Characterising WASP-43b’s interior structure: Unveiling tidal decay and apsidal motion

¹ Institute of Planetary Research, German Aerospace Center (DLR), Rutherfordstrasse 2, 12489 Berlin, Germany
² ELKH-SZTE Stellar Astrophysics Research Group, 6500 Baja, Szegedi út Kt. 766, Hungary
³ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, HUN-REN, MTA Centre of Excellence, Konkoly-Thege Miklós út 15–17., 1121, Hungary
⁴ HUN-REN-ELTE Exoplanet Research Group, Szombathely, Szent Imre h. u. 112., 9700, Hungary
⁵ ELTE Eötvös Loránd University, Doctoral School of Physics, Budapest, Pázmány Péter sétány 1/A, 1117, Hungary
⁶ Institut für Geologische Wissenschaften, Freie Universität Berlin, 12249 Berlin, Germany
⁷ Dipartimento di Fisica, Università degli Studi di Torino, Via Pietro Giuria, 1, 10125 Torino, Italy
⁸ INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
⁹ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
¹⁰ Centre Vie dans l’Univers, Faculté des sciences, Université de Genève, Quai Ernest-Ansermet 30, 1211 Genève 4, Switzerland
¹¹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany

^★ Corresponding author; lia.bernabo@dlr.de; liamarta.bernabo@gmail.com

Received: 26 August 2024
Accepted: 3 January 2025

Abstract

Context. Recent developments in exoplanetary research highlight the importance of Love numbers in understanding the internal dynamics, formation, migration history, and potential habitability of exoplanets. Love numbers represent crucial parameters that gauge how exoplanets respond to external forces such as tidal interactions and rotational effects. By measuring these responses, insights into the internal structure, composition, and density distribution of exoplanets can be gained. The rate of apsidal precession of a planetary orbit is directly linked to the second-order fluid Love numbers. Thus, Love numbers can also offer valuable insights into the mass distribution of a planet.

Aims. In this context, we aim to re-determine the orbital parameters of WASP-43b – in particular, the orbital period, eccentricity, and argument of the periastron – and its orbital evolution. We study the outcomes of the tidal interaction with the host star in order to identify whether tidal decay and periastron precession occur in the system.

Methods. We observed WASP-43b with HARPS, whose data we present for the first time, and we also analysed the newly acquired JWST full-phase light curve. We jointly fit new and archival radial velocity and transit and occultation mid-times, including tidal decay, periastron precession, and long-term acceleration in the system.

Results. We detected a tidal decay rate of Ṗ_a = (−l.99±0.50) ms yr⁻¹ and a periastron precession rate of ω = 0.1727_−0.0089^+0.0083)° d⁻¹ = (621.72_−32.04^+29.88)″ d⁻¹). This is the first time that both periastron precession and tidal decay are simultaneously detected in an exoplanetary system. The observed tidal interactions can neither be explained by the tidal contribution to apsidal motion of a non-aligned stellar or planetary rotation axis nor by assuming a non-synchronous rotation for the planet, and a value for the planetary Love number cannot be derived. Moreover, we excluded the presence of a second body (e.g. a distant companion star or a yet undiscovered planet) down to a planetary mass of ≳0.3 M_J and up to an orbital period of ≲3700 days. We leave the question of the cause of the observed apsidal motion open.