Modeling Stellar Proton Event-induced particle radiation dose on close-in exoplanets

Modeling Stellar Proton Event-induced particle radiation dose on close-in exoplanets

Authors:

Atri et al

Abstract:

Kepler observations have uncovered the existence of a large number of close-in exoplanets and serendipitously of stellar superflares with emissions several orders of magnitude higher than those observed on the Sun. The interaction between the two and its implications on planetary habitability is of great interest to the community. Stellar Proton Events interact with the planetary atmosphere, generate secondary particles and increase the radiation dose on the surface. This effect is amplified for close-in exoplanets and can be a serious threat to potential planetary life. Using Monte Carlo simulations, we model the SPE-induced particle radiation dose on the surface of such exoplanets. We study the dependence of radiation dose on flare energy, planet's orbital distance, magnetic field strength and atmospheric column density, and discuss its implications on constraining planetary habitability.

Habitability of planets on eccentric orbits: the limits of the mean flux approximation

Habitability of planets on eccentric orbits: the limits of the mean flux approximation

Authors:

Bolmont et al

Abstract:

Contrary to Earth, which has a small orbital eccentricity, some exoplanets discovered in the insolation habitable zone (HZ) have high orbital eccentricities (e.g., up to an eccentricity of ∼0.97 for HD~20782~b). This raises the question of the capacity of these planets to host surface liquid water. In order to assess the habitability of an eccentric planet, the mean flux approximation is often used. It states that a planet on an eccentric orbit is called habitable if it receives on average a flux compatible with the presence of surface liquid water. However, as the planets do experience important insolation variations over one orbit and even spend some time outside the HZ for high eccentricities, the question of their habitability might not be as straightforward. We performed a set of simulations using the Global Climate Model LMDz, exploring the limits of the mean flux approximation when varying the luminosity of the host star and the eccentricity of the planet. We computed the climate of tidally locked ocean covered planets with orbital eccentricity from 0 to 0.9 receiving a mean flux equal to Earth's, around stars of luminosity ranging from L⊙ to 10−4 L⊙. Using here a definition of habitability based on the presence of surface liquid water, we find that most of the planets considered can sustain surface liquid water on the dayside with an ice cap on the nightside. However, for high eccentricity and high luminosity, planets cannot sustain surface liquid water during the whole orbital period. They completely freeze at apoastron and when approaching periastron an ocean appears around the substellar point. We conclude that the higher the eccentricity and the higher the luminosity of the star, the less reliable the mean flux approximation.

Introducing the Life Supporting Zone Concept as an Alternative to the Habitable Zone

Effective stellar flux calculations for limits of Life-supporting zones of Exoplanets

Authors:

Ludwig et al

Abstract:

Habitable zones (HZ) are key concepts in the quest for finding extrasolar planets that may host life as we know it. HZs encompass regions around a star that would allow for liquid water to be present on the surface of a rocky planet. However, water may not be the only solvent capable of producing and sustaining biospheres (e.g. Schulze-Makuch & Irwin 2006), so the concept of life-supporting zones (LSZ) was introduced as a generalization of the classical HZ for a broader range of solvents (Leitner et al. 2010) . The aim of this work is to offer a straightforward means of calculating LSZs similar to those presented by Kopparapu et al. (2014) for the HZ. We used a 1D radiative convective model to determine LSZ limits for water/ammonia mixtures and sulfuric acid. A simplified cloud model was used for offline sulfuric acid cloud simulation. Water clouds were accounted for by variations of surface albedo values. Compared to recently updated results by Kopparapu et al. (2014), our results lie well within the uncertainty range of the Toon algorithm (Toon et al. 1989) for flux calculations. We found an inner limit of the LSZ closer and an outer limit further away from the star than the limits for the HZ would be. Recently discovered exoplanets (like Kepler 452-b) are shown to be positioned very well in the LSZ. The concept of LSZs adds additional perspectives to an exoplanet's ability to maintain life on its surface.

How Strong is the Extreme Ultraviolet Light on the TRAPPiST-1 Worlds

Strong XUV irradiation of the Earth-sized exoplanets orbiting the ultracool dwarf TRAPPIST-1

Authors:

Wheatley et al

Abstract:

We present an XMM-Newton X-ray observation of TRAPPIST-1, which is an ultracool dwarf star recently discovered to host three transiting and temperate Earth-sized planets. We find the star is a relatively strong and variable coronal X-ray source with an X-ray luminosity similar to that of the quiet Sun, despite its much lower bolometric luminosity. We find L_x/L_bol=2-4x10^-4, with the total XUV emission in the range L_xuv/L_bol=6-9x10^-4. Using a simple energy-limited model we show that the relatively close-in Earth-sized planets, which span the classical habitable zone of the star, are subject to sufficient X-ray and EUV irradiation to significantly alter their primary and perhaps secondary atmospheres. Understanding whether this high-energy irradiation makes the planets more or less habitable is a complex question, but our measured fluxes will be an important input to the necessary models of atmospheric evolution.