Astronomers using NASA space telescopes say they have observed a giant storm, similar to the Giant Red Spot on Jupiter, swirling across the surface of a distant star.

Further observations have confirmed the star W1906+40, which was thought to be a sunspot when it was first observed in 2011, is a continuously active storm on the star's surface near one of its poles.

"The star is the size of Jupiter, and its storm is the size of Jupiter's Great Red Spot," said John Gizis of the University of Delaware, Newark. "We know this newfound storm has lasted at least two years, and probably longer."

While massive storms have been seen in the cloudy atmosphere of planets, the new observations are the best evidence yet that they can form on stars, the researchers say.

W1906+40 belongs to a class of thermally cool cosmic objects known as L-dwarfs, which hover on the borderline between stars and so-called "Hot Jupiters."

Some are classified as stars because they exhibit evidence of fusing atoms and generating light as our sun does, while others that lack atomic fusion are called brown dwarfs or sometimes "failed stars."

The temperature of W1906+40 is around 3,500 degrees Fahrenheit, relatively cool by stellar standards and cool enough for clouds to form in its uppermost layers.

That creates an environment for planet-like atmospheric phenomena which, in this case, is a large and powerful storm.

The first observations of the star using the Kepler Space Telescope yielded evidence of a large dark area that rotated with the star's spin.

To find out if the dark patch was just an unusually large sunspot, or group of sunspots, the astronomers turned to another space telescope, the Spitzer Space Telescope.

The Spitzer observations proved the rotating dark feature is not being driven by magnetism, which ruled out sunspots.

That left only one candidate: a massive, spinning storm.

Unlike a storm on Earth, there's no water or other liquid vapor making up the storm clouds, says Gizis.

"The L-dwarf's clouds are made of tiny minerals," he explained.

The finding suggests the telescope team of Kepler and Spitzer may help find other stormy stars or brown dwarf "failed stars" in the future, the researchers say.

That could help in understanding exactly what is happening on W1906+40 and whether or not the phenomenon exists elsewhere.

"We don't know if this kind of star storm is unique or common, and we don't [know] why it persists for so long," said Gizis.