A diagram to determine the evaporation status of extrasolar planets

Institut d'Astrophysique de Paris, CNRS, UMR7095: Université Pierre et Marie Curie-Paris6, 98bis boulevard Arago, 75014 Paris, France e-mail: lecaveli@iap.fr

Received: 13 February 2006
Accepted: 26 September 2006

Abstract

Aims.To describe the evaporation status of extrasolar planets, we consider an energy diagram in which the potential energy of the planets is plotted versus the energy received by the upper atmosphere.

Methods.Here we present a basic method to estimate these quantities. For the potential energy, we include the modification of the gravity field by the tidal forces from the parent stars.

Results.This description allows a rapid estimate of both the escape rate of the atmospheric gas and the lifetime of a planet against the evaporation process. In the energy diagram, we find an evaporation-forbidden region in which a gaseous planet would evaporate in less than 5 billion years. With their observed characteristics, all extrasolar planets are found outside this evaporation-forbidden region. The escape rates are estimated to be in the range 10⁵ g s^-1 to 10¹² g s^-1, with a few cases above 10¹¹ g s^-1. The estimated escape rate for HD 209458 b is consistent with the lower limit of 10¹⁰ g s^-1 obtained from interpretation of the H i Lyman-α observations.
This diagram suggests possibilities for the nature of the recently discovered Neptune-mass planets. We find that GJ 436 b, 55 Cnc e and HD 69830 b cannot be low mass gaseous planets. With a density that must be above 0.5 g cm^-3 to survive evaporation, these planets must contain a large fraction of solid/liquid material. We find that GJ 876 d must have a density greater than ~3 g cm^-3 to survive the strong EUV energy flux from its nearby parent star. GJ 876 d must contain a large fraction of massive elements.