Searching for hot water world candidates with CHEOPS: Refining the radii and analysing the internal structures and atmospheric lifetimes of TOI-238 b and TOI-1685 b
Journal article, 2025
Studying the composition of exoplanets is one of the most promising approaches to observationally constrain planet formation and evolution processes. However, this endeavour is complicated for small exoplanets by the fact that a wide range of compositions are compatible with their observed bulk properties. To overcome this issue, we identify triangular regions in the mass-radius space where part of this intrinsic degeneracy is lifted for close-in planets, since low-mass H/He envelopes would not be stable due to high-energy stellar irradiation. Planets in these Hot Water World triangles need to contain at least some heavier volatiles and are therefore interesting targets for atmospheric follow-up observations. We perform a demographic study to show that only few well-characterised planets in these regions are currently known and introduce our CHEOPS GTO programme aimed at identifying more of these potential hot water worlds. Here, we present CHEOPS observations for the first two targets of our programme, TOI-238 b and TOI-1685 b. Combined with TESS photometry and published RVs, we use the precise radii and masses of both planets to study their location relative to the corresponding Hot Water World triangles, perform an interior structure analysis, and study the possible lifetimes of H/He and waterdominated atmospheres under these conditions. We find that TOI-238 blies, at the 1σ level, inside the corresponding triangle. While a pure H/He atmosphere would have evaporated after 0.4-1.3 Myr, it is likely that a water-dominated atmosphere would have survived until the current age of the system, which makes TOI-238 ba promising candidate for a hot water world. Conversely, TOI-1685 b lies below the mass-radius model for a pure silicate planet, meaning that even though a water-dominated atmosphere would be compatible both with our internal structure and evaporation analysis, we cannot rule out the planet being a bare core.
Planets and satellites: interiors
Planets and satellites: individual: TOI-1685
Planets and satellites: formation
Planets and satellites: individual: TOI-238
Techniques: photometric
Author
J. A. Egger
Physics Institute
D. Kubyshkina
Physics Institute
Institut fur Weltraumforschung
Y. Alibert
Physics Institute
University of Bern
H. P. Osborn
University of Bern
Swiss Federal Institute of Technology in Zürich (ETH)
A. Bonfanti
Institut fur Weltraumforschung
T.G. Wilson
The University of Warwick
A. Brandeker
Stockholm University
Maximilian N. Günther
European Space Agency (ESA)
M. Lendl
University of Geneva
D. Kitzmann
University of Bern
Physics Institute
L. Fossati
Institut fur Weltraumforschung
C. Mordasini
University of Bern
Physics Institute
S.G. Sousa
University of Porto
V. Adibekyan
University of Porto
Malcolm Fridlund
Leiden University
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
C. Pezzotti
University of Liège
D. Gandolfi
University of Turin
S. Ulmer-Moll
University of Geneva
University of Liège
Physics Institute
R. Alonso
University of La Laguna
Instituto de Astrofísica de Canarias
T. Bárczy
Admatis
D. Barrado Navascues
European Space Agency (ESA)
S.C.C. Barros
University of Porto
W. Baumjohann
Institut fur Weltraumforschung
W. Benz
University of Bern
Physics Institute
N. Billot
University of Geneva
L. Borsato
Istituto nazionale di astrofisica (INAF)
C. Broeg
Physics Institute
University of Bern
A. Collier Cameron
University of St Andrews
A. Correia
University of Coimbra
Szilard Csizmadia
German Aerospace Center (DLR)
P. E. Cubillos
Istituto nazionale di astrofisica (INAF)
Institut fur Weltraumforschung
Melvyn B. Davies
Lund University
M. Deleuil
Laboratoire d'Astrophysique de Marseille
A. Deline
University of Geneva
O. Demangeon
University of Porto
B.O. Demory
University of Bern
Physics Institute
A. Derekas
Eötvös Loránd University (ELTE)
Billy Edwards
Netherlands Institute for Space Research (SRON)
D. Ehrenreich
University of Geneva
Anders Erikson
German Aerospace Center (DLR)
A. Fortier
University of Bern
Physics Institute
K. Gazeas
University of Athens
Michaël Gillon
University of Liège
M. Gudel
University of Vienna
A. Heitzmann
University of Geneva
Ch. Helling
Technische Universität Graz
Institut fur Weltraumforschung
K. Isaak
European Space Agency (ESA)
L. L. Kiss
Eötvös Loránd University (ELTE)
Hungarian Academy of Sciences
Judith Korth
Lund University
K. W.F. Lam
German Aerospace Center (DLR)
J. Laskar
Paris Observatory
A. L. des Etangs
Institut d 'Astrophysique de Paris
A. Luntzer
University of Vienna
R. Luque
University of Chicago
D. Magrin
Istituto nazionale di astrofisica (INAF)
P. Maxted
Keele University
Bruno Merín
European Space Agency (ESA)
M. Munari
Istituto nazionale di astrofisica (INAF)
Valerio Nascimbeni
Istituto nazionale di astrofisica (INAF)
G. Olofsson
Stockholm University
R. Ottensamer
University of Vienna
I. Pagano
Istituto nazionale di astrofisica (INAF)
Enric Palle
University of La Laguna
Instituto de Astrofísica de Canarias
G. Peter
German Aerospace Center (DLR)
D. Piazza
University of Bern
Giampaolo P. Piotto
Istituto nazionale di astrofisica (INAF)
University of Padua
Don L. 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)
H. Rauer
Freie Universität Berlin
German Aerospace Center (DLR)
I. Ribas
Spanish National Research Council (CSIC)
Centre Tecnològic de Telecomunicacions de Catalunya (CTTC)
N. C. Santos
University of Porto
Gaetano Scandariato
Istituto nazionale di astrofisica (INAF)
D. Segransan
University of Geneva
A.E. Simon
Physics Institute
University of Bern
A. M.S. Smith
German Aerospace Center (DLR)
R. Southworth
European Space Agency (ESA)
M. Stalport
University of Liège
S. Sulis
Laboratoire d'Astrophysique de Marseille
G.M. Szabó
Eötvös Loránd University (ELTE)
S. Udry
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
Physics Institute
D. Wolter
German Aerospace Center (DLR)
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 696 A28Subject Categories (SSIF 2025)
Astronomy, Astrophysics, and Cosmology
DOI
10.1051/0004-6361/202453325