The GAPS Programme at TNG

LXV. Precise density measurement of TOI-1430 b, a young planet with an evaporating atmosphere

¹ Dipartimento di Fisica e Astronomia “Galileo Galilei” – Università degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
² INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, Padova 35122, Italy
³ Centro di Ateneo di Studi e Attività Spaziali “G. Colombo” – Università degli Studi di Padova, Via Venezia 15, 35131 Padova, Italy
⁴ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95060, USA
⁵ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo (PA), Italy
⁶ Dipartimento di Scienza e Alta Tecnologia, Università dell’Insubria, Via Valleggio 11, 22100 Como, Italy
⁷ ESO-European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
⁸ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone (Roma), Italy
⁹ Dipartimento di Fisica, Università degli Studi di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Roma, Italy
¹⁰ Department of Physics and Astronomy, The University of Texas, Rio Grande Valley, Brownsville, TX 78520, USA
¹¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹² INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025, Pino Torinese, Italy
¹³ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
¹⁴ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
¹⁵ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁶ Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro 5, 00185 Roma, Italy
¹⁷ Fundación Galileo Galilei – INAF, Rambla J.A. Fernandez P., 7, 38712 S.C. Tenerife, Spain
¹⁸ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
¹⁹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
²⁰ South African Astronomical Observatory, PO Box 9, Observatory, Cape Town 7935, South Africa
²¹ Kotizarovci Observatory, Sarsoni 90, 51216 Viskovo, Croatia

^★ Corresponding author; domenico.nardiello@unipd.it

Received: 13 September 2024
Accepted: 16 November 2024

Abstract

Context. Small-sized (<4 R_⊕) exoplanets in tight orbits around young stars (10–1000 Myr) give us the opportunity to investigate the mechanisms that led to their formation, the evolution of their physical and orbital properties, and, in particular, their atmospheres. Thanks to the all-sky survey carried out by the TESS spacecraft, many of these exoplanets have been discovered, and have subsequently been characterized with dedicated follow-up observations.

Aims. In the context of a collaboration among the Global Architecture of Planetary Systems (GAPS) team, the TESS-Keck Survey (TKS) team, and the California Planet Search (CPS) team, we measured – with a high level of precision – the mass and the radius of TOI-1430 b, a young (~700 Myr) exoplanet with an escaping He atmosphere orbiting the K-dwarf star HD 235088 (TOI-1430).

Methods. By adopting appropriate stellar parameters, which were measured in this work, we were able to simultaneously model the signals due to strong stellar activity and the transiting planet TOI-1430 b in both photometric and spectroscopic series. This allowed us to measure both the radius and mass (and consequently the density) of the planet with high precision, and to reconstruct the evolution of its atmosphere.

Results. TOI-1430 is an active K-dwarf star born 700 ± 150 Myr ago, with a rotation period of P_rot ~ 12 days. This star hosts a mini-Neptune, whose orbital period is P_b = 7.434133 ± 0.000004 days. Thanks to long-term photometric and spectroscopic monitoring of this target performed with TESS, HARPS-N, HIRES, and APF, we estimate a radius of R_P,b = 1.98 ± 0.07 R_⊕, a mass of M_P,b = 4.2 ± 0.8 M_⊕, and thus a planetary density of ρ_b = 0.5 ± 0.1 ρ_⊕. TOI-1430 b is therefore a low-density mini-Neptune with an extended atmosphere, and is at the edge of the radius gap. Because this planet is known to have an evaporating atmosphere of He, we reconstructed its atmospheric history. Our analysis supports the scenario in which, shortly after its birth, TOI-1430 b was super-puffy, with a radius 5 × −13 × and a mass 1.5 × −2 × the values of today; in ~200 Myr from now, TOI-1430 b should lose its envelope, showing its Earth-size core. We also looked for signals from a second planet in the spectroscopic and photometric series, without detecting any.