Sedna Mystery Deepens With Hubble Images Of Farthest Planetoid

This unexpected result might offer new clues to the origin and evolution of objects on the far edge of the solar system.

Sedna's existence was announced on March 15. Its discoverer, Mike Brown of the California Institute of Technology, Pasadena, Calif., was so convinced it had a satellite, that an artist's concept of Sedna released to the media included a hypothetical moon.

Brown's prediction was based on the fact, Sedna appears to have a very slow rotation that could best be explained by the gravitational tug of a companion object. Almost all other solitary bodies in the solar system complete a spin in a matter of hours.

"I'm completely baffled at the absence of a moon," Brown said. "This is outside the realm of expectation and makes Sedna even more interesting. But I simply don't know what it means." Immediately following the announcement of the discovery of Sedna, NASA astronomers turned the HST toward the new planetoid to search for the expected companion moon. The space-based platform provides the resolving power needed to make such precision measurements in visible light. "Sedna's image isn't stable enough in ground-based telescopes," Brown said.

Surprisingly, the HST images, taken March 16 with the new Advanced Camera for Surveys, only show the single object Sedna, along with a faint, very distant background star in the same field of view.

Even with Hubble's crisp view, it may just be barely resolving the disk of mysterious Sedna, Brown said. It's equivalent to trying to see a soccer ball 900 miles away. This would place an upper limit in the object's size of being approximately three-quarters the size of Pluto, or about 1,000 miles across.

But Brown predicted a satellite would pop up as a companion "dot" in Hubble's precise view. But the object is not there. There is a very small chance, it might have been behind Sedna or transiting in front of it, so it could not be seen separately from Sedna in the HST images.

Brown based this prediction on his earlier observations of apparent periodic changes in light reflecting from Sedna's mottled surface. The resulting light curve gives a rotation period of 40 days. If true, Sedna would be the slowest rotating object in the solar system after Mercury and Venus, whose slow rotation rates are due to the tidal influence of the sun.

One easy way out of this dilemma is the possibility the rotation period is not as slow as astronomers thought. But even with a careful reanalysis, the team remains convinced the period is correct. Brown admits, "I'm completely lost for an explanation as to why the object rotates so slowly."

Small bodies like asteroids and comets typically complete one rotation in a matter of hours. Pluto has a six-day period from being tidally locked to the revolution of its satellite Charon. The HST was the first telescope to resolve Pluto and Charon as two separate bodies. NASA's forthcoming James Webb Space Telescope will provide a platform for further high-resolution studies of infrared light from such distant, cold bodies in our solar system.

Electronic images of Sedna and additional information are available on the Internet, at: http://hubblesite.org/news/2004/14