Habitable zone Earth: 0.95-1.15 AU F. Marzari, Dept. Physics, Padova Univ. Earth: 0.95-1.15 AU

HABITABLE ZONE: DEFINITION A habitable zone for a given star is defined by the range of distances around a central star at which Earth-like planets maintain conditions sufficient for the existence of life (carbon based?) at the surface. A habitable planet is one on which liquid water is stable at the surface (in between 273-373 K).

Outgassing from Earth interior (vulcanic activity) DELIVERY OF WATER AND OTHER VOLATILES TO THE EARTH Main volatiles: H2O, CO2, N2, HCl Outgassing from Earth interior (vulcanic activity) Impacts of water-rich bodies: scattered by outer planets

Jupiter-Saturn planetesimals TNO region Total mass of water on Earth: Planetesimals in the terrestrial zone: less than 0.05% of water. This could also be lost during the accretion phase due to collisional heating Planetesimals from the outer regions of the solar system rich in ices: possible radial mixing 3 possible sources Outer asteroid belt Jupiter-Saturn planetesimals TNO region

1) Outer asteroid belt Formation of planetary embryos – Stirring due to mutual perturbations and Jupiter – Compositional mixing and enrichment of water with impacts of outer embryos (Chambers, Raymond etc...) - asteroid belt depletion

2) Planetesimals from the Jupiter-Saturn region. About 50-100 ME of planetesimals in this region. Estimates assume that 2-20% of water could have been supplied. This assuming that the Earth had its present size, but when Jupiter (and Saturn) were fully formed probably the Earth was small and still losing water by accretional heating.

3) Planetesimals (cometesimals..) from the TNO regions In the Nice model cometesimals are scattered inside by encounters with Neptune and Uranus which migrate outwards. However, estimates of total mass delivered by cometesimals is about 2.5 x 10-5 ME Too low, only about 10% of ocean water mass.

All scenarios require Jupiter All scenarios require Jupiter! If terrestrial planets form in systems where Jupiter is not around 5 AU can they be enriched with water like the Earth? Most extrasolar planets on eccentric orbits: habitable planet needs circular orbit.

Te = 255 +33 = 288 K Temperature and feedbacks Greenhouse effects: IR emission is absorbed by greenhouse gasses (CO2, H2O, CH4 ...) Te = 255 +33 = 288 K

Climate feedbacks If T decreases, saturation pressure drops, less H2O in atmosphere, less greenhouse effect, farther decrease of T: positive feedback If T increases, the Earth emits more IR radiation and cools down: negative feedback Carbonate-silicate cycle: CO2 + H2O → H2CO3 (carbonic acid) which dissolves silicate rocks and goes to the bottom of ocean where it is reprocessed and emitted by vulcanos as CO2. If T grows CO2 precipitation increases (while vulcanism is constant) and CO2 concentration falls.

The sun luminosity changes with time, it was cooler in the past: higher concentration of greenhouse gases.