We've continued to keep a watch for objects in orbit around the Sun that could possibly be called a "planet" -- and it turns out that the most recent candidate has been hiding in plain sight. 2003 EL61 (it has yet to be given a formal name), although officially announced yesterday, was spotted initially in 2003, and images of it have been found in archives as far back as 1955.
2003 EL61 was first spotted by Jose-Luis Ortiz at the Sierra Nevada Observatory in Spain in March of 2003; the team's website includes some of the initial details, as well as the first photo used to identify the body.
Jose-Luis Ortiz and his group were not the only ones to have stumbled across this object. Mike Brown at Cal Tech also found it, in mid-2004 (although he didn't identify it until December, 2004). It turns out that, in astronomy, naming and primacy of discovery accrues to the team that announces first. If Brown and his group had called a press conference for last week, they'd be given credit for the find, even though Ortiz, et al, had spotted it first.
these are extracts from an article on worldchanging on 29th July link
announced on 31st july
A US astronomer has announced the discovery of a 10th planet in the outer reaches of the solar system that could force a redrawing of the astronomical map.
If confirmed, the discovery - by Mike Brown, a professor of planetary astronomy at the respected California Institute of Technology - would be the first new planet since Pluto was identified in 1930 and would shatter the notion that nine planets circle the sun.
"Get out your pens - start rewriting textbooks today," said Professor Brown, announcing what he called "the 10th planet of the solar system", larger than Pluto.
"It's the farthest object ever discovered to orbit the sun," Professor Brown said of the planet, said to be covered in methane ice and which lies nearly 15 billion kilometres from Earth. "It's probably 1½ times the size of Pluto."
About 97 times further from the sun than Earth is, the celestial body (tentatively called "2003-UB313") is the farthest known object in the solar system and the third brightest among the Kuiper Belt objects. It was first spotted in 2003, but not identified as a planet.
as reported in the SMH
note also the haggling over size & distance