Constraining Small Planet Compositions for Future Missions
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.
M–R diagram. Planet results from this study are shown as coloured stars, while archival literature values for the same planets (Table 1) appear as grey symbols with error bars; squares for most planets and a circle for the alternative TOI-1634 b value from Hirano et al. (2021). Other confirmed planets with mass and radius precisions better than 20% and 5%, taken from TepCat (Southworth 2011), are also plotted. Dotted lines show compositions from Zeng et al.(2019) (1000 K), and solid lines from Lopez & Fortney (2014) (1 Gyr, solar metallicity, 1000 𝐹⊕). Points are colour-coded by incident flux. Earth, Neptune, and the approximate radius valley (Fulton et al. 2017) are indicated. Archival points are not individually labelled; exceptions are K2- 79 b and one of the TOI-1634 b measurement, where the closest grey point corresponds to the Cloutier et al. (2021) solution. — astro-ph.EP
Larissa Palethorpe, Annelies Mortier, Jo Ann Egger, Ken Rice, Thomas G. Wilson, Andrew Vanderburg, Aldo S. Bonomo, Walter Boschin, Andrew Collier Cameron, Yoshi Nike Emilia Eschen, Avet Harutyunyan, Luca Malavolta, Aldo F. Martínez Fiorenzano, Alessandro Sozzetti, Manu Stalport, Vincent Van Eylen, Christopher Allan Watson
Comments: 30 pages, 16 figures, 6 tables, accepted for publication in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2603.14552 [astro-ph.EP] (or arXiv:2603.14552v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2603.14552
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Submission history
From: Larissa Palethorpe
[v1] Sun, 15 Mar 2026 19:00:01 UTC (11,397 KB)
https://arxiv.org/abs/2603.14552
Astrobiology, exoplanet,
Astrobiology, exoplanet,
