A new study has doubted the existence of two of the Earth-sized planets that were "discovered" outside our solar system which may have had the potential to have liquid water and life too. But it may have uncovered a way to detect new planets easily.

In 2010, when the discovery of Gliese 581 g was announced, there was a lot of excitement since it was in the habitable zone of its red dwarf star, the "Goldilocks" region which is not too hot and not too cold and where liquid water and life could exist. The planet appeared to be about 1.2 to 1.4 times larger than the Earth and was likely to have a rocky surface.

Another planet in the system called Gliese 581 d which was announced in 2007 was also on the edge of the habitable zone.

"They were very high value targets if they were real", said Paul Robertson, a researcher at Penn state University and the lead author of the new study which was published this week in Science.

The two planets like the Earth have caused controversy since they were discovered using a method that looks for small changes in the colour of light of a star which is caused by the tug of the planet's gravity.

Robertson says this is due to the very low masses of the planets, if they do exist.

Several studies have argued the existence of these planets based on different methods of analyzing the signals. Among them was one study by Phil Gregory, professor emeritus at the University of British Columbia.

Another study by a Swiss team which used additional data caused the scientists who discovered Gliese 581 g, Steve Vogt of the University of California Santa Cruz and Paul Butler of the Carnegie Institution for Science in Washington D. C., to publish a new paper in 2012 criticizing the analysis of the team and attempting to reaffirm the existence of Gliese 581 g with their own analysis.