Unbiasing the density of TTV-characterised sub-Neptunes: Update of the mass-radius relationship of 34 Kepler planets

TL;DR

This study re-analyzed 34 Kepler planets using improved TTV extraction methods, revealing that previous density discrepancies were partly due to measurement biases, and providing a more accurate mass-radius relationship for these planets.

Contribution

It demonstrates that refining TTV analysis reduces density biases, aligning TTV-characterized planets closer to RV-characterized populations, and updates the mass-radius relationship for 34 Kepler planets.

Findings

Re-analysis shows previous low densities were partly due to measurement biases.

Robust mass estimates were obtained for 23 planets.

TTV-characterized planets are more similar to RV planets than previously thought.

Abstract

Transit Timing Variations (TTVs) can provide useful information on compact multi-planetary systems observed by transits, by putting constraints on the masses and eccentricities of the observed planets. This is especially helpful when the host star is not bright enough for radial velocity follow-up. However, in the past decades, numerous works have shown that TTV-characterised planets tend to have a lower densities than RV-characterised planets. Re-analysing 34 Kepler planets in the super-Earth to sub-Neptunes range using the RIVERS approach, we show that at least part of these discrepancies was due to the way transit timings were extracted from the light curve, which had a tendency to under-estimate the TTV amplitudes. We recover robust mass estimates (i.e. low prior dependency) for 23 of the planets. We compare these planets the RV-characterised population. A large fraction of these previously had a surprisingly low density now occupy a place of the mass-radius diagram much closer to the bulk of the known planets, although a slight shift toward lower densities remains, which could indicate that the compact multi-planetary systems characterised by TTVs are indeed composed of planets which are different from the bulk of the RV-characterised population. These results are especially important for obtaining an unbiased view of the compact multi-planetary systems detected by Kepler, TESS, and the upcoming PLATO mission.