What was hailed as the first image of a planet outside our solar system is probably nothing more than a normal star.

Last May, amid a blaze of publicity, Nasa unveiled Hubble Space Telescope images that astronomer Sue Terebey said might show a planet born to a pair of stars some 450 light years from Earth.

Flung into space from its twin parents, it was visible at the end of a filament of gas.

The discovery made headlines all over the world because the images were the first taken of a planet outside our solar system.

But further observations outlined at two recent conferences have revealed that the object is too hot to be a planet.

Instead, it is "almost certainly a normal reddened star", says Keith S Noll of the Space Telescope Science Institute in Baltimore, Maryland.

Terebey acknowledges that new data she has obtained of the faint object, dubbed TMR-1C, reveals that it does not contain water vapour, which should be present if it were a planet with a temperature lower than 2,500 K.

She also suggested that the object might be a failed star, known as a brown dwarf, or a planet that is warmer and possibly younger than first thought.

Holy Grail illusion

"In my opinion, it is a waste of time and bad science to keep pursuing this idea [of a planet] when a much simpler and more likely alternative is supported by all the evidence," says Noll.

"Extrasolar planets are one of current astronomy's holy grails, and so there is strong temptation to see them where one wants to see them. But in this case, the data seems to be saying quite clearly that this extrasolar planet was an illusion."

Nasa has been criticised for publicising the "discovery" so widely before it had been published in a scientific journal when any problems in the interpretation would emerge.