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February 24, 2013

Exoplanet Habitability adjusted for atmospheric pressure and seasonality

A new Energy Balance Model (EBM) provides more insight into the habitability of extrasolar planets. It also has a seasonal model of planetary climate, with new prescriptions for most physical quantities. Researchers use the EBM to investigate the surface habitability of planets with an Earth-like atmospheric composition but diff erent levels of surface pressure. The habitability, defi ned as the mean fraction of the planet's surface on which liquid
water could exist, is estimated from the pressure-dependent liquid water temperature range, taking into account seasonal and latitudinal variations of surface temperature. By running several thousands of EBM simulations they generated a map of the habitable zone (HZ) in the plane of the orbital semi-major axis, a, and surface pressure, p, for planets in circular orbits around a Sun-like star.

As pressure increases, the HZ becomes broader, with an increase of 0.25 AU in its radial extent from p=1/3 bar to p=3 bar. At low pressure, the habitability is low and varies with a; at high pressure, the habitability is high and relatively constant inside the HZ. We interpret these results in terms of the pressure dependence of the greenhouse e ffect, the effi ciency of horizontal heat transport, and the extent of the liquid water temperature range. Within the limits discussed in the paper, the results can be extended to planets in eccentric orbits around non-solar type stars. The main characteristics of the pressure-dependent HZ are modestly aff ected by variations of planetary properties, particularly at high pressure.

Circumstellar habitable zone of planets with Earth-like atmospheres and di fferent levels of surface pressure obtained with our EBM climate simulations. Abscissae: semi-major axis, a (bottom axis), or insolation (top axis). Ordinates: logarithm of the total surface pressure, p. The circles indicate solutions with mean global annual habitability h over 0. The area of the circles is proportional to h; the colors are coded according to the mean annual global surface temperature, Tm. The size and color scales are shown in the legend. The solid lines are contours of equal mean temperature Tm = 273 K (magenta), 333 K (red) and 393 K (black). Results above the contour at Tm = 333 K (red line) are tentative; see Section 3.3. Red crosses: simulations stopped on the basis of the water loss limit criterion; blue crosses: simulations interrupted when Tm less then Tmin;





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