Record spin rate for cosmic body

By Paul Rincon
BBC News science reporter, in Cambridge

2003 EL61 has taken up an elongated shape

Astronomers have discovered what they say is the fastest rotating object in the Solar System for its size.

The object is called 2003 EL61 and rotates once every 3.9 hours.

Rather than being spherical like a planet, the object has a shape much like a squashed rugby ball, its discoverers told a Cambridge meeting.

2003 EL61 is located in the Kuiper belt - a vast, distant region of the Solar System that contains the planet Pluto and other ice-rock bodies.

"It's by far the record," said David Rabinowitz, of the department of astronomy at Yale University, US.

It's the weirdest shape ever seen for an object this size David Rabinowitz, Yale University

"For an object almost the size of Pluto, this has very dramatic consequences."

Flat sphere

Planets are spherical because gravity pulls them in all directions. But as spin rates start to increase, bodies that would otherwise be spherical can begin to stretch into a flat sphere.

However, 2003 EL61, first seen in July, is spinning so fast it has been stretched into the shape of a squashed rugby ball.

"It's the weirdest shape ever seen for an object this size," said Dr Rabinowitz.

By knowing its shape, astronomers have been able to determine the object is about two-and-a-half times denser than ice. And they have also been able to find that its reflectivity is almost that of pure snow.

2003 EL61 is big enough that gravity is the dominant force governing it rather than its internal structure. But, said Chad Trujillo of the California Institute of Technology, US, it could be approaching the kind of rotation rate where a body of its size would get torn apart.

"I think if you took a body that large and spun it twice as fast, you might be getting close to the limit," he explained.

It is common for asteroids to spin as fast as 2003 EL61, but these are small, fairly solid objects with low gravity. 2003 EL61 is distinct because it is a large object dominated by gravity, much like a planet.

Dr Trujillo said a spiralling-in effect of the object and its moon could have caused 2003 EL61 to spin at its fast rate.

Results were presented at the American Astronomical Society's Division of Planetary Sciences meeting in Cambridge.