The 2005 discovery of a solar system object larger than Pluto was a crushing uppercut to the cold world's planetary status. But it's the official weigh-in, reported tomorrow in Science, that scores the technical knockout. Researchers have calculated that the newfound world, Eris, is significantly more massive than its fellow ice giant, closing the chapter on Pluto's fall from king of the Kuiper belt to lowly dwarf planet.
The heavyweight matchup is the latest in a series of battles that have roiled the astronomy community for the past year. Two years ago, Eris's discovery forced the International Astronomical Union (IAU) to take a hard look at its own dictionary: If Eris is larger than Pluto, should it be a planet? If not, how could Pluto possibly retain its planetary status? (ScienceNOW, 1 August 2005) After intense debate and a concomitant media circus, IAU created a new type of object in August 2006, reclassifying Pluto as a dwarf planet, along with Eris and Ceres, a large, rocky body orbiting between Mars and Jupiter (ScienceNOW, 24 August 2006). Forevermore, our solar system would have only eight planets, unless IAU decided to change its mind again.
The decision was a dark day for Pluto lovers. Still, they held out hope that the icy world would remain the heavyweight of the Kuiper belt. Although Eris's diameter is 2400 kilometers, which bests Pluto by about 100 kilometers, there was a chance it would turn out to be less dense than its neighbor.
Alas, it was not to be. Eris's discoverer, Michael Brown, a planetary scientist at the California Institute of Technology in Pasadena, California, along with Caltech colleague Emily Schaller, weighed the dwarf planet by using the Keck Observatory and the Hubble Space Telescope to determine the orbit of its satellite, Dysnomia. Considering the radius of Dysnomia's orbit and the time it takes to complete one lap, the team calculated the mass of Eris to be 16.6 billion trillion kilograms. That bests Pluto by 27% and makes Eris the most massive of the three known dwarf planets.
But Eris's reign may not last long. Portions of the Kuiper belt have yet to be explored, says astronomer Frank Bertoldi of the University of Bonn in Germany, who expects a few more dwarf planets to be found. As to whether demoting Pluto was a good idea in the first place, Bertoldi thinks the decision hasn't fazed at least one segment of the population. "The schoolkids still like Pluto whether it's a minor planet, a dwarf planet, or a planet," he says. "Pluto is Pluto, and it will stay out there no matter what we call it."