Gliese 581 Update: First Good Candidate for Life is Out, but a Second Candidate in Same System is Looking Better...

Today's Gliese 581 update from Space.com concerns new observations that lead researchers to believe that Gliese 581c, a rocky world of about five Earth masses, may now be too hot to support life--the consequence of a runaway greenhouse affect similar to that found on Venus. But all hope is not lost for this system. It turns out that Gliese 581d, slightly further from the primary star, and as massive as about eight Earths, may turn out to be the better candidate for life. Excerpt:

Scientists earlier this year announced they had found a small, rocky planet located just far enough from its star to sustain liquid water on its surface, and thus possibly support life.

Turns out the scientists might have picked the right star for hosting a habitable world, but got the planet wrong. The world known as Gliese 581c is probably too hot to support liquid water or life, new computer models suggest, but conditions on its neighbor, Gliese 581d, might be just right.

The findings are detailed in the May 25 issue of the journal Astronomy & Astrophysics.

Gliese 581c, discovered in April by a team led by Stephane Udry of the Geneva Observatory in Switzerland, is about 50 percent bigger than Earth and about five times more massive. It is located about 20.5 light-years away, and circles a dim red dwarf star called Gliese 581.

Of the more than 200 extrasolar planets, or "exoplanets," discovered since 1995, Gliese 581c was the first found that resides within the habitable zone of its star, if only barely. The habitable, or "Goldilocks" zone is the region around a star where the temperature is neither too hot nor too cold, so water can exist on a planet's surface in its liquid state. Water is a key ingredient for life as we know it.

But new simulations of the climate on Gliese 581c created by Werner von Bloh of the Institute for Climate Impact Research in Germany and his team suggest the planet is no Earthly paradise, but rather a faraway Venus, where carbon dioxide and methane in the atmosphere create a runaway greenhouse effect that warms the planet well above 212 degrees Fahrenheit (100 Celsius), boiling away liquid water and with it any promise of life.

But the same greenhouse effect that squashes prospects for life on Gliese 581c raises the same hope for another planet in the system, a world of eight Earth-masses called Gliese 581d, which was also discovered by Udry's team.

"This planet is actually outside the habitable zone," said Manfred Cuntz, an astronomer at the University of Texas at Arlington and a member of von Bloh's team. "It appears at first sight too cold. However, based on the greenhouse effect, physical processes can occur which are heating up the planet to a temperature that allows for fluid water."

And where this is fluid water, there is the chance of life as well. The researchers speculate that "at least some primitive forms of life" might exist on Gliese 581d. There is no evidence to support that speculation, however.

Jaymie Matthews, an astronomer at the University of British Columbia in Canada, doesn't treat the new findings as conclusive, but finds them "interesting as an illustration of how we can use remote exoplanetary environments as possible test beds for climate models."

Matthews own research, recently presented at the annual meeting of the Canadian Astronomical Society, suggests one reason Gliese 581 is such a promising star for finding habitable planets is that it is similar to our own sun in that it is remarkably stable.

Matthews and his team used a Canadian space telescope called MOST to monitor Gliese 581 for six weeks. During that time, they observed very few instances of the powerful solar flares common among red dwarf stars.

"If the star showed significant variations in brightness during the weeks we monitored it, that would at least complicate the thermal equilibrium of the planets around it," Matthews explained.

The stability of the light also suggests Gliese 581 is old and that is has been around for at least a few billion years. "Young stars, like young people, can have bad cases of acne (large starspots and activity) and spin around," Matthews said in an email interview. "Older stars like the sun have relatively clear complexions and rotate rather sedately."

Gliese 581's advanced age is good news for scientists hoping to find signs of life in the system. "We know it took about three and a half billion years for life on Earth to reach the level of complexity that we call human," Matthews said, "so it's more encouraging for the prospects of complex life on any planet around Gliese 581 if it's been around for at least as long."

These points are extremely important. As is correctly detailed here, young red dwarf stars exhibit flare activity, and this activity would put any planets with anything close to Earth-like conditions at risk of being bathed in radiation, and to have their temperatures skyrocket within short time periods. The fact that this activity is absent in Gliese 581 is a good thing, as it points to the star's stability.

As for the planets in question, Gliese 581c lies only about 7 million miles from the primary (figures obtained from solstation.com--see blog roll), so if it is as massive as the measurements indicate (five Earth masses), then it should come as no surprise to find that the planet's atmosphere will trap the heat of the star in the manner described above. Gliese 581d (eight Earth masses) appears to lie at roughly 23 million miles from the primary. That distance places it just outside the speculative habitable zone, but with its increased mass, 581d may have a thick enough atmosphere to trap the primary's feeble heat, and make it just warm enough for life to evolve.

Keep in mind that Gliese 581 has only about 30 percent of our Sun's mass, and 38 percent of its diameter, and these distances make sense when trying to figure out what we are observing. It is too bad we have no data (at least none I've been able to find) about densities, from which we could extrapolate who strong each planet's gravitational pull would be. One clue is that the star appears to have between 36 and 62 percent as enriched as the Sun is in proportion to elements heavier than hydrogen. That would indicate that these worlds would be less dense than Earth, and therefore their increased mass may not lead to proportionately high gravitational effects.

The hits just keep on coming with Gliese 581. There is still a lot of work to be done, but I'm certain that we are far from finished with this fascinating system.

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