This artist's concept is of the world currently cataloged as 2003 UB313. The "10th planet" was discovered with the Samuel Oschin Telescope at the Palomar Observatory near San Diego.
For now, they're calling it Xena.
Astronomers have discovered what many are saying is our Solar System's 10th planet. It's reportedly to be named Xena, and its discovery is the first of a planet since Pluto was found 75 years ago.
Xena - named after the protagonist of the TV series "Xena, Warrior Princess" -- is an object whose size hasn't been precisely determined, but it could be about 1.5 times the size of Pluto. That makes it a frozen world perhaps a little smaller than our Moon, but one much farther away.
For now, the new world is officially dubbed 2003 UB313. The International Astronomical Union will have to rule on any less-esoteric designation.
By that or any other name, however, it's a truly significant find. It's the largest nonstellar object now known to exist beyond the orbit of Neptune. And, at roughly 97 AU, or Astronomical Units (one AU equals the Earth's distance from the Sun, about 93 million miles) it's also the most distant.
First imaged by Caltech astronomers in 2003 - hence the 2003 in its temporary moniker - it wasn't recognized as part of the Solar System till later. Then scientists re-examined images taken of the region over time and noticed an object that appeared to move against the stellar field. That motion indicated that UB313 was not a distant star but an object orbiting the Sun. And the investigation began in earnest.
Official announcement of "Xena" was made July 29 by discoverers Michael E. Brown of Caltech, Chad Trujillo of Gemini Observatory and David Rabinowitz of Yale University. It was somewhat premature, prompted leaks of the data and images of the new planet.
The disclosure on UB313 came just hours after an announcement that another team had discovered another world beyond Neptune that was somewhat smaller than Pluto.
UB313 is in a highly elliptical orbit inclined about 45 degrees from the main plane of our Solar System. Currently it's near aphelion (farthest distance from the Sun) at some 9 billion miles from the center of the Solar System. It gets as close to the Sun as 38 A.U., or some 3.5 billion miles, during a 557-year orbit.
By contrast, Pluto's mean distance from the Sun is just 39 A.U., or 3.6 billion miles, and it orbits in just 248.5 years.
The new world is thought to be about 2,100 miles in diameter, roughly 1.5 times as large as Pluto. Near-infra-red spectrometry indicates that, like Pluto, it has methane ice on its surface.
"Xena" has not been discovered until now, because no one had thought to look in an orbit so far removed from those of the other planets in our Solar System.