US astronomers have found a planet that has what they consider the most "eccentric" orbit of any yet documented.

The extrasolar planet HD 20782, about 117 light years from Earth, moves in a nearly flattened ellipse, travelling a long path far from its star and then making a fast and furious slingshot around the star at its closest approach.

At the furthest point in its orbit, the planet is separated from its star by 2.5 times the distance between the sun and Earth, while at its closest point, it ventures much closer than Mercury orbits the sun.

"It's around the mass of Jupiter, but it's swinging around its star like it's a comet," said San Francisco State University astronomer Stephen Kane, who led the study.

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While the planets in our solar system have nearly circular orbits, astronomers have discovered several extrasolar planets with highly elliptical or "eccentric" orbits.