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Pathways to Habitability: from disks to active stars, planets and life Colin P. Johnstone PATHWAYS TO HABITABILITY PROJECT What are the astrophysical conditions required for the formation of habitable planets? Department of Astrophysics, University of Vienna - circumstellar disks - planet formation - water transport - stars: magnetic activity - binary star systems Institut für Weltraumforschung, Graz - planetary atmospheres - planetary magnetospheres THE CLASSICAL HABITABILE ZONE The classical definition of planetary habitability is based on the planet's 'equilibrium temperature' STELLAR RADIATION PLANET THERMAL RADIATION Input energy from star: Planet thermal blackbody radiation: Equating input and output energies gives: For the Earth: Teq = 250 K... ...but our surface has a temperature of ~270 K ! For the Earth: Teq = 250 K... ...but our surface has a temperature of ~270 K ! GREENHOUSE EFFECT HEATS SURFACE ABOVE EQUILIBRIUM TEMPERATURE CLASSICAL HABITABILE ZONE STELLAR MASS DEPENDENCE STELLAR AGE DEPENDENCE For the last ~4.5 Gyr, the Sun has been getting bigger and more luminous Solar Mass Half Solar Mass NOW: everything fine In a few Gyr: oceans evaporated A few Gyr ago: oceans frozen THE FAINT YOUNG SUN PARADOX In the past, the Sun was too faint During the initial disk phase, the first stages of planet formation take place Planets formed in the gas disk gain large atmospheres made of hydrogen During the initial disk phase, the first stages of planet formation take place When disk is gone, a distribution of solid bodies remains - how do these bodies form planets? - where is the water? - how is the water transported? TIME During collision, water can be transported between bodies and lost into space red = not water blue = water Without a gas disk, planets are exposed to the radiation of their stars l a c i ity s l s i a l b C ita b a h 60 00 K 50 , 00 0K 1M K 2M K X-ray and extreme ultraviolet irradiation of an atmosphere causes heating and expansion STELLAR RADIATIVE EVOLUTION VISIBLE WAVELENGTHS X-RAY + EUV STELLAR RADIATIVE EVOLUTION VISIBLE WAVELENGTHS X-RAY + EUV SECONDARY ATMOSPHERES As the liquid surfaces of young planets solidify, they release gases forming secondary atmospheres For secondary atmospheres, X-ray and EUV heating casuses little mass loss WE NEED WINDS NON-THERMAL MASS LOSS Interactions with the stellar wind cause erosion of atmosphere red = ions blue = neurals Early Sun Early Earth Stronger XUV Stronger Wind Larger atmosphere Smaller magnetosphere r Ea th s mo At ere h p s o gne t a M h Eart e ph re