A 955-mile dwarf planet has been hiding in our Solar System

Most people reading this will be old enough to still consider Pluto a planet, even after it was humiliatingly downgraded to “dwarf planet” status a decade ago. Textbooks before then listed nine planets in our solar system, but it’s interesting to speculate that in those less fussy times, it could have been ten*, were we aware of a dwarf planet only slightly smaller than Pluto that’s been lurking in our back yard this whole time.

True, that’s a backyard some distance away, hidden

deep in the Kuiper Belt, around twice the distance of Pluto to the Sun, but 2007 OR10 is still very much in our galactic postcode. It turns out that this distant neighbour is a whopping 955 miles in diameter, which isn’t too far off Pluto’s 1,475 miles.  As the name suggests, we’ve actually been aware of the dwarf planet since 2007, but now we’ve learned it’s only slightly smaller than Pluto, we should probably give it a better name.

For nine years, 2007 OR10 was considered of relatively minor interest, estimated to be around 795 miles in diameter. That may not sound like a huge difference, but bear in mind we know of around 200 objects in our system that might be dwarf planets, so in the greater scheme of things, one more wasn’t a big deal.

So how did 2007 OR10 hide in plain sight for so long? Well, like so much in the very distant corners of our solar system, the answer is that it’s very hard to see. It’s a very dark planet – with a deep red surface, probably indicating a coating of methane ice – and as a result it doesn’t reflect much light at all.

On top of that, it spins really slowly, with days lasting a tedious sounding 45 hours, and it has a strange elliptical orbit that makes it hard to keep track of. Check out the fuzzy footage from the Kepler telescope below and see for yourself.

We understand it better thanks to a team of Hungarian researchers at the Konkoly Observatory, who combined K2 mission observations with the Herschel Space Observatory. By calculating light reflection against heat radiation data, they came up with the 955-mile measurement.

“Our revised larger size for 2007 OR10 makes it increasingly likely the planet is covered in volatile ices of methane, carbon monoxide and nitrogen, which would be easily lost to space by a smaller object,” lead researcher András Pál told Phys.org. “It’s thrilling to tease out details like this about a distant, new world – especially since it has such an exceptionally dark and reddish surface for its size.”

The team plan to use the same technique to study some of the other objects around the edge of our solar system, so more surprises could be on the way. For now, though, it’s about time 2007 OR10 had its official christening.

*Yes, yes, probably 11, because if we invited 2007 OR10 to the party, we’d almost certainly include Eris (1,445 miles), but that would have made a massively overrun sentence. Like this one.

READ NEXT: With so many planets in the universe, where are all the aliens?

Image Credit: Konkoly Observatory/András Pál, Hungarian Astronomical Association/Iván Éder, NASA/JHUAPL/SwRI

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