Dwarf planet bigger than Pluto

In 2007, when astronomers measured the mass of the dwarf planet Eris, they found that it’s 27% more massive than Pluto.

Astronomer Mike Brown discovered Eris. He also recently found its tiny moon.

Mike Brown: We got lucky in that Eris has a moon going around it. And by tracking the orbit of the moon around the dwarf planet, you actually get to weigh how much the dwarf planet weighs.

Once they found this moon, astronomers were able to ‘weigh’ Eris. The moon, called Dysnomia, zips around Eris about every 16 Earth-days. By tracking it, Brown determined Eris’ mass.

Mike Brown: And it turns out that it’s about 27% more massive than Pluto. And the uncertainty there is only about 2%. So, this was sort of the last chance that Pluto had, and now it’s definitely smaller and less massive.

When it was discovered in 2005, some thought Eris should be considered the 10th planet of our solar system. Everyone still considered Pluto a planet then. At first, Eris was thought to be slightly larger than Pluto. Now – with the help of Eris’ moon – Eris is known to be 27% more massive than Pluto. If Pluto had remained a planet to the entire community of astronomers, surely Eris would be considered the 10th planet.

In 2006, the International Astronomical Union (IAU) voted to demote Pluto as a full-fledged planet. The IAU now considers both Eris and Pluto to be dwarf planets. And yet there are many similarities between our own Earth and moon and the dwarf planet Eris and its moon.

Mike Brown: So the moon is in a very circular orbit around Eris, and we think that that’s telling us something important. We think that it actually formed much in the same way that the Earth and moon formed, such that some sort of large object hit both of these things in the distant past and made a big cloud of debris that coalesed to form the moon. We think the same thing happened for Eris and Dysnomia.

Thanks today to Research Corporation, America’s first foundation for science advancement.

Our thanks to:
Mike Brown
Division of Geological and Planetary Sciences
California institute of Technology

August 8, 2007

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