paper

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

arXiv:1401.2885 · doi:10.1088/0004-637X/783/1/53

Abstract

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 ( $P_{b} = 45.2 days$ ) and Kepler-51c ( $P_{c} = 85.3 days$ ), and one transiting planet candidate KOI-620.02 ( $P_{02} = 130.2 days$ ), 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 $M_{b} = 2. 1_{- 0.8}^{+ 1.5} M_{\oplus}$ (Kepler-51b), $M_{c} = 4.0 \pm 0.4 M_{\oplus}$ (Kepler-51c), and $M_{02} = 7.6 \pm 1.1 M_{\oplus}$ (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, $I ¨ _{p} ≲ 0.05 g c m^{- 3}$ . 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 $I^ I^c ◯ = - 25. 3_{- 6.8}^{+ 6.2} deg$ , 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.

9 pages, 4 figures, accepted for publication in ApJ