British scientists are trying to determine why a Jupiter-style planet orbiting close to its sun has not spiraled into the star and burned up.

Wasp-18b is so close to the star Wasp-18 that it completes its orbit in less than an Earth-length day, astrophysicists at Keele University in Staffordshire say. In an article in Nature, they say standard astronomical theories hold the planet should have a lifespan of less than a million years, while Wasp-18b is believed to be a billion years old.

Astronomers have a similar problem in our solar system. Phobos, one of the moons of Mars, has an orbit only 5,000 miles up that should, according to theory, have led to a fatal collision with the planet long ago.

Coel Hellier of Keele said there could be reasons for the anomaly. Wasp-18 might have less energy than it appears or the planet may have been spiraling inward and is a recent arrival at its currently observed position.

Douglas Hamilton of the University of Maryland, who wrote a commentary on the article, said there is another possibility: "We're just missing something -- there is some property of stars or tides that we just don't understand."