Spitzer thermal phase curve of WASP-121 b

¹ Department of Space, Earth and Environment, Chalmers University of Technology, 412 96 Gothenburg, Sweden
² Instituto de Astrofísica de Canarias (IAC), Vía Láctea, s/n, 38205 La Laguna, Tenerife, Spain
e-mail: giuseppe.morello-ext@iac.es
³ European Space Agency (ESA), ESA Office, Space Telescope Science Institute (STScI), Baltimore, MD 21218, USA
⁴ Department of Physics and Astronomy, University College London, Gower Street, WC1E 6BT London, UK
⁵ AIM, CEA, CNRS, Université Paris-Saclay, Université Paris-Diderot, Sorbonne Paris-Cité, 91191 Gif-sur-Yvette, France
⁶ Dept. of Astronomy, University of Virginia, 530 McCormick Rd, Charlottesville, VA 22904, USA

Received: 12 April 2023
Accepted: 20 June 2023

Abstract

Aims. We analyse unpublished Spitzer observations of the thermal phase-curve of WASP-121 b, a benchmark ultra-hot Jupiter.

Methods. We adopted the wavelet pixel-independent component analysis technique to remove challenging instrumental systematic effects in these datasets and we fit them simultaneously with parametric light-curve models. We also performed phase-curve retrievals to better understand the horizontal and vertical thermal structure of the planetary atmosphere.

Results. We measured planetary brightness temperatures of ~2700 K (dayside) and ~700–1100 K (nightside), along with modest peak offsets of 5.9° ± 1.6 (3.6 µm) and 5.0°_−3.1^+3.4 (4.5 µm) after mid-eclipse. These results suggest inefficient heat redistribution in the atmosphere of WASP-121 b. The inferred atmospheric Bond albedo and circulation efficiency align well with observed trends for hot giant exoplanets. Interestingly, the measured peak offsets correspond to a westward hot spot, which has rarely been observed. We also report consistent transit depths at 3.6 and 4.5 µm, along with updated geometric and orbital parameters. Finally, we compared our Spitzer results with previous measurements, including recent JWST observations.

Conclusions. We extracted new information on the thermal properties and dynamics of an exoplanet atmosphere from an especially problematic dataset. This study probes the reliability of exoplanet phase-curve parameters obtained from Spitzer observations when state-of-the-art pipelines are adopted to remove the instrumental systematic effects. It demonstrates that Spitzer phase-curve observations provide a useful baseline for comparison with JWST observations, and shows the increase in parameters precision achieved with the newer telescope.