The KELT-7b atmospheric thermal-inversion conundrum revisited with CHEOPS, TESS, and additional data
Journal article, 2025
Context. Early theoretical works suggested that ultrahot Jupiters have inverted temperature-pressure (T-P) profiles in the presence of optical absorbers, such as TiO and VO. Recently, an inverted T-P profile of KELT-7b was detected, in agreement with the predictions. However, the diagnosis of T-P inversions has always been recognized to be a model-dependent process.
Aims. We used the Characterising Exoplanet Satellite (CHEOPS), the Transiting Exoplanet Survey Satellite (TESS), and additional literature data to characterize the atmosphere of KELT-7b, rederive the T-P profile, provide a precise measurement of the albedo of KELT-7b, and search for a possible distortion in the precise CHEOPS transit light curve of the planet.
Methods. We first jointly fitted the CHEOPS and TESS data and measured the occultation depths in these passbands. The CHEOPS transits were also fitted with a model including the gravity-darkening effect. Emission and absorption retrievals were performed to characterize the atmosphere of KELT-7b. The albedo of the planet was calculated in the CHEOPS and TESS passbands.
Results. When adopting a thermochemical-equilibrium atmospheric composition, the emission retrievals return a non-inverted T-P profile, in contrast with previous results. When adopting a free-chemistry atmospheric parameterization, the emission retrievals return an inverted T-P profile with - likely unphysically - high concentrations of TiO and VO. The 3D general circulation model (GCM) supports a TiO-induced temperature inversion. We report for KELT-7b a very low geometric albedo of A(g) = 0.05 +/- 0.06, which is consistent with the heat distribution & varepsilon; being close to zero and also consistent with a 3D GCM simulation, using magnetic drag (tau(drag) = 10(4) s). Based on the CHEOPS photometry, we are unable to place any meaningful constraint on the sky-projected orbital obliquity.
Conclusions. The choice of a free-chemistry approach or a thermochemical-equilibrium chemistry is the main factor determining the retrieval results. Free-chemistry retrievals generally yield better fits; however, assuming free chemistry risks adopting unphysical scenarios for ultrahot Jupiters, such as KELT-7b. We applied a coherent stellar variability treatment on TESS and CHEOPS observations, commensurate with the known stellar activity of the host star. Other observations of KELT-7b would also benefit from a coherent stellar variability treatment.
Aims. We used the Characterising Exoplanet Satellite (CHEOPS), the Transiting Exoplanet Survey Satellite (TESS), and additional literature data to characterize the atmosphere of KELT-7b, rederive the T-P profile, provide a precise measurement of the albedo of KELT-7b, and search for a possible distortion in the precise CHEOPS transit light curve of the planet.
Methods. We first jointly fitted the CHEOPS and TESS data and measured the occultation depths in these passbands. The CHEOPS transits were also fitted with a model including the gravity-darkening effect. Emission and absorption retrievals were performed to characterize the atmosphere of KELT-7b. The albedo of the planet was calculated in the CHEOPS and TESS passbands.
Results. When adopting a thermochemical-equilibrium atmospheric composition, the emission retrievals return a non-inverted T-P profile, in contrast with previous results. When adopting a free-chemistry atmospheric parameterization, the emission retrievals return an inverted T-P profile with - likely unphysically - high concentrations of TiO and VO. The 3D general circulation model (GCM) supports a TiO-induced temperature inversion. We report for KELT-7b a very low geometric albedo of A(g) = 0.05 +/- 0.06, which is consistent with the heat distribution & varepsilon; being close to zero and also consistent with a 3D GCM simulation, using magnetic drag (tau(drag) = 10(4) s). Based on the CHEOPS photometry, we are unable to place any meaningful constraint on the sky-projected orbital obliquity.
Conclusions. The choice of a free-chemistry approach or a thermochemical-equilibrium chemistry is the main factor determining the retrieval results. Free-chemistry retrievals generally yield better fits; however, assuming free chemistry risks adopting unphysical scenarios for ultrahot Jupiters, such as KELT-7b. We applied a coherent stellar variability treatment on TESS and CHEOPS observations, commensurate with the known stellar activity of the host star. Other observations of KELT-7b would also benefit from a coherent stellar variability treatment.
methods: observational
techniques: photometric
planets and satellites: fundamental parameters
planets and satellites: individual
planets and satellites: atmospheres
Author
Z. Garai
Eötvös Loránd University (ELTE)
Slovak Academy of Sciences
A. Krenn
Austrian Academy of Sciences
P. E. Cubillos
Istituto nazionale di astrofisica (INAF)
Austrian Academy of Sciences
G. Bruno
Istituto nazionale di astrofisica (INAF)
A. M. S. Smith
German Aerospace Center (DLR)
T. G. Wilson
The University of Warwick
A. Brandeker
Stockholm University
M. N. Guenther
European Space Agency (ESA)
A. Heitzmann
University of Geneva
L. Carone
Austrian Academy of Sciences
V. Singh
Istituto nazionale di astrofisica (INAF)
M. Lendl
University of Geneva
O. D. S. Demangeon
University of Porto
Y. Alibert
University of Bern
R. Alonso
Instituto de Astrofísica de Canarias
University of La Laguna
J. Asquier
European Space Agency (ESA)
T. Barczy
Admatis
D. Barrado
Spanish National Research Council (CSIC)
S. C. C. Barros
University of Porto
W. Baumjohann
Austrian Academy of Sciences
W. Benz
University of Bern
N. Billot
University of Geneva
L. Borsato
Istituto nazionale di astrofisica (INAF)
C. Broeg
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
A. Deline
University of Geneva
B. -o. Demory
University of Bern
A. Derekas
Hungarian Research Network
Eötvös Loránd University (ELTE)
B. Edwards
Netherlands Institute for Space Research (SRON)
J. A. Egger
University of Bern
D. Ehrenreich
University of Geneva
A. Erikson
German Aerospace Center (DLR)
J. Farinato
Istituto nazionale di astrofisica (INAF)
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
Ch. Helling
Technische Universität Graz
Austrian Academy of Sciences
K. G. Isaak
European Space Agency (ESA)
F. Kerschbaum
University of Vienna
L. L. Kiss
Hungarian Academy of Sciences
Eötvös Loránd University (ELTE)
J. Korth
Lund University
K. W. F. Lam
German Aerospace Center (DLR)
J. Laskar
Sorbonne University
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
University of La Laguna
Instituto de Astrofísica de Canarias
G. Peter
German Aerospace Center (DLR)
D. Piazza
University of Bern
G. Piotto
Istituto nazionale di astrofisica (INAF)
University of Padua
D. Pollacco
The University of Warwick
D. Queloz
University of Cambridge
Swiss Federal Institute of Technology in Zürich (ETH)
R. Ragazzoni
University of Padua
Istituto nazionale di astrofisica (INAF)
N. Rando
European Space Agency (ESA)
H. Rauer
German Aerospace Center (DLR)
Freie Universität Berlin
I. Ribas
Spanish National Research Council (CSIC)
Inst Estudis Espacials Catalunya IEEC
N. C. Santos
University of Porto
G. Scandariato
Istituto nazionale di astrofisica (INAF)
D. Segransan
University of Geneva
A. E. Simon
University of Bern
S. G. Sousa
University of Porto
M. Stalport
University of Liège
S. Sulis
Aix Marseille University
Gy. M. Szabo
Eötvös Loránd University (ELTE)
S. Udry
University of Geneva
S. Ulmer-Moll
University of Liège
University of Bern
University of Geneva
V. Van Grootel
University of Liège
J. Venturini
University of Geneva
E. Villaver
University of La Laguna
Instituto de Astrofísica de Canarias
N. A. Walton
University of Cambridge
S. Wolf
University of Bern
D. Wolter
German Aerospace Center (DLR)
T. Zingales
University of Padua
Istituto nazionale di astrofisica (INAF)
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 700 A5Subject Categories (SSIF 2025)
Astronomy, Astrophysics, and Cosmology
DOI
10.1051/0004-6361/202452560