HEK. VI. On the Dearth of Galilean Analogs in Kepler, and the Exomoon Candidate Kepler-1625b I

Abstract

Exomoons represent an outstanding challenge in modern astronomy, with the potential to provide rich insights into planet formation theory and habitability. In this work, we stack the phase-folded transits of 284 viable moon hosting Kepler planetary candidates, in order to search for satellites. These planets range from Earth- to Jupiter-sized and from ∼0.1 to 1.0 au in separation—so-called “warm” planets. Our data processing includes two-pass harmonic detrending, transit timing variations, model selection, and careful data quality vetting to produce a grand light curve with an rms of 5.1 ppm. We find that the occurrence rate of Galilean analog moon systems for planets orbiting between ∼0.1 and 1.0 au can be constrained to be to 95% confidence for the 284 KOIs considered, with a 68.3% confidence interval of . A single-moon model of variable size and separation locates a slight preference for a population of short-period moons with radii ∼0.5 R ⊕ orbiting at 5–10 planetary radii. However, we stress that the low Bayes factor of just 2 in this region means it should be treated as no more than a hint at this time. Splitting our data into various physically motivated subsets reveals no strong signal. The dearth of Galilean analogs around warm planets places the first strong constraint on exomoon formation models to date. Finally, we report evidence for an exomoon candidate Kepler-1625b I, which we briefly describe ahead of scheduled observations of the target with the Hubble Space Telescope.

Can close-in giant exoplanets preserve detectable moons?

“…Using Hubble data, (hereafter TK18) announced recently the detection of a third anomalous transit of Kepler-1625b. Along with other two transits detected in Kepler data (Teachey, Kipping & Schmitt 2018), this might hint the existence of an unconventional Neptune-sized exomoon. The size of the object, temporarily designated as Kepler-1625b I, is so large that we should better call it a 'double planet'.…”

Ploonets: formation, evolution, and detectability of tidally detached exomoons

“…An important outcome from our simplistic atmospheric model is that at the end of a ploonet life, the size of its gaseous envelope increases dramatically, which makes the planet's effective crosssection to appear up to an order of magnitude larger when compared to its initial size. The combination of deeper transit light curves, and smaller induced radial-velocity measurements of such low-mass voluminous bodies, could be interpreted as two configurations that may occur even when the moon is still orbiting the planet: 1) a super-puff planet in the mini-Neptunes regime, and 2) a massive or voluminous exomoon around a close-in giant planet (Teachey et al 2018). The second system has faint associated TTV/TDV signals, since moon masses are small, and they are located on large semimajor axes as a result of their tidal evolution.…”

Search for possible exomoons with the FAST telescope

“…Now if the possible exomoons are captured they can survive enough time for all stars presented in Table 1 (Barnes & Brien 2002). These captured satellites can be more massive than the formed ones (Porter &Grundy 2011, andTeachey et al 2017). Since we are not aware of theory which can predict such an event we do not consider them.…”