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Top Habitable Exoplanet Candidate Probably Doesn't Exist

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Lisa Winter

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1416 Top Habitable Exoplanet Candidate Probably Doesn't Exist
The Sun as seen through calcium K (blue) and hydrogen alpha (red) filters. Image courtesy Alan Friedman via Penn State

Gliese 581 is an M dwarf star located about 22 light-years away in the constellation Libra. After years of observation, the planet has been suspected to have as many as six planets in its system. Unfortunately, a new study from researchers at Penn State has concluded that two of the planets are actually misinterpreted solar activity and don’t exist at all. This includes Gliese 581g, which was thought to be in the planet’s Goldilocks Zone. The results of the study were published in Science.

The existence of Gliese 581g was announced in the fall of 2010, following 119 observations over the course of a decade. This planet was touted as a potential Earth-like planet residing in its star’s habitable zone. With a mass 3.1-4.3 times greater than Earth, it was regarded as one of the most promising locations to search for extraterrestrial life. Its detection was based on the assumption of the existence of 581d, which is where the problem lies. 


The study analyzed the rotational speed of Gliese 581, which had been unknown up until this point. That data was then used to go back to previous observations and correct for the star’s movement that caused excess noise and distorted signals. Those corrections did boost the signal of many super-Earth planets, but diminished the signal for d incredibly while the signal for g was eliminated entirely. 

“Gliese 581d does not exist,” the paper states, “but is an artifact of stellar activity which, when incompletely corrected, causes the false detection of planet g.”

Our own Sun undergoes cycles in activity that last 11 years, though other stars can have cycles ranging from 1 to 20. During times of increased solar activity, flares, and plasma flows, the radial velocity readings can be altered, confounding signals read by scientists on Earth. Astronomers have previously investigated Gliese 581’s solar activity in an attempt to ensure the existence of planets d and g, though calcium emission and x-ray readings determined the star’s activity was too stable to cause false reading. The starspots were also hard to detect with the available equipment. 

However, some have been skeptical about planets d and g since their respective announcements, as data from HRS on the HET indicated that the rotational velocity might be impacted by activity on the star. The team later used spectrography data from HARPS and HIRES to study the absorption lines of sodium and hydrogen-alpha, which revealed the star’s rotation period. This allowed them to go back and correct previous calculations and clear up the noise and error in the signals. 


This technique will not only be used to rule out planets, but will play a hand in confirming them as well. There are currently 21 other planets in the catalog of nearby potentially habitable planets including recently-discovered Gliese 832c, so there is still much work to do and much left to explore.


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