New study shows no sign of first habitable exoplanet

Things don't look good for Gliese
581g, the first planet found orbiting in the habitable zone of another
star. The first official challenge to the small, hospitable world
looks in the exact same data -- and finds
no significant sign of the planet.

"For the time being, the world does not have data that's good
enough to claim the planet," said astro-statistics expert Philip
Gregory of the University of British Columbia, author of the new
study.

The "first habitable exoplanet" already has a chequered history.
When it was announced last September, Gliese 581g was heralded as
the first known planet that could harbour alien life. The planet
orbits its dim parent star once every 36.6 days, placing it smack
in the middle of the star's habitable zone, the not-too-hot,
not-too-cold region where liquid water could be stable.

Planet G was the sixth planet found circling Gliese 581, a red dwarf star 20 light-years from Earth. A team of astronomers from the Geneva Observatory
in Switzerland found the first four
planets using the HARPS spectrograph on a telescope in Chile. The team carefully
measured the star's subtle wobbles as the planets tugged it back
and forth.

Two more planets, including the supposedly habitable 581g,
appeared when astronomers Steve Vogt of the University of
California, Santa Cruz and Paul Butler of the Carnegie Institution
of Washington added data from the HIRES spectrograph on the Keck
Telescope in Hawaii. They announced their discovery 29
September.

Just two weeks later, the HARPS team announced they found no
trace of the planet in their data, even when they added two more
years' worth of observations. But it was still possible that the
planet was only visible using both sets of data.

Now, the first re-analysis of the combined data from both
telescopes is out, and the planet is still missing.

"I don't find anything," Gregory said. "My analysis does not
want to lock on to anything around 36 days. I find there's just no
feature there."

Unlike earlier studies, Gregory used a branch of statistics
called Bayesian analysis. Classical methods are narrow, testing
only a single hypothesis, but Bayesian methods can evaluate a whole
set of scenarios and figure out which is the most likely.

Gregory wrote a program that analysed the likelihood that a
given planetary configuration would produce the observed
astronomical data, then ran it for various possible
configurations.