for National Geographic News
Poor Pluto just keeps getting downgraded.
The beleaguered former planet has been dwarfed again by a new study that finds its neighbor Eris is significantly bigger.
That makes Eris—formerly known as 2003 UB313 and then Xena—the largest so-called dwarf planet in the solar system.
Eris was the catalyst for Pluto's demotion last summer, when the International Astronomical Union redefined the term "planet" and created the category of dwarf planet for objects such as Eris and Pluto.
Using the Keck Observatory and the Hubble Space Telescope, Michael E. Brown and Emily Schaller at the California Institute of Technology in Pasadena have now put the mass of Eris at a third more than Pluto's.
The paper appears in this week's issue of the journal Science.
The key to finding Eris' mass was its tiny moon, Dysnomia. Brown and Schaller used Keck and Hubble to capture images of the moon's position over time.
The data suggest that Dysnomia is Eris' only moon and orbits every 15 days.
The researchers also calculated the total mass of the pair—about 1.27 times that of Pluto—and revealed that the objects are made of around 70 percent rock, making them as dense as Pluto and Triton, a moon of Neptune.
Pluto itself has been described as a ball of rock and ice just bigger than Texas.
Alan Stern is principal investigator of NASA's New Horizons mission to Pluto. When he first read the paper, he said the researchers compared "apples to oranges" by putting Pluto in the ring with two objects—Eris and its moon.
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