Volume 585, January 2016
|Number of page(s)||5|
|Published online||18 December 2015|
Energy-limited escape revised
The transition from strong planetary winds to stable thermospheres
Hamburger Sternwarte, Universität Hamburg,
2 European Space Research and Technology Centre (ESA/ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
Received: 23 July 2015
Accepted: 10 November 2015
Gas planets in close proximity to their host stars experience photoevaporative mass loss. The energy-limited escape concept is generally used to derive estimates for the planetary mass-loss rates. Our photoionization hydrodynamics simulations of the thermospheres of hot gas planets show that the energy-limited escape concept is valid only for planets with a gravitational potential lower than log 10(−ΦG)< 13.11 erg g-1 because in these planets the radiative energy input is efficiently used to drive the planetary wind. Massive and compact planets with log 10(−ΦG) ≳ 13.6 erg g-1 exhibit more tightly bound atmospheres in which the complete radiative energy input is re-emitted through hydrogen Lyα and free-free emission. These planets therefore host hydrodynamically stable thermospheres. Between these two extremes the strength of the planetary winds rapidly declines as a result of a decreasing heating efficiency. Small planets undergo enhanced evaporation because they host expanded atmospheres that expose a larger surface to the stellar irradiation. We present scaling laws for the heating efficiency and the expansion radius that depend on the gravitational potential and irradiation level of the planet. The resulting revised energy-limited escape concept can be used to derive estimates for the mass-loss rates of super-Earth-sized planets as well as massive hot Jupiters with hydrogen-dominated atmospheres.
Key words: methods: numerical / hydrodynamics / radiation mechanisms: general / planets and satellites: atmospheres / planets and satellites: dynamical evolution and stability
© ESO, 2015
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