Nevertheless, understanding Uranus and Neptune is an important step towards understanding a frequently occurring class of exoplanets. Due to selection effects in exoplanet surveys, however, these numbers refer to the detected planets, while the absolute occurrence rates at large orbital distances (>100 days) are not known yet.
As of June 2019, out of the 3972 confirmed and categorised exoplanets 1, more than 30% are in a size range similar to Neptune and about 30% are intermediate in size between Earth and Neptune. It is important to gain better insight into the thermal structure of Uranus and Neptune and their heat fluxes because the thermal structure largely influences the inferred composition ( Podolak et al. More recent models, using a finite-temperature EOS for water but currently outdated ones for hydrogen and helium, could reproduce Neptune’s brightness but still obtained too high luminosity for Uranus ( Fortney et al. Uranus’ intrinsic flux is consistent with being zero, meaning that it could be in thermal equilibrium with the Sun, while Neptune is clearly still cooling.Įarlier adiabatic cooling calculations using a zero-temp- erature equation of state (EOS) for the ice material have found that both planets are cooling too slowly to explain their present-day luminosity ( Hubbard 1978 Hubbard & McFarlane 1980 Hubbard et al. Neptune’s intrinsic heat flux of F int, N = 0.433 ± 0.046 Wm −2 is about an order of magnitude higher than that of Uranus ( ) ( Guillot & Gautier 2015). Because of their similar characteristics, which are different from the larger primarily hydrogen- and helium-based gas giants Jupiter and Saturn and the smaller rock-based inner planets, they are usually classified in their own category as ice giants. These observations suggest similar structural and evolutionary paths since their time of formation 4.56 × 10 9 yr ago. Uranus and Neptune, the two outermost planets of our solar system, share a large number of very similar observed values such as mean density, surface temperature, atmospheric composition, and magnetic field morphology ( Guillot & Gautier 2015). Key words: planets and satellites: physical evolution / planets and satellites: interiors / planets and satellites: individual: Uranus / planets and satellites: individual: Neptune Our results suggest that in contrast to common assumptions, neither planet is fully adiabatic in the deeper interior. We also find that uncertainties on input parameters such as the level of irradiation matter generally more for Uranus than for Neptune. Our cooling times of about τ U = 5.1 × 10 9 yr for Uranus and τ N = 3.7 × 10 9 yr for Neptune bracket the known age of the planets of 4.56 × 10 9 yr implying that neither planet’s present-day luminosity can be explained by adiabatic cooling. We investigate the influence of albedo, solar energy influx, and equations of state of H and He, and water on the cooling time. For this purpose, we have developed a new planetary model and evolution code. Here we apply revised equation of state data of hydrogen, helium, and water and compute the thermal evolution of Uranus and Neptune assuming an adiabatic interior. The brightness of Neptune is often found to be in accordance with an adiabatic interior, while the low luminosity of Uranus challenges this assumption. Institut für Physik, Universität Rostock,Į-mail: für Planetenforschung, Deutsches Zentrum für Luft- und Raumfahrt, Ludwig Scheibe 1, Nadine Nettelmann 1 ,2 and Ronald Redmer 1 Astronomical objects: linking to databases.
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