Long-term monitoring of WASP-19 b: Signs of apsidal precession and molecular signatures

¹ Instituto de Astronomía y Ciencias Planetarias de Atacama (INCT), Universidad de Atacama, Copayapu 485, Copiapó, Chile
² European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
³ Department of Physics, Washington University, St. Louis, MO 63130, USA
⁴ School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
⁵ Centro de Astronomía (CITEVA), Universidad de Antofagasta, Avenida U. de Antofagasta, 02800 Antofagasta, Chile
⁶ Astrophyscis Group, Keele University, Staffordshire ST5 5BG, UK
⁷ University of Southern Denmark, Department of Physics, Chemistry and Pharmacy, SDU-Galaxy, Campusvej 55, 5230 Odense M, Denmark
⁸ Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
⁹ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
¹⁰ Centre for ExoLife Sciences, Niels Bohr Institute, Jagtvej 155, 2200 Copenhagen, Denmark
¹¹ INAF - Osservatorio Astrofisico di Torino, Via Observatorio 20, 10025 Pino Torinese, Italy
¹² Alma Mater Studiorum - University of Bologna, Dipartimento di Fisica e Astronomia “Augusto Righi”, Via Gobetti 93/2, 40129 Bologna, Italy
¹³ Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, Napoli, Italy
¹⁴ Earth System Sciences, Atmospheric Science, University of Hamburg, Hamburg, Germany
¹⁵ Centre for Exoplanet Science, SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
¹⁶ Millennium Institute of Astrophysics MAS, Nuncio Monsenor Sotero Sanz 100, Of. 104, Providencia, Santiago, Chile
¹⁷ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
¹⁸ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg (ZAH), 69120 Heidelberg, Germany
¹⁹ Astronomy Research Center, Research Institute of Basic Sciences, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea
²⁰ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
²¹ European Southern Observatory (ESO), Alonso de Córdova 3107 Vitacura, Santiago, Chile
²² Departamento de Matemática y Física Aplicadas, Facultad de Ingeniería, Universidad Católica de la Santísima Concepción, Alonso de Rivera 2850, Concepción, Chile
²³ Perimeter Institute for Theoretical Physics, 31 Caroline St N, Waterloo, ON N2L 2Y5, Canada
²⁴ Centre for Electronic Imaging, Department of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK
²⁵ Dipartimento di Fisica, Università degli Studi di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133, Roma, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 11 August 2025
Accepted: 9 February 2026

Abstract

Context. With over 6000 exoplanets discovered to date, approximately 12 % are classified as hot-Jupiters. Due to their large sizes and short orbital periods (P < 10 day), they are easier to detect and provide crucial insights into planetary formation, atmospheric properties, and orbital dynamics. Among these, ultra-short-period exoplanets (P ≤ 1 d) are particularly interesting, as they are expected to undergo orbital decay driven by strong tidal interactions. Despite theoretical predictions, WASP-12 b and WASP-4 b remain the confirmed hot-Jupiters experiencing measurable orbital decay.

Aims. This study presents a homogeneous analysis of WASP-19 b to investigate both its orbital dynamics and atmospheric composition. Leveraging a 15-year dataset, our goal is to assess whether the system exhibits long-term deviations from a constant orbital period and to investigate whether any detected variations are consistent with tidal orbital decay, apsidal precession, or periodic signals indicative of a potential planetary perturber. Additionally, we also construct a photometric transmission spectrum to characterize its atmosphere.

Methods. We analyze multi-wavelength light curves, incorporating starspot modeling with PRISM to account for stellar inhomogeneities. To assess orbital evolution, we fit linear, quadratic, and cubic ephemeris models to transit timing residuals with respect to a non-decaying orbit.

Results. Our analysis, which includes 27 new transits, reveals no statistically significant periodic signal in the transit timings. Although none of the tested ephemeris models fully reproduce the observed timing scatter, the mid-transit times exhibit systematic deviations from a strictly constant orbital period and are best reproduced by the cubic ephemeris in a relative model-comparison sense, indicating a slow, non-periodic long-term trend over the ~ 15-year baseline. This behavior is more consistent with gradual orbital precession than with monotonic tidal decay, for which a dominant quadratic trend would be expected. Fitting a precession model yields a rate of ω̇_obs = (1.00 ± 0.12) × 10⁻⁴ rad/orbit, corresponding to a planetary Love number k_2p = 0.107 ± 0.08, in agreement with recent independent estimates. The transmission spectrum reveals signatures of Na, K, and H₂O, with no strong evidence of TiO/VO, likely due to the resolution limits of the photometric data.

Conclusions. Our results support that apsidal precession could be the dominant process governing the long-term orbital evolution of WASP-19b, possibly sustained by weak eccentricity forcing from the wide companion WASP-19 B. These orbital dynamics can, in turn, impact the atmospheric structure by modulating the irradiation history, potentially altering molecular abundances over time. Our findings highlight the importance of combining TTV analyzes with multi-wavelength atmospheric data, while emphasizing that additional high-quality timing and spectroscopic observations are required to corroborate the fidelity of the proposed orbital model.