Pluto has regained its position as the ninth largest body to orbit the Sun. Probably.

When scientists first discovered the dwarf planet Eris back in 2005, they claimed that the icy body was actually larger than Pluto. But when Eris' orbit passed in front of a dim star late last year, astronomers got their first chance to take a good hard look at the dwarf planet's actual diameter. And as it turns out, Eris and Pluto are almost identical in size; in fact, Pluto might actually be just a little bit bigger.

We first reported on the team's findings about a year ago, but until recently the researchers' analyses had yet to be confirmed by a peer review process. Now, however, their results appear to be official; the team's conclusions were published in the October 26th issue of Nature.


The latest measurements put Eris' diameter at an estimated 2,326 kilometers (1,445 miles), while a similar set of measurements, published back in 2009, estimated that Pluto is at least 2,338 kilometers (1,453 miles) in diameter.

"It could be smaller, it could be larger; basically, it is a twin," said Paris Observatory astronomer Bruno Sicardy, the lead researcher on the investigation.

In other words, the two planets are almost identical in size (even if Eris is actually more massive than Pluto). But Pluto is technically a smidge bigger. You know, if you want to be technical about it. But who's really keeping track, right? It's not like the discovery of Eris was what first prompted astronomers to demote Pluto to dwarf-planet status in the first place, or anything. Oh wait, it totally was.


So what happens from here? Does Pluto regain planet status; do Pluto and Eris both become official planets; or do we simply continue regarding them as non-planets? Amanda Gulbis—a planetary scientist from the South African Astronomical Observatory who was not involved in the Nature study—has a terse answer at the ready for the LA Times:

"Pluto is never going to go back to being a planet. The definition has been set."


[Nature (doi:10.1038/nature10550) via Cosmic Log + The LA Times]
Top image by Alexander Novati/JPL via Cosmic Log

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