Planetary System Gliese 581
July 4, 2014

Astronomers Prove That Two Potentially Habitable Planets Do Not Exist

redOrbit Staff & Wire Reports - Your Universe Online

Gliese 581g, described as one of the most Earth-like planets when it was originally discovered four years ago, probably does not exist, researchers from Penn State University and the University of Texas at Austin report in the latest edition of the journal Science.

Officially, lead author Paul Robertson of Penn State’s Center for Exoplanets and Habitable Worlds and his colleagues write that the existence of Gliese 581g is “disputed,” but they unequivocally state that its less-habitable sister planet Gliese 581d “does not exist.” In truth, the authors report that Gliese 581d is actually “an artifact of stellar activity which, when incompletely corrected, causes the false detection” of Gliese 581g.

According to Michael D. Lemonick of National Geographic, the first evidence of the two supposed worlds came from measurements of their host star, a dim red dwarf that is roughly one-third the size of the sun and approximately 22 light years away from our solar system. Since most exoplanets tend to be too close to their stars to be directly observed, astronomers are typically forced to rely on indirect clues in order to find them.

When it came to Gliese 581g, Paul Butler of the Carnegie Institution for Science and Steven Vogt of the University of California, Santa Cruz looked for subtle wobbles caused by the gravity of an orbiting planet pulling back and forth on the star in a regular pattern, Lemonick added. Since it took 37 days to complete one orbit, the researchers said it was at the ideal distance from its star for liquid water to exist on its surface, and the strength of the gravitational pull revealed that Gliese 581g was approximately three times more massive than the Earth.

Now, however, using techniques designed to confirm the existence of smaller, harder-to detect planets, Robertson and his fellow investigators have actually debunked the existence not only of Gliese 581g, but Gliese 581d as well. Instead, what scientists had believed to be potential habitable worlds are actually caused by physical changes taking place within the star itself, according to the Washington Post’s Sandhya Somashekhar.

The study authors analyzed Doppler shifts in existing spectroscopic observations of Gliese 581 that had been acquired using the European Southern Observatory (ESO) HARPS and WM Keck Observatory HIRES spectrographs. Specifically, the PSU and UT astronomers focused on Doppler shifts that were most sensitive to magnetic activity, and they were able to boost the signals of three additional planets surrounding the star.

By doing so, however, the signals attributed to the other two planets “disappeared, becoming indistinguishable from measurement noise,” co-author Suvrath Mahadevan, an assistant professor of astronomy and astrophysics at Penn State, said in a statement. “The disappearance of these two signals after correcting for the star's activity indicates that these signals in the original data must have been produced by the activity and rotation of the star itself, not by the presence of these two suspected planets.”

The research did confirm the existence of those three other worlds, which are inner-planets that do not fall into the habitable zone, said Somashekhar. Mahadevan told the Washington Post writer that the results of the study were “bittersweet,” because even though the research team was “pleased” that their technique works, they were disappointed to have disproven the existence of these two planets.

A description of this research is available on the Habitable Zone Planet Finder blog.


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