A possible planet outside our solar system captured by the Hubble space telescope has failed to show up in subsequent searches, U.S. astronomers say.

An image taken by Hubble in 2008 of a pinpoint of light orbiting a star called Fomalhaut about 25 light-years from Earth was hailed as the first actual picture of an exoplanet.

Now scientists using the Spitzer Space Telescope say they suspect the dot of light in the Hubble image isn't a planet at all because it doesn't radiate at the infrared wavelengths expected of an exoplanet, ScienceNews.org reported.

The Spitzer data was reported by a team led by Markus Janson of Princeton University.

The object dubbed Fomalhaut b still puzzles astronomers, since it's proved invisible to ground-based infrared telescopes and is following an unexpected path around its star.

Explanations for the pinpoint of light range from a background star to light scattered by a dust cloud that surrounds the star Fomalhaut.

That dust cloud bears an elliptical shape that could be the handiwork of a giant planetary shepherd, a planet that just hasn't been detected yet, researchers said.

"The 'real' Fomalhaut b still hides in the system," they said in their published study.