Caltech astronomer Mike Brown, who led the discovery, said Sedna's slow rotation rate had him convinced there was an unseen satellite exerting a gravitational tug.

Here's why: Most objects in the solar system that don't have companions complete a rotation, or day, in a matter of hours. There are many examples of fast-spinning asteroids and similarly whirling large, round Kuiper Belt objects. Pluto, on the other hand, has had its rotational period slowed to six Earth-days by its companion, Charon.

Sedna spins on its axis once every 20 Earth-days, or perhaps even more slowly, making the presence of a moon practically inevitable, Brown had thought. So shortly after the discovery, Hubble was pointed at Sedna.

"Much to our surprise, there's no satellite," he told reporters today.