Observational Constraints on the Formation and Evolution of Neptune-Class Exoplanets
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Observational Constraints on the Formation and Evolution of Neptune-Class Exoplanets Magali Deleuil1 · Don Pollacco2 · Clément Baruteau3 · Heike Rauer4 · Michel Blanc3
Received: 31 March 2020 / Accepted: 8 August 2020 © Springer Nature B.V. 2020
Abstract Among exoplanets, the small-size population constitutes the dominant one, with a diversity of properties and compositions ranging from rocky to gas dominated envelope. While a large fraction of them have masses and radii similar to or smaller than Neptune, yet none share common properties in term of orbital period and insulation with our ice giants. These exoplanets belong to multi-planet systems where planets are closely packed within the first tenth of AU and often exposed to strong irradiation from their host star. Their formation process, subsequent evolution, and fate are still debated and trigger new developments of planet formation models. This paper reviews the characteristics and properties of this extended sample of planets with radii between ∼1.6 and 4.0 R⊕ . Even though we still lack real Neptune/Uranus analogues, these exoplanets provide us with key observational constraints that allow the formation of our ice giants to be placed in a more general framework than the sole example of our solar system. Keywords Exoplanetary systems · Planetary formation · Neptune · Uranus
1 Introduction The absence of detailed observational constraints, in terms of composition and atmospheric processes, on our ice giants does not allow us to fully understand their role in the evolution and final orbital architecture of the solar system. On the other hand, exoplanets, which are In Situ Exploration of the Ice Giants: Science and Technology Edited by Olivier J. Mousis and David H. Atkinson
B M. Deleuil
[email protected]
1
Laboratoire d’Astrophysique de Marseille, Technopôle de Marseille-Etoile, Aix-Marseille Université, 38, rue Frédéric Joliot-Curie, 13388, Marseille cedex 13, France
2
Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
3
IRAP, Université de Toulouse, CNRS, UPS, 31400 Toulouse, France
4
German Aerospace Center, Institute of Planetary Research, Berlin, Germany
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now routinely detected, provide us with an enlarged sample of planets to better understand the planetary formation process. In this paper, we review our current knowledge of exoplanets that could be seen as Neptune-class planets, their similarities and dissemblance with our ice giants, and the observational constraints they bring to current formation models. We emphasize that this class of planets does not correspond to a properly defined planet family, but because we aim at placing our ice giants in a more general context, we focus this review on exoplanets whose radius or mass are in the same range of values as those of our giants. Among the few thousands of exoplanets discovered to date, nearly three quarters have indeed broad characteristics comparable to those of Uranus and Neptune, i.e. radii in the range ∼1
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