Astronomers are having a war of words across the Atlantic over the existence -- or not -- of the first exoplanet ever discovered in a star's habitable zone, the nearby GJ581d.

British astronomers said a nearby Earth-like planet, dismissed last year as stellar noise, probably exists.

The possible planet GJ 581d was discovered in 2009 orbiting the star Gliese 581, a red dwarf 20 light-years away. It was the first Earth-like exoplanet discovered in a star's habitable zone, neither too hot not too cold for liquid water.

But an article published last year in the journal Science dismissed the discovery as "stellar activity masquerading as planets."

Researchers from Queen Mary University of London and the University of Hertfordshire this week published a response in the same journal, questioning the recent study's methods, saying they are inadequate for detecting planets that small. The British researchers said their more accurate methods support the existence of GJ 581d, which they say is likely a "super-Earth" planet, larger than Earth and more rocky.

"The existence (or not) of GJ 581d is significant because it was the first Earth-like planet discovered in the 'Goldilocks'-zone around another star and it is a benchmark case for the Doppler technique," said Dr Guillem Anglada-Escudé, lead author of the British research. "There are always discussions among scientists about the ways we interpret data but I'm confident that GJ 581d has been in orbit around Gliese 581 all along."

But the Penn State University scientists who wrote the 2014 study published a response to the British researchers' response.

"Our work clearly established the rotation period of GJ 581 to be 130 days -- twice the period of "planet d" -- independently of the statistical arguments raised in the Comment. Without compelling evidence otherwise, a periodic RV signal at the stellar rotation period or its integer fractions should be assumed to be associated with stellar activity instead of a planet."

"As recently noted by the High Accuracy Radial Velocity Planet Searcher (HARPS) team -- who originally discovered "planet d" -- had the rotation period of GJ 581 been previously known, it is unlikely that the 66-day RV signal would ever have been ascribed to an exoplanet "