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New Horizons Probe Reveals That Pluto is Bigger Than Expected

But so far it still won’t be considered a “planet”

A color image of Pluto and its moon Charon captured on July 11, 2015 by New Horizons (NASA-JHUAPL-SWRI)

As NASA’s New Horizons probe draws closer to Pluto, fans back on Earth are being rewarded with clearer images of the dwarf planet. Researchers are also learning more about the rocky body out in the furthest reaches of our solar system. The newest tidbit of information might excite people who still wish Pluto was a planet: Pluto is bigger than scientists thought.

For Slate, astronomer Phil Plait reports:

The more recent images from New Horizons show that Pluto is 2,370 kilometers ± 20 km across. Previously, it was measured to be 2,368 ± 20 km (some estimates put that diameter a bit lower, too). The size of Pluto is important, because we already know its mass — the time it takes Pluto and its moon Charon to orbit each other give us that, since the orbits depend on the gravity and therefore the masses of the two objects. The size gives us the density, and that in turn gives insight into what Pluto is made of. Ice is less dense, rock more.

The difference makes Pluto just 0.1 percent larger (just over a mile) than previously estimated. This might seem small, but it's still an enticing bit of data. In a press release from NASA mission scientist Bill McKinnon explained why this is exciting.  “The size of Pluto has been debated since its discovery in 1930. We are excited to finally lay this question to rest,” he said. 

The measurement also means that Eris, another Kuiper Belt object, is actually smaller in size than Pluto. When Eris was discovered in 2005, its brightness led scientists to suspect that it was larger than Pluto. They knew for sure that it was 1.25 times more massive than Pluto. But in 2010, when Eris crossed in front of a faint star in 2010, astronomers watching were able to make a better estimate of its size, and decided that perhaps it wasn't larger after all. In fact it was tough to tell which object held the title for largest in the Kuiper Belt, explains Emily Lakdawalla in a blog post for The Planetary Society

With the newest measurement, the answer is clear. "Pluto is almost certainly the largest object in the Kuiper Belt," Plait writes for Slate. But Eris is still more dense and therefore more massive.

Will this new information promote the icy body back to planet status? After all, the discovery of Eris was one of the reasons Pluto was eventually reclassified. Plait argues that it doesn’t really matter — the word "planet" is a bit hazy in terms of a definition. "Pluto doesn’t care what you call it! We need to study it for what it is, not what we want it to be," he writes.

When Pluto was "demoted," the International Astronomical Union laid out their criteria for a planet. Pluto checks off the first two (it orbits around the sun and has enough mass to assume a nearly round shape), but it still doesn’t fulfill the last: It has not cleared the neighborhood around its orbit of other large objects. 

So the IAU probably won’t reverse their decision. Plus, scientists are just beginning to explore the reaches of the Kuiper Belt. Some suspect other planets or dwarf planets could still be discovered there. Pluto may not hold this title of "largest in the Kuiper Belt" forever.


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