Is Pluto the biggest dwarf planet after all?

Pluto’s controversial demotion from planetary status came in 2006 after the rapid discovery of comparably sized bodies—now named Haumea, Makemake and Eris—made Pluto look rather ordinary. In particular, Eris was found to be larger in diameter than Pluto, raising the question of what separated a planet from numerous smaller bodies. The International Astronomical Union decided on a new definition for planets that resulted in a paring down the solar system’s tally of planets to eight, relegating Pluto and its ilk to dwarf planet status.

Pluto lovers of the world may take some small comfort in a new look at Eris that puts Pluto back in the running for the largest dwarf planet, diameter-wise. (Eris seems to retain a lock on the title of most massive dwarf planet for the time being.) Measurements taken as Eris temporarily blotted out the light of a distant star indicate that the dwarf planet’s diameter is on par with, and maybe even smaller than, that of Pluto.

Eris is extremely distant, orbiting much farther from the sun than even Pluto does, and it is difficult to get a good look at the relatively small world. Although initial thermal readings pegged Eris at about 3,000 kilometers (km) in diameter, later infrared observations taken with the Spitzer Space Telescope indicated a diameter of roughly 2,600 kilometers (km), whereas Hubble Space Telescope measurements pointed to a diameter of 2,400 km. Pluto, in comparison, is about 2,300 kilometers across.

On the night of November 5, a fortuitous alignment provided the new data point. As Eris cruised through its orbit, some 14 billion km from Earth, it passed in front of a distant star from Earth’s vantage point, casting a small shadow across our planet in an event known as an occultation. By timing the duration of the occultation at multiple sites, researchers can estimate the size of the shadow and hence the size of the object.

According to Sky & Telescope, three teams witnessed the occultation from sites in Chile. Based on those measurements, astronomer Bruno Sicardy of the Paris Observatory told the magazine that Eris’s diameter is “almost certainly” smaller than 2,340 km.

Mike Brown of the California Institute of Technology, one of the co-discoverers of Eris who seems to relish his role in the Pluto controversy (his Twitter handle is plutokiller), noted on his Web site that the results, though preliminary, are tantalizing. For if Pluto and Eris are approximately the same diameter, yet Eris is substantially more massive, their composition must be fundamentally different. “How could Eris and Pluto look so similar in size and exterior composition yet be totally unalike on the inside?” Brown wrote. “As of today I have absolutely no idea.”

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