* Astronomy

Title: Very Low-Density Planets around Kepler-51 Revealed with Transit Timing Variations and an Anomaly Similar to a Planet-Planet Eclipse Event
Author: Kento Masuda

We present an analysis of the transit timing variations (TTVs) in the multi-transiting planetary system around Kepler-51 (KOI-620). This system consists of two confirmed transiting planets, Kepler-51b (Pb=45.2days) and Kepler-51c (Pc=85.3days), and one transiting planet candidate KOI-620.02 (P02=130.2days), which lie close to a 1:2:3 resonance chain. Our analysis shows that their TTVs are consistently explained by the three-planet model, and constrains their masses as Mb=2.1+1.5-0.8 earth masses (Kepler-51b), Mc=4.0±0.4 earth masses (Kepler-51c), and M02=7.6±1.1 earth masses (KOI-620.02), thus confirming KOI-620.02 as a planet in this system. The masses inferred from the TTVs are rather small compared to the planetary radii based on the stellar density and planet-to-star radius ratios determined from the transit light curves. Combining these estimates, we find that all three planets in this system have densities among the lowest determined, Rhop<0.05gcm-3. With this feature, the Kepler-51 system serves as another example of low-density compact multi-transiting planetary systems. We also identify a curious feature in the archived Kepler light curve during the double transit of Kepler-51b and KOI-620.02, which could be explained by their overlapping on the stellar disk (a planet-planet eclipse). If this is really the case, the sky-plane inclination of KOI-620.02's orbit relative to that of Kepler-51b is given by deltaOmega=-25.3+6.2-6.8deg, implying significant misalignment of their orbital planes. This interpretation, however, seems unlikely because such an event that is consistent with all of the observations is found to be exceedingly rare.

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