Related Links More about the Tenth Planet Samuel Oschin Telescope Planetary Scientists Discover Tenth Planet PASADENA, Calif.--A planet larger than Pluto has been discovered in the outlying regions of the solar system with the Samuel Oschin Telescope at Palomar Observatory, California Institute of Technology planetary scientist Mike Brown announced today. The planet is a typical member of the Kuiper belt, but its sheer size in relation to the nine planets already known means that it can only be classified as a planet, Brown says. Currently about 97 astronomical units from the sun (an astronomical unit is the distance between the sun and Earth), the planet becomes the farthest-known object in the solar system, and the third brightest of the Kuiper belt objects. "It will be visible over the next six months and is currently almost directly overhead in the early-morning eastern sky, in the constellation Cetus," says Brown, who made the discovery with colleagues Chad Trujillo, of the Gemini Observatory, and David Rabinowitz, of Yale University, on January 8. Brown and Trujillo first photographed the new planet with the 48-inch Samuel Oschin Telescope on October 31, 2003. However, the object was so far away that its motion was not detected until they reanalyzed the data in January of this year. In the last seven months, the scientists have been studying the planet to better estimate its size and its motions. "It's definitely bigger than Pluto," says Brown, who is professor of planetary astronomy. Scientists can infer the size of a solar-system object by its brightness, just as one can infer the size of a faraway light bulb if one knows its wattage. The reflectance of the planet is not yet known--in other words, it's not yet possible to tell how much light from the sun is reflected away--but the amount of light the planet reflects puts a lower limit on its size. "Even if it reflected 100 percent of the light reaching it, it would still be as big as Pluto," says Brown. "I'd say it's probably one and a half times the size of Pluto, but we're not sure yet of the final size. "But we are 100 percent confident that this is the first object bigger than Pluto ever found in the outer solar system." Determination of the upper limit of the size of the planet is constrained by results from the Spitzer Space Telescope, which has already proved its mettle in studying the heat of dim, faint, faraway objects such as the Kuiper-belt bodies. Because the Spitzer is unable to detect the new planet, the overall diameter must be less than 3,000 kilometers, Brown says. A name for the new planet has been proposed by the discoverers to the International Astronomical Union, and they are awaiting the decision of this body before announcing the name. The research is funded by NASA. For more information see http://www.gps.caltech.edu/~mbrown Samuel Oschin Telescope: http://www.astro.caltech.edu/palomarnew/sot.html
La planète a été reconnue sur des photos prises en 2003, mais ce n'est qu'en 2005 que l'objet stellaire a vraiment été découvert par l'équipe de  Michael Brown, qui travaille au "California Institute of Technology" (Caltech) . L'équipe n'avait absolument pas l'intention de divulguer la trouvaille avant d'avoir de plus amples informations, mais une fuite est à l'origine de l'information.  

Pour l'instant l'objet planétoïde est dénommée ''2003 UB313''. Il se situe à 14,5 milliards de kilomètres du soleil et mesurerait une fois et demi la taille de Pluton (2600 kilomètres de diamètre). Les estimations les plus prudentes considèrent que ''2003 UB313'' fait 2300 kilomètres de diamètre :   C'est un objet transneptunien (plus éloigné du Soleil que Neptune) et qui appartient à la ceinture de Kuipper comme Sedna ou même Pluton.

C'est d'abord à l'observatoire Gemini à Hawaii qu'on peut apercevoir une trace brillante en déplacement. Mais ce n'est à l'observatoire du mont Palomar, près de San Diego que ''2003 UB313'' a été réellement classifiée et photographiée depuis.

Cette découverte relance le débat sur la définition de l'expression ''planète''. Alors que certains puristes parlent de huit planètes, ne considérant pas Pluton comme telle, il faut se demander si c'est seulement la taille qui fait d'un objet quelconque de la ceinture de Kuipper une planète à part entière. M.Brown estime que  . En effet, sa surface serait constituée de glace de méthane, ce qui est une première pour un objet aussi éloigné du Soleil.

La petite nouvelle pourrait s'appeler Xena (héroïne de la série télévisée éponyme), ce que Michael Brown appelle de ses vœux :  .