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.
So much
promise
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.
Another
contender
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.
Jury
still out
David
Charbonneau, an astronomer at the Harvard-Smithsonian Center for Astrophysics
(CfA) who was not involved in the study, said the results from von Bloh's team
are "probably a sound calculation but we don't actually know if it's
correct."
Gliese 581d
demonstrates the importance of taking a planet's atmospheric conditions into
account when considering its potential for habitability. The concept of a
habitable zone "is a very useful thing because it does inform us a great
deal, and it explains a lot in the solar system. But it's not the whole
story," Charbonneau said.
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."
The models
made by von Bloh's team could be tested if scientists can measure thermal
emissions and the reflectivity, or "albedo," of the planets, Matthews said.
Scientists
"have done this already for HD 209458b, a hot
Jupiter, but we will need to do this for possibly 'Earthy' planets to truly
assess their habitability," he added.
A stable
star
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."
advertisement
