Astronomers searching the skies for pulsars instead found a mysterious burst of radio activity, nothing like anything any of them had seen before.

According to Reuters, the researchers were trawling through a back catalogue of data from the Parkes telescope in Australia when they came across the signal. It was five milliseconds long and might be the calling card of a single, highly energetic event such as a supernova, the team said.

Although the burst was very brief, it was very strong. The team tracked it back to a point of origin roughly three billion light years away.

Little green men are categorically ruled out: the burst is far too powerful, according to Maura McLaughlin of West Virginia University. "We think it has got to be some sort of catastrophic event happening in another galaxy - like two stars colliding and merging, or maybe a black hole. Something kind of exotic," she said.

McLaughlin speculated that many more of these bursts could be happening everyday, but because radio telescopes typically survey a very narrow field of the sky, we could just be missing them.

The work is published in the journal Science. ®

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