Dark skies of the slightly eccentric WASP-18 b from its optical-to-infrared dayside emission☆
Journal article, 2025

Context. Ultra-hot Jupiters (UHJs) are gas giant exoplanets that are strongly irradiated by their star, setting intense molecular dissociation that leads to atmospheric chemistry dominated by ions and atoms. These conditions inhibit day-to-night heat redistribution, which results in high temperature contrasts. Phase-curve observations over several passbands offer insights on the thermal structure and properties of these extreme atmospheres.
Aims. We aim to perform a joint analysis of multiple observations of WASP-18 b from the visible to the mid-infrared, using data from CHEOPS, TESS, and Spitzer. Our purpose is to characterise the planetary atmosphere with a consistent view over the large wavelength range covered, including JWST data. Methods. We implemented a model for the planetary signal including transits, occultations, phase signal, ellipsoidal variations, Doppler boosting, and light travel time. We performed a joint fit of more than 250 eclipse events and derived the atmospheric properties using general circulation models (GCMs) and retrieval analyses.
Results. We obtained new ephemerides with unprecedented precisions of 1 second and 1.4 millisecond on the time of inferior conjunction and orbital period, respectively. We computed a planetary radius of R (p )= 1.1926 +/- 0.0077 R (J )with a precision of 0.65% (or 550 km). Based on a timing inconsistency with JWST, we discuss and confirm the orbital eccentricity (e = 0.00852 +/- 0.00091). We also constrain the argument of periastron to omega = 261.9(-1.4) (+1.3) deg. We show that the large dayside emission implies the presence of magnetic drag and super-solar metallicity. We find a steep thermally inverted gradient in the planetary atmosphere, which is common for UHJs. We detected the presence of strong CO emission lines at 4.5 mu m from an excess of dayside brightness in the Spitzer/IRAC/Channel 2 passband. Using these models to constrain the reflected contribution in the CHEOPS passband, we derived an extremely low geometric albedo of A(g) (CHEOPS) = 0.027 +/- 0.011.
Conclusions. The orbital eccentricity remains a potential challenge for planetary dynamics that might require further study given the short-period massive planet and despite the young age of the system. The characterisation of the atmosphere of WASP-18 b reveals the necessity to account for magnetic friction and super-solar metallicity to explain the full picture of the dayside emission. We find the planetary dayside to be extremely unreflective; however, when juxtaposing TESS and CHEOPS data, we get hints of increased scattering efficiency in the visible, likely due to Rayleigh scattering.

techniques: photometric

planets and satellites: individual: WASP-18 b

planets and satellites: atmospheres

Author

A. Deline

University of Geneva

P. E. Cubillos

Istituto nazionale di astrofisica (INAF)

Austrian Academy of Sciences

L. Carone

Austrian Academy of Sciences

B. -o. Demory

University of Bern

M. Lendl

University of Geneva

W. Benz

University of Bern

A. Brandeker

Stockholm University

M. N. Guenther

European Space Agency (ESA)

A. Heitzmann

University of Geneva

S. C. C. Barros

University of Porto

L. Kreidberg

Max Planck Society

G. Bruno

Istituto nazionale di astrofisica (INAF)

D. Kitzmann

University of Bern

A. Bonfanti

Austrian Academy of Sciences

M. Farnir

University of Geneva

Carina Persson

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics

S. G. Sousa

University of Porto

T. G. Wilson

The University of Warwick

D. Ehrenreich

University of Geneva

V. Singh

Istituto nazionale di astrofisica (INAF)

N. Iro

German Aerospace Center (DLR)

Y. Alibert

University of Bern

R. Alonso

Instituto de Astrofísica de Canarias

University of La Laguna

T. Barczy

Admatis

D. Barrado Navascues

Spanish National Research Council (CSIC)

W. Baumjohann

Austrian Academy of Sciences

M. Bergomi

Istituto nazionale di astrofisica (INAF)

N. Billot

University of Geneva

L. Borsato

Istituto nazionale di astrofisica (INAF)

C. Broeg

University of Bern

M. -d. Busch

University of Bern

A. Collier Cameron

University of St Andrews

A. C. M. Correia

University of Coimbra

Sz. Csizmadia

German Aerospace Center (DLR)

M. B. Davies

Lund University

M. Deleuil

Aix Marseille University

L. Delrez

University of Liège

KU Leuven

O. D. S. Demangeon

University of Porto

A. Derekas

Eötvös Loránd University (ELTE)

B. Edwards

Netherlands Institute for Space Research (SRON)

A. Erikson

German Aerospace Center (DLR)

A. Fortier

University of Bern

L. Fossati

Austrian Academy of Sciences

Malcolm Fridlund

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics

D. Gandolfi

University of Turin

K. Gazeas

National and Kapodistrian University of Athens

M. Gillon

University of Liège

M. Guedel

University of Vienna

J. Hasiba

Austrian Academy of Sciences

Ch. Helling

Technische Universität Graz

Austrian Academy of Sciences

K. G. Isaak

European Space Agency (ESA)

L. L. Kiss

Eötvös Loránd University (ELTE)

Hungarian Academy of Sciences

J. Korth

Lund University

K. W. F. Lam

German Aerospace Center (DLR)

J. Laskar

Université Paris PSL

A. Lecavelier des etangs

Pierre and Marie Curie University (UPMC)

D. Magrin

Istituto nazionale di astrofisica (INAF)

P. F. L. Maxted

Keele University

B. Merin

European Space Agency (ESA)

C. Mordasini

University of Bern

V. Nascimbeni

Istituto nazionale di astrofisica (INAF)

G. Olofsson

Stockholm University

R. Ottensamer

University of Vienna

I. Pagano

Istituto nazionale di astrofisica (INAF)

E. Palle

Instituto de Astrofísica de Canarias

University of La Laguna

G. Peter

German Aerospace Center (DLR)

D. Piazza

University of Bern

G. Piotto

University of Padua

Istituto nazionale di astrofisica (INAF)

D. Pollacco

The University of Warwick

D. Queloz

University of Cambridge

Swiss Federal Institute of Technology in Zürich (ETH)

R. Ragazzoni

Istituto nazionale di astrofisica (INAF)

University of Padua

N. Rando

European Space Agency (ESA)

F. Ratti

European Space Agency (ESA)

H. Rauer

Freie Universität Berlin

German Aerospace Center (DLR)

I. Ribas

Inst Estudis Espacials Catalunya IEEC

Spanish National Research Council (CSIC)

N. C. Santos

University of Porto

G. Scandariato

Istituto nazionale di astrofisica (INAF)

D. Segransan

University of Geneva

A. E. Simon

University of Bern

A. M. S. Smith

German Aerospace Center (DLR)

M. Stalport

University of Liège

S. Sulis

Aix Marseille University

Gy. M. Szabo

Eötvös Loránd University (ELTE)

S. Udry

University of Geneva

V. Van Grootel

University of Liège

J. Venturini

University of Geneva

E. Villaver

Instituto de Astrofísica de Canarias

University of La Laguna

N. A. Walton

University of Cambridge

K. Westerdorff

German Aerospace Center (DLR)

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 699 A150

Subject Categories (SSIF 2025)

Astronomy, Astrophysics, and Cosmology

DOI

10.1051/0004-6361/202450939

More information

Latest update

8/21/2025