Those packing their bags for a trip to the two potentially habitable exoplanets previously claimed to be orbiting the red dwarf star Gliese 581 had better rethink their travel plans. Astronomers at Pennsylvania State University say the planets, Gliese 581 d and Gliese 581 g, don't actually exist.
In 2007, Gliese 581 d was being touted as first exoplanet discovered to sit in the habitable – or "Goldilocks" – zone, where water could exist on the planet's surface in liquid form. This was followed in 2010 with the purported discovery of Gliese 581 g, which was thought to lay right in the middle of the habitable zone. I say "purported" because astronomers weren't able to confirm its existence.
Both of the planets were detected indirectly using the radial velocity method. This is where the gravitational pull of an orbiting exoplanet causes variations in the velocity of its parent star, which can be detected using Doppler spectroscopy.
Now a team at Penn State led by Paul Robertson claims that the signals previously thought to indicate the existence of Gliese 581 d and g weren't due to the gravitational pull of these exoplanets, but were in fact signals that resulted from magnetic activity of the star, similar to the sunspots our Sun experiences.
The team arrived at this conclusion after making two observations. The first was finding that the strength of the signals indicating Gliese 581 d increased at times that the star experienced higher magnetic activity and decreased at times when the star was less active. Secondly, the team revised the star's rotational period, calculating it to be twice as long as Gliese 581 d's orbital period, suggesting the signal was coming from the star rather than a planet.
Because the observations indicating the existence of Gliese 581 g were dependent on the existence of Gliese 581 d, the team was also able to discount the existence of that already disputed exoplanet.
The findings call into question other potential exoplanets discovered using the radial velocity method and reduces the number of exoplanets believed to be orbiting Gliese 581 to just three: Gliese 581 b, c, and e. Unfortunately, none of these are located in the star's habitable zone.
The team's research is published in the journal Science.
An animation created by the Penn State team can be viewed below. The background is a composite image of our Sun, with the left side showing the view through a filter that makes only light in the deep-blue range visible, while the right side uses a filter that blocks all wavelengths except those in the red range. The blue dots indicate candidate exoplanets near or within the habitable zone, orange indicates candidates too close to the star, while green indicates candidates too far from the star.