Volume 598, February 2017
|Number of page(s)||9|
|Section||Planets and planetary systems|
|Published online||07 February 2017|
Aeronomical constraints to the minimum mass and maximum radius of hot low-mass planets
1 Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
2 Federal Research Center “Krasnoyarsk Science Center” SB RAS, “Institute of Computational Modelling”, Krasnoyarsk 36, Russian Federation
3 Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
4 Institut für Geophysik, Astrophysik und Meteorologie, Karl-Franzens-Universität, Universitätsplatz 5, 8010 Graz, Austria
Received: 14 September 2016
Accepted: 15 December 2016
Stimulated by the discovery of a number of close-in low-density planets, we generalise the Jeans escape parameter taking hydrodynamic and Roche lobe effects into account. We furthermore define Λ as the value of the Jeans escape parameter calculated at the observed planetary radius and mass for the planet’s equilibrium temperature and considering atomic hydrogen, independently of the atmospheric temperature profile. We consider 5 and 10 M⊕ planets with an equilibrium temperature of 500 and 1000 K, orbiting early G-, K-, and M-type stars. Assuming a clear atmosphere and by comparing escape rates obtained from the energy-limited formula, which only accounts for the heating induced by the absorption of the high-energy stellar radiation, and from a hydrodynamic atmosphere code, which also accounts for the bolometric heating, we find that planets whose Λ is smaller than 15–35 lie in the “boil-off” regime, where the escape is driven by the atmospheric thermal energy and low planetary gravity. We find that the atmosphere of hot (i.e. Teq ⪆ 1000 K) low-mass (Mpl ⪅ 5 M⊕) planets with Λ< 15–35 shrinks to smaller radii so that their Λ evolves to values higher than 15–35, hence out of the boil-off regime, in less than ≈500 Myr. Because of their small Roche lobe radius, we find the same result also for hot (i.e. Teq⪆ 1000 K) higher mass (Mpl ⪅ 10 M⊕) planets with Λ< 15–35, when they orbit M-dwarfs. For old, hydrogen-dominated planets in this range of parameters, Λ should therefore be ≥15–35, which provides a strong constraint on the planetary minimum mass and maximum radius and can be used to predict the presence of aerosols and/or constrain planetary masses, for example.
Key words: planets and satellites: atmospheres / planets and satellites: fundamental parameters / planets and satellites: gaseous planets
© ESO, 2017
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