THE HUNT FOR EXOMOONS WITHKEPLER(HEK). IV. A SEARCH FOR MOONS AROUND EIGHT M DWARFS

Abstract

With their smaller radii and high cosmic abundance, transiting planets around cool stars hold a unique appeal. As part of our on-going project to measure the occurrence rate of extrasolar moons, we here present results from a survey focussing on eight Kepler planetary candidates associated with M-dwarfs. Using photodynamical modeling and Bayesian multimodal nested sampling, we find no compelling evidence for an exomoon in these eight systems. Upper limits on the presence of such bodies probe down to masses of ∼ 0.4 M ⊕ in the best case. For KOI-314, we are able to confirm the planetary nature of two out of the three known transiting candidates using transit timing variations. Of particular interest is KOI-314c, which is found to have a mass of 1.0 +0.4 −0.3 M ⊕ , making it the lowest mass transiting planet discovered to date. With a radius of 1.61 +0.16 −0.15 R ⊕ , this Earth-mass world is likely enveloped by a significant gaseous envelope comprising ≥ 17 +12 −13 % of the planet by radius. We also find evidence to support the planetary nature of KOI-784 via transit timing, but we advocate further observations to verify the signals. In both systems, we infer that the inner planet has a higher density than the outer world, which may be indicative of photo-evaporation. These results highlight both the ability of Kepler to search for sub-Earth mass moons and the exciting ancillary science which often results from such efforts.

Insights into Planet Formation from Debris Disks

“…The mechanism to find such low mass planets involves looking for small perturbations to the timing of the transit of another large planet in front of the star due to the dynamical perturbation of the smaller object (so-called transit timing variations, or TTVs). This may be the most promising way of detecting extrasolar moons, although none are known yet (Kipping et al 2014). The formation of a moon through giant impact requires an intermediate stage of a circumplanetary disk.…”

Multiple planets or exomoons inKeplerhot Jupiter systems with transit timing variations?

“…During the submission process of this paper, Kipping et al (2013) published a set of Kepler planet candidates, seeking for scenarios that are compatible with moons. The two studies are based on different methods: Kipping et al analyzed TTV distributions, and here we applied a period analysis.…”

“…The two studies are based on different methods: Kipping et al analyzed TTV distributions, and here we applied a period analysis. The two candidate lists are also disjointed, because Kipping et al (2013) restricted their analysis to candidates smaller than six Earth-radii. There are nevertheless prominent similarities between the two studies: neither we nor Kipping et al (2013) succeeded in firmly concluding that TTV signals have a clear dynamical origin, although a few strong candidates survived all tests.…”

Numerical and Analytical Modeling of Transit timing variations

“…However, the conversion of a TTV signal into planet properties is often plagued by a degeneracy between planet masses and eccentricities (Lithwick et al 2012, hereafter LXW). Nonetheless, N-body analyses have provided a number of TTV systems in which planet masses are apparently well constrained and not subject to the predicted degeneracies (e.g., Cochran et al 2011;Carter et al 2012;Sanchis-Ojeda et al 2012;Huber et al 2013;Lissauer et al 2013;Masuda et al 2013;Nesvornỳet al 2013;Jontof-Hutter et al 2014Kipping et al 2014;Schmitt et al 2014). Nesvornỳ& Vokrouhlický (2014) and Deck & Agol (2015a) show that the the mass-eccentricity degeneracy can be broken provided that the effects of the planets' successive conjunctions are seen with sufficient signal to noise in their TTVs.…”

“…Bayesian parameter estimation via Markov chain Monte Carlo (MCMC) analysis is well suited to handlingsuch problems and has been applied to the analysis of TTVs previously by numerous authors (e.g., Sanchis-Ojeda et al 2012;Huber et al 2013;Masuda et al 2013;JontofHutter et al 2015;Schmitt et al 2014). (Also see Kipping et al (2014) for an alternative Bayesian approach to dynamical modeling of TTVs. )…”