Removing biases on the density of sub-Neptunes characterised via transit timing variations

Update on the mass-radius relationship of 34 Kepler planets^★

¹ Observatoire de Genève, Université de Genève, Chemin Pegasi, 51, 1290 Versoix, Switzerland
e-mail: adrien.leleu@unige.ch
² School of Physics and Astronomy, Monash University, Victoria 3800, Australia
³ Laboratoire de Météorologie Dynamique/IPSL, CNRS, Sorbonne Université, École Normale Supérieure, PSL Research University, École Polytechnique, 75005 Paris, France
⁴ Physikalisches Institut, Universität Bern, Gesellschaftsstr. 6, 3012 Bern, Switzerland
⁵ DISAITEK, 10 rue Achille Antheaume, 95190 Fontenay-en-Parisis, France

Received: 27 May 2022
Accepted: 13 July 2022

Abstract

Transit timing variations (TTVs) can provide useful information on compact multi-planetary systems observed by transits by setting constraints on the masses and eccentricities of the observed planets. This is especially helpful when the host star is not bright enough for a radial velocity (RV) follow-up. However, in the past decade, a number of works have shown that TTV-characterised planets tend to have lower densities than planets characterised on the basis of RVs. Re-analysing 34 Kepler planets in the super-Earth to sub-Neptunes range using the RIVERS approach, we show that at least some of these discrepancies were due to the way transit timings were extracted from the light curve, as a result of their tendency to underestimate the TTV amplitudes. We recovered robust mass estimates (i.e. with low prior dependency) for 23 of the planets. We compared these planets the RV-characterised population and found that a large fraction of those that previously had unusually low density estimates were adjusted, allowing them to occupy a place on the mass-radius diagram much closer to the bulk of known planets. However, a slight shift toward lower densities remains, which could indicate that the compact multi-planetary systems characterised by TTVs are indeed composed of planets that are different from the bulk of the RV-characterised population. These results are especially important in the context of obtaining an unbiased view of the compact multi-planetary systems detected by Kepler, TESS, and the upcoming PLATO mission.