Dark skies of the slightly eccentric WASP-18 b from its optical-to-infrared dayside emission
Artikel i vetenskaplig tidskrift, 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.
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: atmospheres
planets and satellites: individual: WASP-18 b
Författare
A. Deline
Université de Genève
P. E. Cubillos
Österreichische Akademie der Wissenschaften
Istituto nazionale di astrofisica (INAF)
L. Carone
Österreichische Akademie der Wissenschaften
B. -o. Demory
Universität Bern
M. Lendl
Université de Genève
W. Benz
Universität Bern
A. Brandeker
Stockholms universitet
M. N. Guenther
Europeiska rymdorganisationen (ESA)
A. Heitzmann
Université de Genève
S. C. C. Barros
Universidade do Porto
L. Kreidberg
Max-Planck-Gesellschaft
G. Bruno
Istituto nazionale di astrofisica (INAF)
D. Kitzmann
Universität Bern
A. Bonfanti
Österreichische Akademie der Wissenschaften
M. Farnir
Université de Genève
Carina Persson
Chalmers, Rymd-, geo- och miljövetenskap, Astronomi och plasmafysik
S. G. Sousa
Universidade do Porto
T. G. Wilson
The University of Warwick
D. Ehrenreich
Université de Genève
V. Singh
Istituto nazionale di astrofisica (INAF)
N. Iro
Deutsches Zentrums für Luft- und Raumfahrt (DLR)
Y. Alibert
Universität Bern
R. Alonso
Instituto de Astrofísica de Canarias
Universidad de la Laguna
T. Barczy
Admatis
D. Barrado Navascues
Consejo Superior de Investigaciones Científicas (CSIC)
W. Baumjohann
Österreichische Akademie der Wissenschaften
M. Bergomi
Istituto nazionale di astrofisica (INAF)
N. Billot
Université de Genève
L. Borsato
Istituto nazionale di astrofisica (INAF)
C. Broeg
Universität Bern
M. -d. Busch
Universität Bern
A. Collier Cameron
University of St Andrews
A. C. M. Correia
Universidade de Coimbra
Sz. Csizmadia
Deutsches Zentrums für Luft- und Raumfahrt (DLR)
M. B. Davies
Lunds universitet
M. Deleuil
Aix-Marseille Université
L. Delrez
KU Leuven
Universite de Liège
O. D. S. Demangeon
Universidade do Porto
A. Derekas
Eötvös Loránd University (ELTE)
B. Edwards
Netherlands Institute for Space Research (SRON)
A. Erikson
Deutsches Zentrums für Luft- und Raumfahrt (DLR)
A. Fortier
Universität Bern
L. Fossati
Österreichische Akademie der Wissenschaften
Malcolm Fridlund
Chalmers, Rymd-, geo- och miljövetenskap, Astronomi och plasmafysik
D. Gandolfi
Universita degli Studi di Torino
K. Gazeas
National and Kapodistrian University of Athens
M. Gillon
Universite de Liège
M. Guedel
Universität Wien
J. Hasiba
Österreichische Akademie der Wissenschaften
Ch. Helling
Österreichische Akademie der Wissenschaften
Technische Universität Graz
K. G. Isaak
Europeiska rymdorganisationen (ESA)
L. L. Kiss
Magyar Tudomanyos Akademia
Eötvös Loránd University (ELTE)
J. Korth
Lunds universitet
K. W. F. Lam
Deutsches Zentrums für Luft- und Raumfahrt (DLR)
J. Laskar
Université de recherche Paris Sciences et Lettres
A. Lecavelier des etangs
Université Pierre et Marie Curie (UPMC)
D. Magrin
Istituto nazionale di astrofisica (INAF)
P. F. L. Maxted
Keele University
B. Merin
Europeiska rymdorganisationen (ESA)
C. Mordasini
Universität Bern
V. Nascimbeni
Istituto nazionale di astrofisica (INAF)
G. Olofsson
Stockholms universitet
R. Ottensamer
Universität Wien
I. Pagano
Istituto nazionale di astrofisica (INAF)
E. Palle
Universidad de la Laguna
Instituto de Astrofísica de Canarias
G. Peter
Deutsches Zentrums für Luft- und Raumfahrt (DLR)
D. Piazza
Universität Bern
G. Piotto
Istituto nazionale di astrofisica (INAF)
Università di Padova
D. Pollacco
The University of Warwick
D. Queloz
Eidgenössische Technische Hochschule Zürich (ETH)
University of Cambridge
R. Ragazzoni
Istituto nazionale di astrofisica (INAF)
Università di Padova
N. Rando
Europeiska rymdorganisationen (ESA)
F. Ratti
Europeiska rymdorganisationen (ESA)
H. Rauer
Deutsches Zentrums für Luft- und Raumfahrt (DLR)
Freie Universität Berlin
I. Ribas
Consejo Superior de Investigaciones Científicas (CSIC)
Inst Estudis Espacials Catalunya IEEC
N. C. Santos
Universidade do Porto
G. Scandariato
Istituto nazionale di astrofisica (INAF)
D. Segransan
Université de Genève
A. E. Simon
Universität Bern
A. M. S. Smith
Deutsches Zentrums für Luft- und Raumfahrt (DLR)
M. Stalport
Universite de Liège
S. Sulis
Aix-Marseille Université
Gy. M. Szabo
Eötvös Loránd University (ELTE)
S. Udry
Université de Genève
V. Van Grootel
Universite de Liège
J. Venturini
Université de Genève
E. Villaver
Instituto de Astrofísica de Canarias
Universidad de la Laguna
N. A. Walton
University of Cambridge
K. Westerdorff
Deutsches Zentrums für Luft- und Raumfahrt (DLR)
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 699 A150Ämneskategorier (SSIF 2025)
Astronomi, astrofysik och kosmologi
DOI
10.1051/0004-6361/202450939