'Habitable' planet GJ 581d might exist; astronomers at odds

Chuck Bednar for redOrbit.com – @BednarChuck

Planet candidate GJ 581d, discovered in 2009 and dismissed by researchers last summer, does actually exist, a team of UK astronomers insist in a report published this week in Science.

GJ 581d, a super-Earth planet first discovered in the habitable zone of a distant star six years ago, was originally detected using a spectrometer which measured the wobble, or small changes in the wavelength of light emitted by a star as a planet orbits it.

In July 2014, researchers from Pennsylvania State University and the University of Texas at Austin published a paper disputing the discovery, claiming that what astronomers believed was a planet was actually nothing more than noise in the data caused by starspots.

“Correcting for activity greatly diminishes the signal of GJ 581d (to 1.5 standard deviations) while significantly boosting the signals of the other known super-Earth planets,” the authors wrote. “GJ 581d does not exist, but is an artifact of stellar activity which, when incompletely corrected, causes the false detection of planet g.”

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Now, astronomers from Queen Mary University of London and the University of Hertfordshire are firing back, claiming that last year’s research challenging the planet’s existence was due to an insufficient analysis of the data. They argue that the statistical technique that was used to account for stellar activity is inadequate when it comes to identifying smaller planets.

The method used has previously worked when identifying larger planets because they impact they had on their stars was significant enough to negate errors in the findings, the authors added. However, it makes it nearly impossible to find the smallest planet signals within the noise caused by stellar variability, and they are confident that using a more accurate model based on existing data will prove that the signal attributed to GJ 581d is real, despite that noise.

So which one is it?

The team behind the 2014 study “claimed that activity-induced variability is responsible for the Doppler signal of the proposed planet candidate GJ 581d. We point out that their analysis using periodograms of residual data is inappropriate and promotes inadequate tools,” the authors of the new paper wrote. “Because the claim challenges the viability of the method to detect exo-Earths, we encourage reanalysis and a deliberation on what the field-standard methods should be.”

“The existence (or not) of GJ 581d is significant because it was the first Earth-like planet discovered in the ‘Goldilocks’-zone around another star and it is a benchmark case for the Doppler technique,” explained lead author Dr. Guillem Anglada-Escudé. “There are always discussions among scientists about the ways we interpret data but I’m confident that GJ 581d has been in orbit around Gliese 581 all along.”

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“In any case, the strength of their statement was way too strong,” Dr. Anglada-Escudé added. “If their way to treat the data had been right, then some planet search projects at several ground-based observatories would need to be significantly revised as they are all aiming to detect even smaller planets. One needs to be more careful with these kind of claims.”

The authors of the disputed study responded, stating that while they agreed that “improvements in multiparametric radial velocity (RV) modeling” were necessary for the detection of planets with a mass similar to that of Earth’s, “key physical points we raised were not challenged. We maintain that activity on Gliese 581 induces RV shifts that were interpreted as exoplanets.”

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Source: http://www.redorbit.com/news/space/1113348261/habitable-planet-gj-581d-might-exist-astronomers-at-odds-030715/