When astronomer Clyde Tombaugh discovered Pluto in 1930, he probably didn't imagine just how much controversy and interest the orb then classified as a planet would generate in 2015. But thanks to NASA's New Horizons spacecraft, we've finally gotten our first really good look at the dwarf planet.
Because Pluto has been such a mystery, we had no idea what the New Horizons spacecraft might discover in its fly-by. But as the data and images come in, we're learning more about Pluto than ever.
Of course, nothing compares with the latest image we received of the planet, which shows a beautiful near true-color perspective of Pluto never before seen by human eyes.
So what more do we know about this mysterious dwarf planet that spins on the outskirts of our solar system? As it turns out, we've learned a lot and will keep learning more as scientists continue to analyze the data from New Horizons.
Pluto has a cold heart
Once images started arriving from Pluto, including the true-color photo seen here, we discovered that Pluto has a heart-shaped area on its surface — as if it were always celebrating Valentine's Day. This shape made such an impact on astronomers that they named the area Tombaugh Regio, after the man who discovered the dwarf planet.
Thanks to New Horizons' measurements, we now know that the heart-shaped area is made of frost.
Pluto's surface is mostly icy mountains and plains
The new images we acquired from New Horizons show that Pluto has icy mountain ranges that reach as high as 11,000 feet. Pluto also has smooth ice plains, as well as ice flows and nitrogen ice glaciers moving across its surface, which suggest that the planet remains geologically active.
"We knew that a mission to Pluto would bring some surprises, and now – 10 days after closest approach – we can say that our expectation has been more than surpassed," said NASA's John Grunsfeld. "With flowing ices, exotic surface chemistry, mountain ranges and vast haze, Pluto is showing a diversity of planetary geology that is truly thrilling."
Pluto is larger than we thought
Perhaps the most controversial discovery is how large Pluto is: we now believe that it measures about 1,473 miles in diameter, much bigger than expected. Of course, some would argue that this alone should get Pluto reinstated as a planet, following its downgrade to dwarf in 2006. But we've also discovered that in spite of its size, Pluto still has less mass than Eris, a similar dwarf planet not far away in the Kuiper Belt.
Pluto's surface is younger than we thought
If Pluto's surface were older, it would have the impact craters to show for it. Pluto's face contains no such craters, which suggests that the dwarf planet's surface is only around 100 million years old.
"This is one of the youngest surfaces we've ever seen in the solar system," said NASA's Geology, Geophysics and Imaging (GGI) team leader Jeff Moore.
Pluto's moon Charon also has a younger surface than expected
It makes sense that if the surface of Pluto is younger than we thought, then perhaps the same is true of its moons. New close-up images of Charon certainly provide evidence to that end, as the moon also lacks craters.
Pluto's atmosphere is thin and hazy
Because Pluto's mass fell by half in just a few years, scientists now believe that the loss in mass is a result of Pluto's thin, potentially vanishing atmosphere.
"That's pretty astonishing, at least to an atmospheric scientist," said New Horizons science team member Michael Summers to the New York Times. "That's telling you something is happening."
We know that Pluto's atmosphere shrank because we've been studying it from ground-based telescopes — watching what happens when the dwarf planet passes in front of a star, and studying how it blocks out the star's light. And now we know more about that atmosphere because images taken by New Horizons show Pluto's silhouette surrounded by a ring of sunlight, revealing a hazy atmosphere reaching out to about 100 miles above the surface.
This marks the first time we've seen any hint of weather on the dwarf planet. This new information has changed our minds about just how far beyond the dwarf planet the atmosphere extends: previously, it was thought that it only reached 20 or so miles above Pluto's surface.
Pluto is red
Thanks to images taken by New Horizons, we now know that Pluto's color is a reddish-brown. Although that makes Pluto seem similar to Mars, scientists believe that the causes for their red color are different. Mars is red because of iron oxide, a.k.a. rust. However, scientists believe Pluto's reddish color comes from something that happens to the atmosphere: when cosmic rays and ultraviolet light hit the methane in Pluto's atmosphere, hydrocarbon molecules form, creating a red color.
Pluto is really cold
Pluto's location in the farthest reaches of the solar system is cold — but until now, we didn't really know just how cold things get out there. Pluto's average temperature hovers around the -390 degree Fahrenheit mark, which explains why it's covered in frost, ice plains and icy mountains.
Pluto has a comet-like tail
Another thing we learned about Pluto's atmosphere is that it sweeps behind the planet, similar to a comet's tail. Even though it's so far away from the sun, the solar wind still affects the dwarf planet's mostly nitrogen atmosphere — so much that this wind blows back part of the thin atmosphere to form a tail.
Pluto is still full of surprises
Although New Horizons recently said goodbye to Pluto, we're really only just saying hello. It will probably take scientists over a year to analyze all the data sent back from New Horizons about Pluto, so it's likely that what we've learned so far is only the tip of the iceberg. Pluto will continue to surprise us as we learn more about the dwarf planet with a heart that has captured our hearts.