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Gliese 582g ~ Similarity to Earth (ESI): 0.92 ~ Minimum mass: 2.2 times that of Earth ~ Distance: 20.3 light-years
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By Paul Rincon
Science editor, BBC News website
It's one of the
big questions: Are we alone on this blue marble or is there life
elsewhere in the cosmos?
To shed some light, astronomers are searching
for habitable worlds circling far-off stars.
A team has now published updated evidence for a planet that
could be the most Earth-like yet. According to the US Planetary
Habitability Laboratory, it would be the fifth potentially habitable
world known outside our Solar System.
So what do we know about these five Earth-like planets, and how likely is it that they could support life?
The discovery of Gliese 581g was announced in
September 2010 by a US-led team. But as soon as they made the
announcement, doubts began to surface. The team at the Geneva
observatory which had discovered all four other planets around the star
Gliese 581 failed to detect it in their own data.
However, the original
discoverers of 581g
have now published an analysis using a greater amount of data to provide more promising evidence for its existence.
This would be significant because the
Earth Similarity Index (ESI),
devised by a team including Dirk Schulze-Makuch from Washington State
University and Abel Mendez from the University of Puerto Rico (UPR) in
Arecibo, shows that Gliese 581g is the most Earth-like planet discovered
to date. The ESI measures characteristics of exoplanets on a scale from
zero to one, with one being identical to Earth. Accordingly, the online
Habitable Exoplanets Catalog, based at UPR, has decided to include it in their list of the most promising worlds to support life.
Like the other worlds in the catalogue, Gliese 581g orbits in
a "sweet spot" around its star - the habitable zone, or Goldilocks zone
- that is neither too hot nor too cold to allow for liquid water. It is
just over twice the mass of Earth and, although the planet is closer to
its parent star than is Earth, it receives about the same light flux (a
measure of the star's apparent brightness) as our planet because Gliese
581 is a red dwarf star and therefore dimmer than our own Sun. Steven
Vogt, from University of California, Santa Cruz, US, one of original
discoverers, said Gliese 581g orbits "squarely in the star's Habitable
Zone at 0.13 AU, where liquid water on planetary surfaces is a distinct
possibility".
But it remains to be seen whether the new evidence will
convince the doubters.
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