Greenhouse effect may give exoplanet liquid water

Ars Technia : The red dwarf Gliese 581, only 20 light years from Earth, is host to at least five planets, three of which have been considered potentially habitable. The latest addition to this list is GJ581d, writes John Trimmer for Ars Technica. The planet is a super-Earth that was previously thought to support neither liquid water nor enough of an atmosphere to produce a significant greenhouse effect. Although GJ581d lies at the far edge of the star’s habitable zone, it’s still close enough for tidal locking to ensure that only one side of the planet ever faces the star. The planet’s poles and far side are therefore extremely cold; below a certain temperature an atmosphere collapses as its constituents freeze out. Robin Wordsworth of the Institut Pierre Simon Laplace in Paris and colleagues have developed a general circulation model (GCM) that can be applied to a range of planets. In the case of a planet like GJ581d, the team found that denser atmospheres lead to horizontal heat transport and a greenhouse effect strong enough to keep the atmosphere intact and allow for surface temperatures above the melting point of water. Although it’s not known whether GJ581d has a dense atmosphere, Wordsworth and his team have devised a test. Using their model, the team synthesized emission spectra that could be compared to real spectra once they become available.