth masses). No question, then, that HEK will be looking for large moons, moons bigger than any we see in our own system.
Image: The view from a large exomoon would be like nothing we’ve seen in our own system, especially if that world proved suitable for life. Credit: Dan Durda.
Of course, binary planets also fall within the scope of this study — Kipping draws the line between a binary planet and a true planet-moon pair at the point where the center of mass of the two bodies is outside the radius of both bodies, but HEK can work comfortably with both scenarios. The paper runs through the likelihood that such large objects might exist, forming either around the host planet as it undergoes planetary growth, or (more likely) being captured by the host — here we think of moons like Triton in our own system, or of impact scenarios between planetesimals or young planets like that thought to have produced our own Moon.
Other scenarios are also possible, as the paper announcing HEK notes:
For planets which do not migrate through a proto-Kuiper belt or under the assumption that such objects will never reach suﬃcient mass to qualify as large moons, an alternative source of terrestrial mass objects is required. This object could be an inner terrestrial planet encountered during the gas giant’s inward migration or even a large, unstable Trojan which librates too close to the planet. Indeed, Eberle et al. (2010) have shown that a gas giant planet (in their case HD 23079b) can capture an Earth-mass Trojan into a stable satellite orbit, occurring in 1 out of the 37 simulations they ran.
How long would such a system be stable? The capture process would produce what the paper describes as ‘very loosely-bound initial orbits,’ but there has been work showing that captured moons have relatively high survival rates, as high as 50 percent in various configurations. Producing binary planets through the same methods is plausible, and the paper notes that a Jupiter orbited by an Earth-class planet could be considered an example of an extreme binary.
Examining these origin scenarios as well as the evolution of large moons in detail, the paper goes on to note the project’s objectives:
1. The primary objective of HEK is to search for signatures of extrasolar moons in transiting systems.
2. The secondary objective of HEK will be to derive posterior distributions, marginalised over the entire prior volume, for a putative exomoon’s mass and radius, which may be used to place upper limits on such terms (where conditions permit such a deduction).
3. The tertiary objective of HEK is to determine… the frequency of large moons bound to the Kepler planetary candidates which could feasibly host such an object (in an analogous manner to η⊕ – the frequency of Earth-like planets).
We know that in our own system, Europa, Titan and even tiny Enceladus are possible candidates for life. The Hunt for Exomoons with Kepler project won’t be able to tell us anything about astrobiology on an exoplanet’s moon, but if we begin to find Earth-sized objects orbiting gas giants in the habitable zone, we’ll have taken a first step toward learning whether exomoons could be just as viable a place for life as a planetary surface. The HEK home page goes so far as to speculate that planet-based life could actually be outnumbered by life on habitable moons. Step one, of course, is to find out if such moons actually exist, using Kepler’s crucial data.
The paper is Kipping et al., “The Hunt for Exomoons with Kepler (HEK): I. Description of a New Observational Project,” submitted to the Astrophysical Journal (preprint). The 2009 study is Kipping et al., “On the detectability of habitable exomoons with Kepler-class photometry,” Monthly Notices of the Royal Astronomical Society, published online 24 September, 2009 (abstract).