Surface Temperatures of Exoplanets

Abstract:

In the search for habitable exoplanets, the planet’s surface temperature plays a crucial role. Unfortunately, direct measurements of surface temperature are not available at this time. Many physical processes influence the surface temperature distribution of a planet. However, the dominating influence is an energy balance between the stellar radiation input and the radiative surface loss of heat. With the further assumptions of a uniform planetary surface temperature, no filtering of the incoming radiation, and black body emission, the only variables are the stellar luminosity and the radial distance of the exoplanet from the star. For the solar system, agreement with observations is quite good except for Venus. The agreement is good for both the inner planets and the outer planets.

In this paper we systematically look at methods of improving the zero order approach given above.

1. We consider the filtering of the incoming radiation and the grey body emission. This accounts for the greenhouse effect and can explain the surface temperature of Venus. We systematically vary the filtering of incoming radiation and the emissivities of the daytime and nighttime surfaces. There is evidence that greenhouse heating on the Earth is primarily at nighttime. Different emissivities can explain this effect.
2. It is straightforward to extend the energy balance analysis to include the latitude dependence of surface temperature. Good agreement is obtained at low latitudes but temperature buffering and heat transport by the oceans and atmosphere are clearly important at high latitudes.
3. It is also straightforward to estimate the difference between the daytime and nighttime temperatures. The important parameter is the rotation rate of the exoplanet. The roles of the oceans and the atmosphere in moderating this difference on the Earth will be discussed.
4. Some exoplanets are sufficiently close to their star to have temperatures above the melting temperatures and even the vaporization temperatures of silicate rocks. The roles of a global magma ocean and rates of vaporization will be discussed.