7 Surprising Facts About Pluto

Until 2015, Pluto, the former ninth planet of the classical solar system, was largely a mystery—a tiny speck 3.6 billion miles from the sun. When NASA's New Horizons spacecraft arrived at the diminutive object in the far-off Kuiper Belt, planetary scientists discovered a geologist’s Disneyland: a mind-blowing world of steep mountains, smooth young surfaces, ice dunes, and a stunning blue atmosphere.

In 2017, Mental Floss spoke to Kirby Runyon, then a planetary geomorphologist at the Johns Hopkins University Applied Physics Laboratory and a scientist on the NASA New Horizons geology team, to learn more. Here is what you need to know about Pluto, the small world with the biggest heart in the solar system.

1. One Pluto year equals 248 Earth years.
2. The Disney dog was named after the dwarf planet (not the other way around).
3. Technically, a Pluto system space elevator is possible.
4. Its “heart” is in the right place.
5. Pluto’s bizarre orbit and rotation are unexplained.
6. Astronomers were wrong about its atmosphere ...
7. ... which is electric blue.

One Pluto year equals 248 Earth years.

At 1473 miles in diameter—about half the width of the United States—Pluto is the smallest of the nine classical planets and the largest discovered “trans-Neptunian object” (an object beyond the planet Neptune), though it was demoted to dwarf planet status in 2006. As could be expected, it is a freezing -375°F on Pluto's surface. Its gravity is about 1/15 that of Earth. It has five moons: Charon, Nix, Hydra, Kerberos, and Styx. Charon is the largest of the moons by far, with a diameter about half that of Pluto. It takes roughly 248 Earth years for Pluto to circle the sun, and during that time, its highly elliptical orbit takes it as far as 49 astronomical units from our star, and as close as 30.

The Disney dog was named after the dwarf planet (not the other way around).

Pluto the not-yet-dwarf planet was discovered on February 18, 1930 by astronomer Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona. It was named later that year by Venetia Burney, an 11-year-old girl in England. She first learned of the new, nameless planet from her grandfather, who mentioned it while reading the newspaper. Burney was interested in Greek and Roman mythology at the time, and she immediately suggested Pluto.

Her grandfather was impressed, and mentioned it in a note to a friend of his, who taught astronomy at Oxford. The astronomy professor passed word to Lowell Observatory, and the astronomers there took an immediate liking to it. It helped that the first two letters of Pluto are the initials of the observatory’s late founder, Percival Lowell. Note that Burney did not get the name from the Disney dog. Just the opposite: The dog, which premiered the same year as Pluto was discovered, was likely named by Walt to ride the planet’s publicity wave. Scientists and cartoonists alike have yet to explain how the then-unseen planet and dog ended up being more or less the same color.

Technically, a Pluto system space elevator is possible.

Space elevators are a science fiction staple, and advances in carbon nanotubes have made their prospects, if not likely, then certainly possible. The idea is to bring a large object such as an asteroid into a geostationary orbit at Earth’s equator and connect that object and Earth with a cable or structure. You could then lift things into orbit without the need for rockets. According to Runyon, the unique orbital characteristics of Pluto and Charon create interesting opportunities for the very, very distant future of engineering.

The two worlds are tidally locked. Charon's orbit is precisely the same duration as Pluto’s rotation, meaning that if you stood on Pluto’s surface, the moon would hover over the same spot, never rising or setting. “Because they are binary, tidally locked, literally orbiting each other in a perfect circle, you could build a space elevator that goes from one planet to the other, from Pluto to Charon,” Runyon told Mental Floss. “And it would touch the ground in both places, physically linking them. And you could literally climb a ladder from one to the other.”

Its “heart” is in the right place.

In 2015, the New Horizons spacecraft arrived at the Pluto system and turned the little speck into a real world. The famous first image released by NASA is not a straight-on shot from Pluto’s side, with the top being the North Pole and bottom being the south; it’s a view from Pluto's higher latitudes, looking down. (The heart, in other words, is higher up on the planet than the picture suggests.) Because New Horizons was a flyby craft and not an orbiter, planetary scientists don't know what 40 percent of the planet looks like.

Pluto’s bizarre orbit and rotation are unexplained.

The traditional classroom solar system model of a light bulb as the sun and planets on wires extending from it represents a flat orbital plane known as the ecliptic, and for most of the solar system, that's pretty close to the truth. But not for Pluto, which has a 17-degree inclination relative to the ecliptic. Moreover, like Uranus, its rotation is tipped on its side, and it rotates backward (east to west). No one knows why, according to Runyon. “It’s probably the result of an ancient impact,” he said. “One not strong enough to disrupt planet but enough to tip on its side. This might have been the Charon-forming impact, which would be similar to how our moon is formed.”

Astronomers were wrong about its atmosphere ...

Astronomers have long known that Pluto has an atmosphere. During stellar occultations (that is, when it moves in front of stars), astronomers can see the star dim, and then completely go out, and then reappear dimly, and then return to its full brightness. That dimming is caused by the planet’s atmosphere. Astronomers are further able to track its density over time. Because Pluto is so far from the sun, the ice on its surface sublimates: It goes from a solid directly to a gas without first becoming a liquid. When Pluto reached perihelion (as close to the sun as its gets in an orbit) in 1989, the expectation was that the atmosphere would begin to collapse entirely: that it would freeze out, basically, and fall to the surface.

“A good comparison is when it snows on Earth,” Runyon said. “Snow is basically the water vapor in the atmosphere freezing out and falling to the surface, leaving Earth’s atmospheric density slightly lower than it would be otherwise.” In Pluto’s case, the thought was that the complete atmosphere would freeze out and fall onto the dwarf planet’s surface.

It didn’t happen. “Pluto’s atmosphere is denser than we thought it would be,” Runyon explained. “Even now as it’s moving farther from the sun, its atmosphere is puffier than ever.” One model says that while the atmosphere does thin as ices fall to the surface, it never completely freezes and falls.

... which is electric blue.

Scientists on the New Horizons team didn’t expect to see Pluto’s atmosphere during the flyby. “When we spun New Horizons around after closest approach and looked back at Pluto—being basically backlit from the sun—we could see the atmosphere,” he said. “We knew we’d be able to detect it, but to see it, and to see that the sunrise and sunset on Pluto is this ethereal electric blue—nobody anticipated that.” Runyon says that the New Horizons found discrete atmospheric layers that could be traced for hundreds of miles. “Pluto has what’s called a stably stratified atmosphere. The coldest layer is on the bottom and it gets warmer as you go up,” he said.

“In science, you test hypotheses, but before you can even do that you need to figure out what’s there in the first place. To me, that’s the most exciting part of science. The most exciting part of space exploration is to see something for the first time, and that’s what New Horizons was. And to turn around and look back at the sun and see a beautiful atmosphere with the gorgeous layers through it is just astonishing.”

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A version of this story was published in 2015; it has been updated for 2025.