Discovery and Characterisation of Two Exoplanets Orbiting the Metal-poor, Solar-type Star TOI-5788 with TESS, CHEOPS, and HARPS-N

We present the discovery and characterisation of two transiting exoplanets orbiting the metal-poor, solar-type star TOI-5788.

From our analysis of six TESS sectors and a dedicated CHEOPS programme, we identify an inner planet (TOI-5788 b; 𝑃 = 6.340758 ± 0.000030 d) with radius 1.528 ± 0.075 R_⊕ and an outer planet (TOI-5788 c; 𝑃 = 16.213362 ± 0.000026 d) with radius 2.272 ± 0.039 R_⊕. We obtained 125 radial-velocity spectra from HARPS-N and constrain the masses of TOI-5788 b and c as 3.72 ± 0.94 M_⊕ and 6.4 ± 1.2 M_⊕, respectively.

Although dynamical analyses indicate that a third planet could exist in a stable orbit between 8 and 14 days, we find no evidence of additional planets. Since the TOI-5788 system is one of the few systems with planets straddling the radius gap, and noting that there are even fewer such systems around metal poor stars, it is a promising system to constrain planet formation theories.

We therefore model the interior structures of both planets. We find that TOI-5788 b is consistent with being a rocky planet with almost no envelope, or having an atmosphere of a high mean molecular weight. We find that TOI-5788 c is consistent with both gas-dwarf and water-world hypotheses of mini-Neptune formation. We model the atmospheric evolution history of both planets.

Whilst both scenarios are consistent with the atmospheric evolution of TOI-5788 c, the gas-dwarf model is marginally preferred. The results of the atmospheric evolution analysis are not strongly dependent on stellar evolution. This makes the system a promising target to test internal structure and atmospheric evolution models.

Ben S. Lakeland, A. Mortier, R. D. Haywood, S. Ulmer-Moll, Z. Garai, A. Vanderburg, J A. Egger, D. A. Turner, D. Kubyshkina, A. C. M. Correia, H. P. Osborn, L. A. Buchhave, L. Malavolta, A. Bonfanti, W. Boschin, A. Cameron, A. Castro-González, R. Cosentino, M. Damasso, X. Dumusque, D. Ehrenreich, Z. Essack, S. Filomeno, L. Fossati, D. Gandolfi, M. Gillon, C. Hedges, M. López-Morales, G. Lacedelli, M. Lendl, J. Maldonado, G. Mantovan, A. F. Martínez Fiorenzano, P. F. L. Maxted, C. Mordasini, B. Nicholson, S. M. O’Brien, L. Palethorpe, E. Palle, M. Pinamonti, D. Rapetti, I. Ribas, N. C. Santos, A. M. Silva, A. Sozzetti, M. Stalport, G. Szabó, S. Udry, M. Vezie, C. A. Watson, T. G. Wilson

Comments: 17 pages, 13 figures, 7 tables, accepted for publication in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2601.14045 [astro-ph.EP] (or arXiv:2601.14045v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2601.14045
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Submission history
From: Ben Lakeland
[v1] Tue, 20 Jan 2026 15:00:16 UTC (12,260 KB)
https://arxiv.org/abs/2601.14045
Astrobiology