The NCORES Program: Precise planetary masses, null results, and insight into the planet mass distribution near the radius gap

TL;DR

The NCORES program used the ESO HARPS spectrograph to measure the masses of small, Neptune-like exoplanets discovered by TESS, providing insights into their composition and the planet mass distribution near the radius gap.

Contribution

This study presents new mass measurements for 35 planets, including marginal signals and a non-transiting candidate, and compares the observed mass distribution with planet formation models.

Findings

Planets below the radius gap are consistent with rocky composition.

The mass distribution shows a sharp cutoff at around 10 Earth masses.

Good agreement with pebble- and planetesimal-driven formation models.

Abstract

NCORES was a large observing program on the ESO HARPS spectrograph, dedicated to measuring the masses of Neptune-like and smaller transiting planets discovered by the TESS satellite using the radial velocity technique. This paper presents an overview of the programme, its scientific goals and published results, covering 35 planets in 18 planetary systems. We present spectrally derived stellar characterisation and mass constraints for five additional TOIs where radial velocity observations found only marginally significant signals (TOI-510.01, $M_p=1.08^{+0.58}_{-0.55}M_\oplus$), or found no signal (TOIs 271.01, 641.01, 697.01 and 745.01). A newly detected non-transiting radial velocity candidate is presented orbiting TOI-510 on a 10.0d orbit, with a minimum mass of $4.82^{+1.29}_{-1.26}M_\oplus$, although uncertainties on the system architecture and true orbital period remain. Combining the NCORES sample with archival known planets we investigate the distribution of planet masses and compositions around and below the radius gap, finding that the population of planets below the gap is consistent with a rocky composition and ranges up to a sharp cut-off at $10M_\oplus$. We compare the observed distribution to models of pebble- and planetesimal-driven formation and evolution, finding good broad agreement with both models while highlighting interesting areas of potential discrepancy. Increased numbers of precisely measured planet masses in this parameter space are required to distinguish between pebble and planetesimal accretion.