Wednesday, October 13, 2010

Gliese 581g? Maybe Not

"Doubt Cast on Existence of Potentially Habitable Planet Gliese 581g"
Leslie Mullen, (October 12, 2010)

"Last month, astronomers announced the discovery of the first potentially habitable extrasolar planet. But this week at an International Astronomical Union meeting, doubts were raised about the existence of this exciting new planet said to be orbiting the star Gliese 581.

"Called Gliese 581g, the planet was determined to be about three times the mass of Earth, meaning it was a rocky world, not a gas giant like Jupiter.

"Rocky extrasolar planets have been found before, but the unique trait about this planet was that it orbited within the red dwarf star's habitable zone, that region of space where temperatures are sufficient for water to remain as a liquid on a planetary surface...."

Even without Gliese 581g, the Gliese planetary system is pretty exciting: with several known (and confirmed) planets in nearly-circular orbits, it's a fairly close match - on a small scale - of our Solar system.

The problem with Gliese 581g is that it's existence was found, based on some really, really, exact measurements. Or, rather, measurements of exact values - plus noise.

As the article puts it:

"...Francesco Pepe, an astronomer who works on HARPS data at the Geneva Observatory, said at the IAU meeting this week that his team could not confirm the existence of Gliese 581g.

"In e-mail correspondence with Astrobiology Magazine, Pepe said that they could not confirm the existence of planet 'f' either.

"The Geneva team, led by Michel Mayor, announced in 2009 the discovery of planet 'e' in the Gliese 581 solar system. At approximately 1.9 Earth masses, this 'e' planet is the lowest mass extrasolar planet yet found, and has a 3.15-day orbital period around the star.

" 'Since Mayor's announcement in 2009 of the lowest-mass planet Gliese 581e, we have gathered about 60 additional data points with the HARPS instrument for a total of 180 data points spanning 6.5 years of observations,' said Pepe. 'From these data, we easily recover the four previously announced planets b, c, d, and e.'

"However, he said they do not see any evidence for planet 'g,' the fifth planet in the system as announced by Vogt and his team.

" 'The reason for that is that, despite the extreme accuracy of the instrument and the many data points, the signal amplitude of this potential fifth planet is very low and basically at the level of the measurement noise, said Pepe...."

Pepe and company have a good point: the spectroscopic radial velocity measurements used to detect Gliese 581a's wobbles are measurements of very small values. There's going to be some uncertainty about the data.

Or, maybe, it's a case of 'Not Discovered Here,' a variation on the 'Not Invented Here' syndrome, where an organization denies the existence of anything its workers didn't develop.

The Lemming doesn't think so: this looks like the sort of double-checking (and triple-checking, and quadruple-checking, and so on) of data that's supposed to happen in scientific inquiry.

It's still something of a letdown.

Now, we wait to see if a third party comes up with a 'we found it,' using another method of analysis - or maybe new data.

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