Photo: Wikipedia, CC
Goldilocks Conditions - Not too Warm, Not Too Cold
There's a lot that we've learned about the Earth by studying other planets (for example, some of what we know about global warming was learned by analyzing the atmosphere of Venus), but until recently, we only had the planets of the solar system to look at. That has changed thanks to better instruments that allowed astronomers to discover distant planets orbiting faraway stars (aka exoplanets). One of the most promising so far is Gliese 581d, an exoplanet about twice as big as Earth that was at first thought to be too cold to allow life as we know it to exist, but a new study reveals that its atmosphere may keep it warm enough for liquid water.
Photo: Wikipedia, CC
Christine has written a good post about Gliese 581d last year. She wrote:
European scientists announced yesterday that they have determined the plant Gliese 581d may be habitable. At a minimum, its orbit lies within the "habitable zone," a band at a distance from the planet's sun in which the planet is warm enough to support life, but not too warm. In fact, scientists on the team that made the discovery went so far as to speculate that the planet may have a "large and deep" ocean.
But this was based only on the orbit of the exoplanet. Now a new study of its atmosphere in the Astrophysical Journal Letters says that its atmosphere is probably also in the 'Goldilocks' zone:
To their surprise, they found that with a dense carbon dioxide atmosphere -- a likely scenario on such a large planet -- the climate of Gliese 581d is not only stable against collapse, but warm enough to have oceans, clouds and rainfall. One of the key factors in their results was Rayleigh scattering, the phenomenon that makes the sky blue on Earth. In the Solar System, Rayleigh scattering limits the amount of sunlight a thick atmosphere can absorb, because a large portion of the scattered blue light is immediately reflected back to space. However, as the starlight from Gliese 581 is red, it is almost unaffected. This means that it can penetrate much deeper into the atmosphere, where it heats the planet effectively due to the greenhouse effect of the CO2 atmosphere, combined with that of the carbon dioxide ice clouds predicted to form at high altitudes. Furthermore, the 3D circulation simulations showed that the daylight heating was efficiently redistributed across the planet by the atmosphere, preventing atmospheric collapse on the night side or at the poles.
An important implication of this discovery is that potential life-hosting planets might not need to be all that similar to Earth in size and orbit. Now of course this doesn't mean that there's life on Gliese 581d or that we'll go there any time soon, but this exoplanet can help us learn about the universe and about our planet.
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