TESS phase curve of ultra-hot Jupiter WASP-189 b^★

¹ Department of Astronomy, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
² Weltraumforschung und Planetologie, Physikalisches Institut, Universität Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
³ Center for Space and Habitability, Universität Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
⁴ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁵ Observatoire astronomique de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁶ INAF, Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy

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

Received: 8 September 2025
Accepted: 3 December 2025

Abstract

Context. The thermal structures of highly irradiated ultra-hot Jupiters can deviate substantially from those of cooler hot Jupiters. For planets orbiting host stars that are rapidly rotating (and, thus, oblate), photometric light curves provide a unique opportunity to measure the spin-orbit angle. Moreover, in systems with significant spin-orbit misalignment, the stellar oblateness can induce an observable orbital precession.

Aims. We wish to study the atmosphere and orbital architecture of an ultra-hot Jupiter WASP-189 b, orbiting a hot A-type star.

Methods. We used the photometric phase curves and gravity-darkened transits of WASP-189 b observed with the Transiting Exoplanet Survey Satellite (TESS). We complemented these data with archival observations from CHaracterising ExOPlanet Satellite (CHEOPS).

Results. We detected a phase-curve signal with significant occultation depth of 203.4_−16.3^+16.2 ppm, while the nightside flux, −71.8_−36.0^+36.4 ppm, is consistent with zero at 2σ. We inverted the phase-curve signal to construct the temperature map of the planet. The map was subsequently used to estimate the Bond albedo and heat redistribution efficiency, whose expected median ranges were found to be 0.19–0.35 and 0.09–0.41, respectively. Finally, we analysed gravity-darkened transits to find that the planet is in polar orbit with the spin-orbit angle of 89.46_−1.08^+1.08 deg. We found no hint of any orbital precession when comparing our results with the literature.

Conclusions. Our observations, together with atmospheric modelling, suggest that the dayside emission of WASP-189b in TESS and CHEOPS bandpasses is dominated by thermal emission from an atmosphere with extremely inefficient heat transport and a negligible contribution from reflected light.