Thursday, September 30, 2010

Gliese 581 Planetary System: Possible Habitable Planet

"A Habitable Exoplanet — for Real This Time"
Lisa Grossman, Wired Science (September 29, 2010)

"After years of saying habitable exoplanets are just around the corner, planet hunters have finally found one. Gliese 581g is the first planet found to lie squarely in its star's habitable zone, where the conditions are right for liquid water.

" 'The threshold has now been crossed,' said astronomer R. Paul Butler of the Carnegie Institution of Washington, one of the planet's discoverers, in a press briefing September 29. 'The data says this planet is at the right distance for liquid water, and the right mass to hold on to a substantial atmosphere.'

"The discovery is both 'incremental and monumental,' comments exoplanet expert Sara Seager of MIT, who was not involved in the new study. When a recent study predicted the first habitable world should show up by next May, Seager rightly said the real answer was more like 'any day now.'

" 'We've found smaller and smaller planets that got closer and closer to the habitable zone,' she said. 'But this is the first that's in the habitable zone.'..."

Gliese 581 is practically next door, on a galactic scale: about 20 light years away. It's also quite dim, a red dwarf star, so Gliese 581g orbits once every 36.6 days - about 13,000,000 out from its star.

Red dwarf stars haven't always been seen as particularly good candidates for hosting a habitable planet: but that view has been changing as we got to know more about the dim end of the stellar continuum. (July 7, 2010)
"Alien World Tour: The Exoplanets Around Star Gliese 581" (September 29, 2010)

"...Gliese 581 is a red dwarf located 20.5 light-years from Earth, in the constellation Libra. Like other red dwarfs, it's smaller and much dimmer than our sun. Scientists believe Gliese 581 is old - at least a few billion years - and relatively stable. Both are qualities conducive to the evolution of life, scientists have said...."

"...Gliese 581g is three to four times as massive as Earth, is most likely rocky, and may have an atmosphere, scientists say. It orbits about 0.146 AU from the central star.

"Liquid water could exist on some part of the planet's surface, which seems to have an average temperature between minus 24 and minus 10 degrees Fahrenheit (minus 31 to minus 12 degrees Celsius). Gliese 581g completes an orbit every 37 days or so...."

The length of Gliese 581g's year's been reported as 36.6 days and "37 days or so." Which could likely enough be two different editorial teams' way of expressing the same value.

Whatever the exact length of Gliese 581g's year is, the average temperature is on the chilly side: sort of like Minnesota during the winter. But that's an estimated average - and taking Earth as an example, we could expect quite a lot of variation in climate on the planet.

Whether Gliese 581g has an atmosphere, liquid water - and life? Good questions, that'll have to wait until we've got better ways of studying planets around other stars.

At the rate things have been happening, the wait may not be all that long.

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