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Welcome to the habitable zone

9 May 2007 by Karen

Planetarium operator John Moran explains all the fuss over the latest planetary discovery.

a space image of a red planet with a red sun in the distance

An artist’s impression of an exoplanet. Image couresy of NASA.

After discovering some 200 or so planets orbiting distant stars, scientists have finally found what looks like an Earth type planet. Gliese 581c is the smallest “Extrasolar planet” ever discovered. Most Exoplanets discovered so far have been many times the size of Earth and more resemble gas giants like Jupiter. But the detection of a world so close in size to the Earth, has got the scientific community very excited.
But even more significant is the planets location around its parent star: Gliese 581c orbits around its Sun in a narrow band of space known as the “Habitable zone”, defined as the region around a star where liquid water is stable. And as we all know, where there is water, there is a good chance that some kind of life may have formed.
Over the last few years, scientists have discovered hundreds of Exoplanets. How they do this is by studying a star’s spectrum and looking for periodic shifts. These are tell-tale signs that the star is wobbling – rocking to and fro because of the gravitational pull of an orbiting planet. This wobble was detected in a small and quite dim star called Gliese 581, which is in our galactic  neighbourhood, 20.5 light years away. This wobble was the sign that it had a companion, and this companion turned out to be a Neptune sized planet orbiting very close to the star and completing its course every 5.4 days. But they noticed that the wobble did not fit the usual pattern, something  else seemed to be tugging at the star. As it turned out there where two more low mass planets  orbiting Gliese 581 and it is the middle of the three that has caught the attention. Not only is it the lowest mass planet ever found, but the distance from its Sun means it may  have a surface temperature of around 0 to 40 degrees Celsius and consequently water would be liquefied not frozen.
The fact that a Red dwarf is of much lower mass than our Sun makes the detection of small rocky planets that much easier. All in all, both the low luminosity and the low mass of Red dwarfs favour the detection of Earth-like planets around such stars, and with Red dwarfs being the most abundant stars in the Galaxy, the possibility for similar discoveries is very real. 
John Moran

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