# HARPS-N radial velocities confirm the low densities of the Kepler-9 planets

## TL;DR

This study uses HARPS-N radial velocity data to confirm the low densities of Kepler-9 planets, resolving previous discrepancies between TTV and RV measurements and demonstrating TTV's reliability in mass determination.

## Contribution

The paper provides the first high-precision RV confirmation of Kepler-9 planet densities, reconciling TTV and RV methods and refining planetary mass estimates.

## Key findings

- RV data supports lower planetary masses than previous estimates.
- Confirmed low densities place planets in the super-Neptunes / inflated sub-Saturns region.
- TTV method aligns with high-precision RV measurements.

## Abstract

We investigated the discrepancy between planetary mass determination using the transit timing variations (TTVs) and radial velocities (RVs), by analysing the multi-planet system Kepler-9. Despite being the first system characterised with TTVs, there are several discrepant solutions in the literature, with those reporting lower planetary densities being apparently in disagreement with high-precision RV observations. To resolve this, we gathered HARPS-N RVs at epochs that maximised the difference between the predicted RV curves from discrepant solutions in the literature. We also re-analysed the full Kepler data-set and performed a dynamical fit, within a Bayesian framework, using the newly derived central and duration times of the transits. We compared these results with the RV data and found that our solution better describes the RV observations, despite the masses of the planets being nearly half that presented in the discovery paper. We therefore confirm that the TTV method can provide mass determinations that agree with those determined using high-precision RVs. The low densities of the planets place them in the scarcely populated region of the super-Neptunes / inflated sub-Saturns in the mass-radius diagram.