An unusual dwarf planet discovered in the outer Solar System could be en route to becoming the brightest comet ever known.

2003 EL61 is a large, dense, rugby-ball-shaped hunk of rock with a fast rotation rate.

Professor Mike Brown has calculated that the object could be due a close encounter with the planet Neptune.

If so, Neptune's gravity could catapult it into the inner Solar System as a short-period comet.

"If you came back in two million years, EL61 could well be a comet," said Professor Brown, from the California Institute of Technology (Caltech) in Pasadena.

"When it becomes a comet, it will be the brightest we will ever see."

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