Astronomer Michael Brown of the University of California at Berkeley has more bad news for Pluto. Everyone’s favorite former planet, it turns out, is not even the heftiest dwarf. That distinction now belongs to Eris (formerly Xena). Brown’s discovery of Eris in 2003 kicked off the what-is-a-planet-anyway fracas last year at the annual meeting of the International Astronomical Union, which reclassified Pluto as a “dwarf planet.”

While most astronomers suspected that Eris was bigger than Pluto, no one was sure. Brown’s previous observations suggested that Eris was about 5 percent larger than Pluto, but “the uncertainty was 4 percent, awfully close to being the same size,” he says. It was very plausible that if Eris were made of just barely lighter material, it would weigh a little less.” Now, using the method pioneered by Sir Isaac Newton when he determined the mass of Jupiter, Brown and his colleagues have determined that Eris, a far-flung rocky mass with a wildly eccentric orbit, is 27 percent more massive than Pluto. “Eris is my baby,” says Brown. “It was like having a child. I feel almost protective, and it’s hard not to keep meddling with it.”

To zero in on the mass, Brown and his colleagues looked at the interaction of Eris with its moon, Dysnomia, through the Keck Observatory in Hawaii and the Hubble Space Telescope. They used models detailing the gravitational pull between the two objects to deduce that Eris weighs in at 16.6 billion trillion kilograms, making it the most massive dwarf planet known.

That still won’t land it in the sought-after planet category, but all is not lost, says Brown. Both Pluto and Eris move through space in the Kuiper belt, a collection of rocky objects just beyond Neptune’s orbit. While it is unlikely that astronomers will continue to find larger objects in the belt, Brown says that the region outside the belt, in the coldest hinterland of the solar system, could very well hold planet-size rocks. “Beyond Neptune’s orbit is the solar system’s next frontier,” says Bob Millis, head of Arizona’s Lowell Observatory. “There are really interesting objects out there that remain to be discovered.”

If astronomers do find a big enough body, the debates over the word “planet” could reignite. “I suspect that if we find something bigger than Mercury in the region out there, then all this will start all over again,” Brown says. “The astronomers will sit around and argue endlessly; first they’ll say that this isn’t important and then proceed to tell you for three hours why they’re right.”