A discovery of the first potentially-habitable exoplanet may have been in error, according to a new study.

Gliese 581g was announced as a potential "Goldilocks planet" back in 2010, when it was the first planet found orbiting an alien star in the "habitable zone," where liquid water is likely to exist.

Astronomers questioned why the signal from the planet was so weak. Despite this, the excitement of finding a planet that might support life was so exciting, the news spread quickly, among both professional and armchair astronomers. Gliese 581g was named time and again as the best spot where alien life might be found outside the solar system.

A new study examined the finding, revealing the discovery could have been a false signal, caused by a sunspot. This star is a mere 20 light years from Earth, but even at that distance, it is impossible to view the planet directly. Therefore, astronomers measure the light that radiates from stars, recording how it changes over time.

As planets come between their stellar companion and the Earth, the star is pulled a little in the direction of our planet, and the light becomes bluer. On the opposite side of the orbit, the stellar body is pulled away from your field of view, and the light turns more red, in a process called the Doppler effect. This technique now only shows changes in stars caused by orbiting planets, but can also be affected by conditions on the stellar surface, including sunspots. In all, three planets, including Gliese 581g, were thought to be located in the habitable zone of the star.

Penn State University astronomers have now shown that at least two of these "worlds" were only optical illusions.

"This result is exciting because it explains, for the first time, all the previous and somewhat conflicting observations of the intriguing dwarf star Gliese 581, a faint star with less mass than our Sun that is just 20 light years from Earth," Paul Robertson, a postdoctoral fellow at Penn State and lead author of the study, said.

Robertson and his team examined the data collected of Doppler shifts from the stars, and boosted the signals caused by the three inner-most planets. Once this was done, evidence of two of the "habitable planets" disappeared into the background.

A video showing how the discovery of the planets was refuted is available on the Penn State YouTube page.

Investigation of Gliese 581g, exploring the idea of a false signal possibly induced by a sunspot was published in the journal Science.