The planet was first spotted on 31 October 2003 with the Samuel Oschin Telescope at Palomar Observatory near San Diego, California.
US astronomer Mike Brown said he and two colleagues had discovered what appears to be a 10th planet that would force a redrawing of the celestial map and the rewriting of school text books.
The astronomers have proposed a name for the "planet" to the science's governing body, the International Astronomical Union (IAU), and are awaiting the decision of this body before announcing it.
"Get out your pens. Start re-writing textbooks today," said Brown, a professor of planetary astronomy, announcing what he called "the 10th planet of the solar system," one that is larger than Pluto.
The following is a list of the nine accepted planets in Earth's solar system - starting with the closest body to the sun and ending with the furthest - and their dates of discovery:
1) Mercury: Second smallest planet, known since at least the time of the Sumerians in the third millennium BC,
2) Venus: Known since before recorded history,
Planet 2003UB313 was spotted
from Palomar Observatory
4) Mars: The red planet, seen by stargazers since prehistoric times,
5) Jupiter: The largest planet in solar system, known since ancient times,
6) Saturn: The ringed planet, second-largest in the solar system, rings first observed by Galileo Galilei in 1610,
7) Uranus: Third largest planet by diameter, first planet not known since ancient times, first recorded sighting by John Flamsteed in 1690,
8) Neptune: Fourth largest planet by diameter, first discovered by Galileo on December 28, 1612 after he mistook it for a fixed star,
9) Pluto: The smallest planet in the known solar system, discovered by Clyde Tombaugh at Lowell Observatory in US state of Arizona on February 18, 1930
10) 2003UB313: Unconfirmed new planet discovered 8 January 2005 by Mike Brown, Chad Trujillo and David Rabinowitz from Palomar Observatory near San Diego, California.