Astronomers led by Dr Paul Robertson of the Pennsylvania State University say that signals that were suspected to be coming from two Earth-like exoplanets orbiting in the habitable zone of a nearby star called Gliese 581 actually are created by activity within the star itself.
The upper image shows the location of six candidate exoplanets believed to orbit Gliese 581 as of 2010. Blue indicates candidate exoplanets in the habitable zone. The bottom image shows the location of the three exoplanets remaining in 2014. Image credit: NASA / Penn State University.
Gliese 581 is a main sequence red star with spectral type M3V located in the constellation Libra, about 20 light-years from Earth.
Beginning in 2005, astronomers discovered a series of progressively smaller exoplanets around this star, eventually finding evidence for 4 low-mass exoplanets.
Four years later, they concluded the outermost of these exoplanets, labeled Gliese 581d, orbited at in the star’s habitable zone. With a mass of 6 times the mass of the Earth, this exoplanet has been considered by many to be the first truly habitable exoplanet ever discovered.
In 2010, scientists announced the discovery of two additional exoplanets – Gliese 581f and 581g. The exoplanet Gliese 581g was believed to have a mass of about 3 times the Earth’s mass and orbit in the middle of its star’s habitable zone.
Now Dr Robertson and his colleagues have found that Gliese 581d and 581g do not actually exist and that astronomers were confused by sunspots on their parent star.
“We have proven that controversial signals are not coming from two additional proposed Goldilocks exoplanets in the star’s habitable zone, but instead are coming from activity within the star itself,” said Dr Suvrath Mahadevan of the Pennsylvania State University, who is a co-author on the paper published in the journal Science.
“As a result, the exoplanets now confirmed to be orbiting Gliese 581 total exactly three.”
Dr Robertson’s team made its discovery by analyzing Doppler shifts in existing spectroscopic observations of Gliese 581 obtained with ESO’s HARPS and Keck HIRES spectrographs.
The Doppler shifts that they focused on were the ones most sensitive to magnetic activity. Using careful analyses and techniques, they boosted the signals of the three innermost exoplanets around the star, but the signals attributed to the existence of the two exoplanets disappeared, becoming indistinguishable from measurement noise.
“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 exoplanets,” Dr Mahadevan explained.
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Paul Robertson et al. Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581. Science, published online July 03, 2014; doi: 10.1126/science.1253253
