Pluto Is Bigger Than We Thought

Scientists determined it is roughly 1,473 miles wide – which makes it bigger than Eris.

By Nadia Drake
Pluto, as seen by New Horizons on July 12, 2015
Photograph by NASA, Jhuapl, SwRI

Pluto, the small, frozen dwarf planet at the edge of the solar system, has been officially declared the largest known world in its neighborhood. The planet is just one of many icy bodies in the Kuiper Belt, a vast debris ring beyond the orbit of Neptune. There has been some uncertainty over the last decade about whether Pluto is the biggest among those worlds, or if recently discovered dwarf planet Eris might take that title.

Today, we wonder no longer.

“Pluto is a little bit larger than we anticipated,” said New Horizons principal investigator Alan Stern at a press conference this morning. “That settles the debate about the largest object in the Kuiper Belt.”

Scientists on the New Horizons team determined that Pluto is roughly 1,473 miles wide (give or take 12 miles) – which makes it bigger than Eris by about 30 miles. But Eris is still more massive, which means the two worlds are made of somewhat different ingredients. In fact, Pluto’s diameter is large enough that scientists are having to reconsider how it is put together, and what that means for characteristics such as its atmosphere.

Though Pluto was discovered in 1930, its exact size has been a mystery until now. The planet’s puffy nitrogen atmosphere that makes it difficult for distant telescopes to see all the way down to the world’s surface. As such, Pluto’s precise width couldn’t be determined from afar. Now that New Horizons is in Pluto’s immediate neighborhood, it can answer this persistent question — and others.

Today, scientists also shared some more data from the spacecraft. For starters, the recently spotted presumptive north polar cap is, in fact, a polar cap. Results suggest it’s made of nitrogen and methane ice, both constituents of Pluto’s surface frosts. Next, the spacecraft has been detecting Pluto’s escaping nitrogen atmosphere for five days now – which is much sooner than anticipated.

Of course, the New Horizons spacecraft is scheduled to fly through the Pluto system in less than 24 hours – and all signs suggest the spacecraft is right on course to fly within 8,000 miles of the dwarf planet at 12:50 pm. BST.

“We are good to go for this last part of the trip,” said New Horizons project manager Glen Fountain.

Family Portrait

Images released yesterday also provide the best looks we’ve gotten so far of Pluto and its large moon Charon, two worlds that would look totally unrelated if they weren’t locked in a whirling dance with one another.

Pluto, which is large and orangey-red, is covered in dark splotches and some bright patches. So far, its mottled surface appears relatively smooth and devoid of cliffs and craters – but that could change as New Horizons gets closer.

“Pluto has these large amoeboid and blobby things all over it that look very different from the other ice worlds we’ve looked at to date – with the possible exception of Triton,” said the Lunar and Planetary Science Institute’s Paul Schenk, referring to the Neptunian moon most scientists guessed would closely resemble Pluto. But as New Horizons gets closer to Pluto, Schenk said, the worlds are looking curiously different from one another.

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