X-ray induced mass loss effects on exoplanets orbiting dM stars

INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy e-mail: [tpenz;giusi]@astropa.inaf.it

Received: 18 October 2007
Accepted: 7 November 2007

Abstract

Aims.We study the influence of the X-ray luminosity distribution of dM stars on mass loss from planets on close-in orbits.

Methods.Using the X-ray luminosity of the Pleiades, the Hyades, and field dM stars, we construct a scaling law for the radiation environment of dM stars for ages between 0.1 and 10 Gyr. An energy-limited escape approach is used to calculate the influence of thermal mass loss on planetary distribution functions.

Results.We show that the X-ray luminosity distribution of nearby dM stars can be described by using a scaling law derived from observations of open clusters with a given age. It is shown that the X-ray flux from dM stars is significantly less than the flux from dG stars for a given orbital distance. Therefore, loss processes have less of an impact on the mass evolution of planets orbiting dM stars. We found that the mass loss is negligible for hydrogen-rich Jupiter-mass planets at orbits >0.02 AU, while Neptune-mass planets are influenced up to 0.05 AU. At orbits of 0.02 AU, Roche lobe effects are also having a strong impact on the mass-loss evolution. Because of the low mass of dM stars, Roche lobe effects are less effective for loss processes at planets orbiting these stars. Finally, if we use only the X-ray luminosity of their host stars for the energy input to the atmosphere, we obtain a lower limit for the mass loss of GJ876d and GJ674b. This does not allow us to conclude whether they are remnants of eroded Jupiter-mass planets.