Exomoons: Rare in Inner Stellar Systems?

Exomoons — moons around planets in other star systems — are an exhilarating and at the same time seemingly inevitable prospect. There is little reason to assume our Solar System is unique in its menagerie of moons, with the gas giants favoring us particularly with interesting mission targets, and then there’s that fascinating double system at Pluto/Charon. If we visualize what we expect to find in any given stellar system, surely moons are part of the mix, and investigations like the Hunt for Exomoons with Kepler will doubtless find them.

An actual exomoon detection would be a triumph for exoplanet science, especially given how recently it was that we nailed down the first confirmed exoplanet, 51 Pegasi b, in 1995 (or, if you prefer, the 1992 detection of terrestrial-mass planets orbiting the pulsar PSR B1257+12). We’re new at this, and what huge strides we’ve made! Given the small size of the transit signal and its changing relation to the body it orbits, exomoons offer a particularly difficult challenge, although David Kipping’s team at HEK has plenty of Kepler data to work with.

Image: A star with a transiting planet and its moon. The angled area shows the inclination of the moon orbit. Orbit positions beyond the dashed line are not undergoing transit, and are thus not observable. Credit: Michael Hippke.

With all this in mind, every paper that comes out of HEK gets my attention. Kipping (Columbia University), working with graduate student Alex Teachey and citizen scientist Allan Schmitt, has now produced a paper that takes a significant step as the investigation proceeds. We have no detection yet — more about that in a moment — but we do have a broader result showing that exomoons are unusual in the inner regions of the systems surveyed.

Kipping and Teachey looked at 284 viable moon-hosting Kepler planetary candidates to search for moons around planets from Earth to Jupiter in size and distances from their stars of 0.1 to 1 AU. This finding seems to be getting less attention in the press than it deserves, so let’s dig into the paper on it:

Our results place new upper limits on the exomoon population for planets orbiting within about 1 AU of their host star, upper limits that are remarkably low. We have also analyzed subsets of the ensemble to test the effect of various data cuts, and we have identified the regime in which the OSE model presented in Heller (2014) breaks down, which we call the “Callisto Effect” — beyond 20 planetary radii, discrepancies appear in the results.

OSE stands for Orbital Sampling Effect, developed by René Heller in 2014 and described by Michael Hippke in Exomoons: A Data Search for the Orbital Sampling Effect and the Scatter Peak. OSE stacks multiple planet transits to search for an exomoon signature. What the paper is referring to as the ‘Callisto effect’ is the disagreement between OSE predictions and moons like Callisto. Even so, the authors continue to see OSE as a useful tool, and learning about an area in which it breaks down is helpful as we fine-tune our capabilities.

Back to the paper:

Our analysis suggests that exomoons may be quite rare around planets at small semi-major axes, a finding that supports theoretical work suggesting moons may be lost as planets migrate inward. On the other hand, if the dearth of exomoons can be read as a reliable indicator of migration, our results suggest a large fraction of the planets in the ensemble have migrated to their present location.

And that is a pointer to which we need to pay attention. Is a lack of exomoons a marker for planetary migration? If further analysis determines that it is, then we’ve found an extremely handy tool for studying the formation history of other stellar systems.

The Kepler data did yield one exomoon candidate in the Kepler-1625 system for which the authors have set up plans for follow-up observations with Hubble this fall. There is no way to know at this point whether we’ve got a genuine exomoon here or not. And I much appreciate the thorough job that Alex Teachey did in getting this point across to the public in his article Are Astronomers on the Verge of Finding an Exomoon? We learn here that the authors put their paper online earlier than intended because a media outlet was going to publish news about the upcoming Hubble study (Hubble proposals are publicly posted online).

And Teachey’s point is sound at a time when ideas whip around the Internet at lightspeed:

Peer review is a critical part of the scientific process, and we are not terribly comfortable putting out our results before they have been examined by a qualified referee. Unfortunately, we feel the circumstances have forced us to make our results freely available to the public before such a review, so that everyone may see for themselves what we are claiming and what we are not. While David and I are both big proponents of engaging with the public and boosting interest in the incredible things happening every day in astronomy, we have serious concerns about the potential for sensational headlines misleading the public into thinking a discovery has been made when it is really too early to say that for sure.

It’s a solid point. But I also want to emphasize that this paper’s findings about the apparent rarity of exomoons in the inner systems of the stars being studied is quite significant. To my knowledge this is the first time we’ve developed a constraint on exomoon formation. We doubless have moons hiding in the data (recall that the authors are looking for analogs to the Galilean moons of Jupiter), and we can also suspect they are going to be much more common in outer stellar systems, which is certainly the case in our own Solar System.

Don’t expect an immediate result from the Hubble observations. According to this article in Nature, Kipping and team will take about six months to analyze the work before making any announcements. Steady, painstaking effort is how this job gets done.

The paper is Teachey, Kipping & Schmitt, “HEK VI: On the Dearth of Galilean Analogs in Kepler and the Exomoon Candidate Kepler-1625b I,” submitted to AAS journals and available as a preprint. For helpful background, check Kipping, “The Transits of Extrasolar Planets with Moons,” PhD thesis, University College London (March 14, 2011), available online.