Servizio Comunitario di Informazione in materia di Ricerca e Sviluppo - CORDIS

Final Activity Report Summary - ISHERPA (The Influence of Stellar High Energy Radiation on Planetary Atmospheres)

The goal of the project was the investigation of the effects of the stellar high energy radiation, mainly X-rays, on the formation and evolution of the planetary atmospheres. Since the high energy radiation evolves with the stellar age, it has been necessary to determine its intensity and properties as a function of stellar age. In particular we have determined the X-ray luminosity and temperature of emission from very young stars, when the planets are just forming from the circumstellar material. This emission is very variable, with a large number of flares, powerful and energetic explosions that may strongly affect and destroy the planetary atmospheres that are forming at these young ages.

We have investigated different star formation regions of comparable age that result to share similar properties. We found that, while most of the massive (Jovian) planets may survive, even if very close to the host star, a large fraction of smaller planets at small distances from the star are destroyed or loose a large fraction of their mass leaving only small cores, without any atmospheres. These effects decrease if the planets form at larger distances from the star, even if some influence is always present. Indeed up to several Astronomical Units (and at the Earth distance) it is possible that the atmosphere is heated by the X-ray stellar emission, changing the conditions for life origin and development (habitability conditions). These effects are particularly relevant at young stellar ages, much younger than the age of our Sun and may have had an important role on the onset of life in our planet.

The high energy effects are more destructive for planets orbiting solar-like stars, while are less effective in destroying planets around smaller, less massive stars, although the latter may have brighter energetic flares, therefore the survival capability and the possible habitability of a given planet strongly depends on the mass, activity and evolutive history of the host star. Since the high energy radiation effects are relevant mainly at young ages, it is very important to estimate how many young stars there are in the solar neighbourhood. Indeed all the present and planned observational programs for the search of extrasolar planets are based on surveys of nearby stars. In order to interpret correctly the results of such surveys is necessary to understand very well the properties of the observed stars, in particular their mass and age. While the stellar mass determination is relatively easy, the age measurement is much more difficult.

We have adopted a new approach that does not determine the age of individual stars, but estimate in a statistical way the fraction of young stars close to the Sun (less than 1000 light years), reconstructing the recent star formation history in the solar neighbourhood. We use statistical age indicators as those obtained by unbiased X-ray surveys or rotation measurements of unbiased stellar samples (i.e. from the French-ESA satellite CoRoT). The results are very intriguing since we found a burst in star formation rate by a factor of 3-4 that occurred less than one billion of years ago producing a significant excess of young stars close to the Sun. The observations of planets around such young stars will be very important to understand the planet formation process. In fact they host primordial planets that may be destroyed or partially evaporated at older age, preventing us to observe them when stars reach an age similar to that of the Sun.

Finally we have estimated the very high energy emission from young stars that cannot be observed with present day instruments and we found that next generation observatories (i.e. the Italian-French satellite Simbol-X or the ESA project Xeus/IXO) can be able to probe this important emission that may very deeply penetrate in the circumstellar material, both in the circumstellar disk and in the planetary atmosphere, significantly modifying their evolution, chemistry and physical conditions.

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