What defines a planet? New finds put the answer in doubt.
The discovery of an icy world beyond Pluto, and a moon circling it, points to an unforeseen diversity of objects.
The discovery of a tiny moon circling the most distant object seen in the solar system is further proof that the view of a tidy solar system with nine planets - enshrined in science-fair dioramas and school textbooks - is headed toward almost certain revision.
In July, astronomers announced the discovery of what they considered the 10th planet, an icy world that swings 9 billion miles away from the sun and is almost certainly larger than Pluto. This weekend, they declared this object, informally known as Xena, also has a most planetlike feature: a moon.
Whether Xena is in fact a planet will be the decision of the International Astronomical Union (IAU), which could instead begin a far more fundamental reexamination of what a planet is.
Whatever its final classification, though, Xena is but one in a series of new discoveries in the solar system that point to an unforeseen diversity of intriguing objects beyond the nine planets.
In the asteroid belt between Mars and Jupiter, for instance, scientists recently found the first "triple asteroid" - two asteroids orbiting a third.
In addition, they discovered that the largest known asteroid, Ceres, is probably a failed planet. It is not just a glob of rock. It is almost perfectly round, suggesting that it has a rocky core separated from an outer crust - like Earth. Only Jupiter's disruptive gravity prevented Ceres from accumulating more mass and becoming a planet.
Then again, Xena could redefine what a planet is. "It's going to reignite the planet debate," says Marcos van Dam, who helped discover Xena's moon, Gabrielle.
The planet debate dates back to 1930 and the discovery of Pluto. Even then, Pluto was seen as an oddity - a tiny ball of ice wheeling among gas giants on an unusually elliptical orbit tilted far above and below the plane of the other eight planets. Yet in 1930, Pluto was unique, so it was deemed a planet.
Now, astronomers have found other worlds like Pluto in the Kuiper Belt - a band of frigid and far-flung objects beyond Neptune. Last year, they found Sedna, a curiously reddish body with its own moon. Now, with Xena, they have found a Kuiper Belt object larger than Pluto, and they could find scores more such "planets" - leading IAU to reconsider the term.
None of them, however, will probably be named after TV show characters, like Xena the Warrior Princess. Xena is technically named 2003 UB313 and will remain so until the IAU decides whether it is a planet. And IAU chooses the name itself.