It's also way, way out there — some six billion kilometres from the sun. And for a variety of reasons it was kicked out of our planetary family in 2006, demoted by astronomers to a mere "dwarf planet."
Until recently, Pluto was thought to be little more than a frozen rock; a dull footnote at the end of the solar system.- QUIZ | 10 questions about the mighty dwarf planet Pluto
Well, Pluto is still remote and still not a planet. But the "dull" part was crossed out this week thanks to the latest pictures from the New Horizons probe that left NASA scientists giddily throwing around words like "amazing" and "mind-blowing."
The pictures appear to upend some of our previous ideas about the presumed dead, dwarf planet. Most notably, it might not be dead.
Where are the craters?
A close-up of Pluto's equatorial region revealed a big surprise: an icy mountain range with peaks as high as 3,500 metres, comparable in size to some of the smaller Rocky Mountains.
NASA scientists believe the mountains may still be forming, which suggests the region might still be geologically active, throwing up some combination of water and gases from a heated core.
"Who would've supposed there were ice mountains? It's just blowing my mind," project scientist Hal Weaver said at a news conference.
Elsewhere in the solar system, surface features are formed on icy moons by heat caused by gravitational interactions with heavyweight planets like Jupiter and Saturn. But there's nothing that big anywhere near Pluto.
NASA says "some other process" must be making those mountains.
"This may cause us to rethink what powers geological activity on many other icy worlds," said John Spencer of the Southwest Research Institute.
Another surprise is that Pluto and at least one of its moons lack the many craters one would expect to find on an object that's been around for some 4.5 billion years, taking hits from meteorites and the like.
Earth's moon is about the same age, and looks it.
But Pluto has "one of the youngest surfaces we've ever seen in the solar system," according to Jeff Moore, leader of the geology and geophysics team at NASA's Ames Research Centre.
NASA is suggesting that recent geologic activity may have given the region "a facelift, erasing those pockmarks." Scientists will now go looking for geysers and so-called "ice volcanos."
Pluto's largest moon, Charon, also showed a surprising lack of craters, suggesting it too might be geologically active.
Small, but not that small
New Horizons also settled the long-running question about Pluto's exact size, which, since its discovery almost a century ago, has been hard to measure because of its atmosphere.
Previous estimates put its diameter somewhere above 2,300 kilometres, but mission scientists this week said it is a slightly larger 2,370.
"We are excited to finally lay this question to rest," said mission scientist Bill McKinnon in a statement.
Charon has no significant atmosphere and New Horizons confirmed it is 1,208 kilometres in diameter.
New Horizons also snuck a peek at two of Pluto's smaller moons, Nix and Hydra, which are estimated to be about 35 and 45 kilometres across, respectively.
Perhaps because it's an outcast, or because it shares a name with a cartoon dog, people get sentimental about Pluto. And it turns out the not-quite planet also has a very big sentimental side. That is found about 1,600 kilometres across and just above Pluto's equator.
The bright, heart-shaped region was first spotted on July 7 and came into sharper focus on July 13 — getting a "wow" from New Horizons principal investigator Alan Stern.
"This is proof that good things really do come in small packages," Stern said.
The region has been named the Tombaugh Regio in honour of Clyde Tombaugh, the astronomer who discovered Pluto in 1930, a gesture that brought a heart-felt round of applause at the NASA news conference.