Mass-loss rates for transiting exoplanets

Abstract

Exoplanets at small orbital distances from their host stars are submitted to intense levels of energetic radiations, X-rays and extreme ultraviolet (EUV). Depending on the masses and densities of the planets and on the atmospheric heating efficiencies, the stellar energetic inputs can lead to atmospheric mass loss. These evaporation processes are observable in the ultraviolet during planetary transits. The aim of the present work is to quantify the mass-loss rates (ṁ), heating efficiencies (η), and lifetimes for the whole sample of transiting exoplanets, now including hot jupiters, hot neptunes, and hot super-earths. The mass-loss rates and lifetimes are estimated from an "energy diagram" for exoplanets, which compares the planet gravitational potential energy to the stellar X/EUV energy deposited in the atmosphere. We estimate the mass-loss rates of all detected transiting planets to be within 10 6 to 10 13 g s −1 for various conservative assumptions. High heating efficiencies would imply that hot exoplanets such the gas giants WASP-12b and WASP-17b could be completely evaporated within 1 Gyr. We further show that the heating efficiency can be constrained whenṁ is inferred from observations and the stellar X/EUV luminosity is known. This leads us to suggest that η ∼ 100% in the atmosphere of the hot jupiter HD 209458b, while it could be lower for HD 189733b. Simultaneous observations of transits in the ultraviolet and X-rays are necessary to further constrain the exospheric properties of exoplanets.

Heating efficiency in hydrogen-dominated upper atmospheres

“…Based on the idea of Lecavelier des Etangs (2007), who produced an energy diagram by comparing the stellar XUV energy received by the upper atmospheres to the gravitational energies of exoplanets, but by introducing the heating efficiency η hν as in Lammer et al (2009) However, as shown in our detailed study, the XUV-related η hν value should be within 10-20% and will therefore not reach the high values necessary to reproduce the observation-based mass-loss rates for HD 209458b with the hypothesis presented in Ehrenreich & Désert (2011). The most likely reason why the hypothesis of Ehrenreich & Désert (2011) will not give accurate η hν values is related to the fact that similarly to Lecavelier des Etangs (2007) or Lammer et al (2009), they assumed that the effective XUV absorption radius R XUV lies close to R p , the radius used in the energy-limited mass-loss formula for all of the studied exoplanets. This assumption is more or less valid for massive and compact exoplanets (Erkaev et al 2007), such as HD 189733b with an average density ρ ∼ 0.95 g cm −3 , but will yield less accurate mass-loss rates for less compact objects with lower average densities, such as HD 209458b with 0.37 g cm −3 .…”

“…R XUV can exceed the planetary radius R p quite substantially for a planetary body with a low average density when its atmosphere is exposed to high XUV fluxes. Depending on the distribution of the XUV volume-heating rate and the related density profile of the upper atmosphere, the mass-loss rate can then be higher, as estimated with the assumptions in Ehrenreich & Désert (2011).…”

Effects of M dwarf magnetic fields on potentially habitable planets

“…The Earth also does not have a significant atmospheric escape ( a few kg/s, Fahr & Szizgal 1983). Mass loss from planets can be significant, however, especially for close-in gas giant planets orbiting solar-type stars (or hotter), as a consequence of high stellar irradiation (Lecavelier des Etangs 2007;Murray-Clay et al 2009;Ehrenreich & Désert 2011;Owen & Jackson 2012). Here, we assume that ram pressure due to atmospheric escape of terrestrial planets (with limited gas reservoir) orbiting in the HZ of dM stars is negligible.…”

Mass of WASP-33b

“…Another interesting aspect is that A-stars are much brighter than solar-like stars and exhibit stronger winds (Babel 1995;Simon et al 2002). The investigation of close-in planets also constrains models of the evaporation and inflation of planets (Ehrenreich & Désert 2011), and they are important for testing the mass-radius relation of the exoplanets (e.g. Batygin et al 2011;Enoch et al 2012).…”