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."

Cosmic oddball

2003 EL61 is a large object; it is as big as Pluto along its longest dimension. It is one of the largest of a swarm of icy objects that inhabit a region of the outer Solar System known as the Kuiper Belt.

But it is extremely unusual: spinning on its axis every four hours, it has developed an elongated shape.

2003 EL61 is apparently composed of rock with just a thin veneer of water-ice covering its surface. Other Kuiper Belt Objects (KBOs) contain much more water-ice.

Professor Brown's computer simulations show that the object is on a very unstable orbit and set for a close encounter with Neptune.

The eighth planet's gravitational force could either sling the icy rock ball into the inner Solar System as a comet, out into the distant Oort Cloud region, or even into interstellar space.

Orbits of Kuiper Belt Objects tend to be very stable, but the region is thought to be a reservoir for short-period comets.

Occasionally, some of these objects must get tossed inward to become the fizzing lumps of ice and dust that criss-cross our cosmic neighbourhood.

Shedding surface

Mike Brown and his colleagues have come up with a scenario to explain 2003 EL61's physical characteristics and behaviour.

About 4.5 billion years ago, the object that became 2003 EL61 was a ball, half composed of ice and half of rock - like Pluto - and about the same size as Pluto.

Some time early in its history, it was smacked, edge on, by another large KBO. This broke off much of 2003 EL61's icy mantle, which coalesced to form several satellites.

As expected, the satellites seem to be composed of very pure water-ice.

Professor Brown suggested that some of 2003 EL61's mantle may already have made it into the inner Solar System as cometary material.

The oblique impact also caused 2003 EL61 to spin rapidly. This rapid rotation elongated 2003 EL61 into the rugby ball shape we see today.

"It's a bit like the story of Mercury," Professor Brown explained.

"Mercury got hit by a large object early in the Solar System. It left mostly a big iron core, with a little bit of rock on the outside. This is mostly a rock core with a little bit of ice on the outside."

Mike Brown outlined details of his work during a plenary lecture at the recent American Astronomical Society meeting in Seattle.