TOI-544 b: a potential water-world inside the radius valley in a two-planet system
H. L. M. Osborne, V Van Eylen, Elisa Goffo, D. Gandolfi, G. Nowak, C. M. Persson, John H. Livingston, A. Weeks, Ε. Πάλλη, R. Luque, C. Hellier, I. Carleo, Seth Redfield, Teruyuki Hirano, M. Garbaccio Gili, Javier Alarcón, Oscar Barragán, N. Casasayas-Barris, M. R. Díaz, Marco Esposito, Emil Knudstrup, J. S. Jenkins, F. Murgas, J. Orell-Miquel, F. Rodler, L. M. Serrano, M. Stangret, Simon Albrecht, A. Alqasim, William D. Cochran, H. J. Deeg, M. Fridlund, A. P. Hatzes, J. Korth, K. W. F. Lam
Abstract
ABSTRACT We report on the precise radial velocity follow-up of TOI-544 (HD 290498), a bright K star (V = 10.8), which hosts a small transiting planet recently discovered by the Transiting Exoplanet Survey Satellite (TESS). We collected 122 high-resolution High Accuracy Radial velocity Planet Searcher (HARPS) and HARPS-N spectra to spectroscopically confirm the transiting planet and measure its mass. The nearly 3-yr baseline of our follow-up allowed us to unveil the presence of an additional, non-transiting, longer-period companion planet. We derived a radius and mass for the inner planet, TOI-544 b, of 2.018 ± 0.076 R⊕ and 2.89 ± 0.48 M⊕, respectively, which gives a bulk density of $1.93^{+0.30}_{-0.25}$ g cm−3. TOI-544 c has a minimum mass of 21.5 ± 2.0 M⊕ and orbital period of 50.1 ± 0.2 d. The low density of planet-b implies that it has either an Earth-like rocky core with a hydrogen atmosphere, or a composition which harbours a significant fraction of water. The composition interpretation is degenerate depending on the specific choice of planet interior models used. Additionally, TOI-544 b has an orbital period of 1.55 d and equilibrium temperature of 999 ± 14 K, placing it within the predicted location of the radius valley, where few planets are expected. TOI-544 b is a top target for future atmospheric observations, for example with JWST, which would enable better constraints of the planet composition.