The Dwarf Planet Known as Eris is More Massive than Pluto, New Data Shows PASADENA, Calif.--Die-hard Pluto fans still seeking redemption for their demoted planet have cause for despair this week. New data shows that the dwarf planet Eris is 27 percent more massive than Pluto, thereby strengthening the decree last year that there are eight planets in the solar system and a growing list of dwarf planets. According to Mike Brown, the discoverer of Eris, and his graduate student Emily Schaller, the data confirms that Eris weighs 16.6 billion trillion kilograms. They know this because of the time it takes Eris's moon, Dysnomia, to complete an orbit. "This was Pluto's last chance to be the biggest thing found so far in the Kuiper belt," says Brown, a professor of planetary astronomy at the California Institute of Technology. "There was a possibility that Pluto and Eris were roughly the same size, but these new results show that it's second place at best for Pluto." Eris was discovered in 2005 with Palomar Observatory's 48-inch Samuel Oschin Telescope, an instrument specially adapted to do comprehensive searches for objects in the sky. When it became apparent that Eris was similar in size if not larger than Pluto, Brown and others called for the International Astronomical Union to rule on its planetary status. The end result was demotion of Pluto and the redesignation of it and other Kuiper-belt objects as dwarf planets. Schaller says that the new results, obtained with Hubble Space Telescope and Keck Observatory data, indicate that the density of the material making up Eris is about two grams per cubic centimeter. This means that Eris very likely is made up of ice and rock, and thus is very similar in composition to Pluto. Past results from the Hubble Space Telescope had already allowed planetary scientists to determine that its diameter is 2,400 kilometers, also larger than Pluto's. "Pluto and Eris are essentially twins--except that Eris is slightly the pudgier of the two," says Brown. "And a little colder," adds Schaller. The reason for Eris's blustery surface conditions is its sheer distance from the sun. Currently 97 astronomical units from the sun (an astronomical unit being the distance between the sun and Earth), Eris hovers at temperatures well below 400 degrees Fahrenheit and is pretty dark. However, things get a little better on Eris now and then. Orbiting the sun on a highly elliptical 560-year journey, Eris sweeps in as close to the sun as 38 astronomical units. But at present it is nearly as far away as it ever gets. Pluto's own elliptical orbit takes it as far away as 50 astronomical units from the sun during its 250-year revolution. This means that Eris is sometimes much closer to Earth than Pluto, although never closer than Neptune. Based on spectral data, the researchers think Eris is covered with a layer of methane that has seeped from the interior and frozen on the surface. As in the case of Pluto, the methane has undergone chemical transformations, probably due to the faint solar radiation, causing the methane layer to redden. But the methane surface on Eris is somewhat more yellowish than the reddish-yellow surface of Pluto, perhaps because Eris is farther from the sun. As for Dysnomia, the tiny satellite remains the only moon discovered orbiting Eris so far. Dysnomia is about 150 kilometers in diameter, is about 37,000 kilometers from Eris, and has a lunar "month" that lasts 16 days. "But every year is 560 Earth-years," says Brown. "So on Eris they have a lot more months in their calendar." Like the Earth-moon system, Eris-Dysnomia probably formed about 4.5 billion years ago following a massive collision. Brown and Schaller are the authors of a paper, "The Mass of Dwarf Planet Eris," appearing in the June 15 issue of the journal Science. The search for new planets and other bodies in the Kuiper belt is funded by Caltech and NASA. For more information on the program, see the Samuel Oschin Telescope's website at http://www.astro.caltech.edu/palomarnew/sot.html. For more information on Mike Brown's research, see http://www.gps.caltech.edu/~mbrown. To learn more about Eris, see http://www.planeteris.com. An Eris image is available at http://www.gps.caltech.edu/~mbrown/planetlila/moon/hst.jpg Contact: Robert Tindol tindol@caltech.edu (626) 395-3631
On savait depuis bientôt six mois qu'Éris était légèrement plus grand que Pluton, de nouvelles observations viennent de montrer qu'il est également 27 % plus massif que son congénère, enfonçant le clou de la rétrogradation de Pluton du statut de planète à celui de planète naine en août dernier.
Tailles comparées d'Éris, Pluton et quelques-uns des plus gros objets connus (la Terre en bas).

Pour mémoire : depuis sa découverte en 1930 jusqu'à 2006, Pluton a été considéré comme la neuvième planète du système solaire, la plus éloignée du Soleil mais également la plus petite. Au début des années 2000, les découvertes de plus en plus fréquentes d'objets similaires en taille aux confins du système solaire ont alors amené l'instance chargée de coordonner les travaux des astronomes à travers le monde (Union astronomique internationale) à se poser la question de ce qu'est réellement une planète. Ces réflexions ont aboutit à une refonte de la nomenclature des objets orbitant autour du Soleil, avec la création d'un nouveau type d'objet : les planètes naines, dont font désormais partie Pluton et Éris.

Cette décision de déchoir Pluton de son rang de planète avait provoqué l'émoi d'une petite partie de la communauté scientifique et du grand public, qui conservait l'espoir de voir cette décision annulée ultérieurement.

Ces mesures récentes tendent à confirmer que Pluton n'est pas le « roi de la ceinture de Kuiper » comme on l'a longtemps cru, mais un objet somme toute assez commun dans cette région reculée.

Mike Brown — co-découvreur d'Éris en 2005, et à l'origine de ces nouvelles mesures — a trouvé un allié précieux pour calculer sa masse : Dysnomie, le satellite naturel d'Éris. L'observation très précise de l'orbite du satellite à l'aide du télescope spatial Hubble et du Keck de Hawai a permis, par déduction, de remonter à la masse de chacune des composantes, ce qui donne pour Éris 1,67 x 10 ± 0.02 x 10 kilogrammes.

Ces résultats sont publiés dans la revue scientifique ''Science'' datée du 15 juin 2007.