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Habitable Zone Dramatically Bigger Around Red Dwarfs

The zone that supports liquid water may be 30 per cent bigger than previously thought, say astrobiologists

In looking for Earth-like planets around other stars, astrobiologists search for planets that can support liquid water. So these planets must have a temperature in the relatively narrow range that exists on Earth. The general thinking is that these conditions can only exist at a certain distance from the star–the so-called habitable zone.

In our Solar System, the habitable zone stretches from about 0.7 to 3 AU, approximately from the orbit of Venus to about twice the orbit of Mars.

But determining the size of a star’s habitable zone is not straightforward. Obviously, the size and temperature of the star are crucial but much depends on conditions on the exoplanet itself, in particular how much light is reflected back into space, the albedo.

Today, Manoj Joshi at the National Centre for Atmospheric Science in Reading, UK, and Robert Haberle at the NASA Ames Research Centre near San Francisco point out an important new factor that dramatically extends the habitable zone around an important class of stars.

These guys say that the amount of light that snow and ice reflects depends on the fraction emitted at different wavelengths. The Sun produces much of its light at visible wavelengths. The albedo at these wavelengths for snow and ice is 0.8 and 05 respectively.

But the vast majority of stars are red dwarfs and these emit far more of their light at longer wavelengths. “The albedos of ice and snow on planets orbiting M-stars are much lower than their values on Earth,” say Joshi and Haberle.

These guys calculate the albedo for snow and ice on planets orbiting two nearby red dwarfs–Gliese 436, just 33 light years from here, and GJ 1214 some 40 light years away. Both are known to have exoplanets, although not in the habitable zone. The wavelengths that these stars emit mean that snow and ice here have albedos of about 0.4 and 0.1 respectively.

In other words, water-bearing planets orbiting these stars ought to absorb far more energy than Earth. Therefore, this extends the radius of the potential habitable zone by as much as 30 per cent. “The outer edge of the habitable zone around M-stars may be 10-30% further away from the parent star than previously thought,” they say.

That will be of more than passing interest to astrobiologists. Not only are red dwarfs by far the most common type of star, they are also the most likely to provide us with our first view of Earth 2.0 (if we haven’t seen it already). That’s because they are smaller, which makes it easer to see planets orbiting close to them.

Having an extended zone makes it just that little more likely than we’ll find another Earth sooner rather than later.

Ref: arxiv.org/abs/1110.4525: Suppression Of The Water Ice And Snow Albedo Feedback On Planets Orbiting Red Dwarf Stars And The Subsequent Widening Of The Habitable Zone

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