Published online 9 September 2005 | Nature | doi:10.1038/news050905-17


Cosmic 'cigar' spins at astonishing pace

Pluto's neighbour has a very unexpected shape.

Although it doesn't look like it here, Santa has spun itself out into a cigar shape. Read more about odd planets in our blog from the Cambridge conference.Although it doesn't look like it here, Santa has spun itself out into a cigar shape. Read more about odd planets in our blog from the Cambridge conference.© M. Brown et al / Caltech / Keck

A recently discovered planetoid on the outskirts of our Solar System is turning so fast that it seems to have been squeezed into the shape of a cigar. And this cigar looks as if it is spinning not around its long axis, but around its middle.

This is such a bizarre situation that some refuse to believe it. "It's a really cool result... if it's true," says Tommy Grav, a sceptical astronomer from the University of Hawaii.

The object, known as 2003 EL61 and nicknamed 'Santa', is a bit smaller than Pluto, has its own moon, and spends half of its time outside Pluto's orbit, and half of its time closer to the Sun.

Its existence was announced in July by two teams of astronomers (see 'Santa and little helper seen beyond Pluto' ). But further observations by one of the teams have now revealed the strangest characteristic of this new planetoid: it is spinning at an unprecedented speed for something of its size, giving it a 'day' of just 3.9 hours.

"That rotation makes it stretch," says David Rabinowitz of Yale University, Connecticut, one of the object's discoverers.

The Earth bulges similarly at the equator because of its rotation, but it has spread out evenly, like a rugby ball. Theory predicts that a very rapidly spinning body could bulge out along just one axis, and this seems to be what has happened to 2003 EL61.

One possibility is that the planetoid recently collided with a massive chunk of rock, suggests Grav, speeding it up and encouraging elongation along one axis. Rabinowitz's team hopes that further observations with the Hubble Space Telescope will confirm its shape.

Trick of the light?

The team has considered the possibility that the elongation might be an optical illusion, caused by having another moon orbiting extremely close to the planet that periodically blocks reflected light. "But that system wouldn't be stable," says Rabinowitz, who presented the discovery at the American Astronomical Society's Division of Planetary Sciences meeting in Cambridge on 8 September.


Others say it might be an illusion caused by variation in surface materials on different sides of the planet.

The astronomers are not yet sure what the planetoid is made of, although it seems to be a mix of water ice and rock. The surface is mostly covered with ice, says Rabinowitz, which is probably cracked from the stresses of the rotation.

If it is indeed the shape it looks, then it might some day snap in two. "It's very close to being unstable," says Rabinowitz.

"If you spun a body twice as fast, it would probably turn into two bodies," adds Rabinowitz's colleague Chad Trujillo of the California Institute of Technology in Pasadena.