Two teams of astronomers have snapped the first pictures of alien planets outside the solar system.

The pictures show four likely planets that appear as specks of white, nearly indecipherable, except to the most eagle-eyed experts.

All are trillions of kilometres away - three of them orbiting the same star, and the fourth circling a different star.

None of the four gaseous planets are habitable like Earth. But they raise the possibility of others more hospitable.

"It is only a matter of time before we get a dot that's blue and Earthlike" astronomer Bruce Macintosh of the Lawrence Livermore National Lab, who led one of the two teams of photographers, said.

"It is a step on that road to understand if there are other planets like Earth and potentially life out there", he said.

Macintosh's team used two ground-based telescopes, while the second team relied on photos from the 18-year-old Hubble Space Telescope to gather images of the exoplanets - planets that don't circle our sun.

Exciting finding

In the past 13 years, scientists have discovered more than 300 planets outside the solar system, but they have done so indirectly, by measuring changes in gravity, speed or light around stars.

The research from both teams was published in Thursday's online edition of the journal Science.

Ed Weiler, Nasa's space sciences chief, said the new photos were important and compared the effort to a hunt for elusive elephants. 

"For years we've been hearing the elephants, finding the tracks, seeing the trees knocked down by them, but we've never been able to snap a picture. Now we have a picture," he said.

But doubts still persist about whether these were the first exoplanet photos.

Others have made earlier claims, but those pictures haven't been universally accepted yet.

The photos released on Thursday have still not convinced all the experts.

Alan Boss, an exoplanet expert at the Carnegie Institution of Washington, and Lisa Kaltenegger, a Harvard exoplanet hunter, both said more study was needed to confirm the new set of photos were those of proven planets and not just brown dwarf stars.