Constraining Small Planet Compositions for Future Missions
Abstract
Accurate mass and radius measurements of small transiting exoplanets are essential for probing their compositions, formation histories, and potential habitability. We present a uniform analysis of six planetary systems (each hosting at least one small transiting planet): K2-79, K2-106, K2-111, K2-222, K2-263, and TOI-1634. Our study combines new CHEOPS transit observations with archival photometry from K2, TESS, and ground-based facilities, alongside new and archival radial velocity data from HARPS-N, HIRES, ESPRESSO, and others. For each system, we perform joint transit and RV modelling, achieving typical precisions better than 15% and 5% for mass and radius, respectively, and thus enabling precise bulk density determinations. These reveal a range of compositions, including rocky planets near the radius valley (e.g. K2-106 b, TOI-1634 b), intermediate-density planets requiring steam-rich or mixed volatile envelopes (e.g. K2-111 b, K2-263 b), and low-density regimes, consistent with gas dwarfs or water-worlds (e.g. K2-79 b, K2-222 b). Several systems show evidence of additional companions detectable via RVs but not seen in transit. The results highlight the value of coordinated CHEOPS and HARPS-N observations in delivering some of the most precise bulk densities for small planets to date and support the preparation for future atmospheric characterisation missions.
Cite
@article{arxiv.2603.14552,
title = {Constraining Small Planet Compositions for Future Missions},
author = {Larissa Palethorpe and Annelies Mortier and Jo Ann Egger and Ken Rice and Thomas G. Wilson and Andrew Vanderburg and Aldo S. Bonomo and Walter Boschin and Andrew Collier Cameron and Yoshi Nike Emilia Eschen and Avet Harutyunyan and Luca Malavolta and Aldo F. Martínez Fiorenzano and Alessandro Sozzetti and Manu Stalport and Vincent Van Eylen and Christopher Allan Watson},
journal= {arXiv preprint arXiv:2603.14552},
year = {2026}
}
Comments
30 pages, 16 figures, 6 tables, accepted for publication in MNRAS