Unless you live under a rock (or in a different solar system), you've probably heard two important pieces of news about Pluto by now: (1) Pluto has been downgraded to a dwarf planet, and (2) NASA just took some pretty epic close-up images of Pluto.

But that doesn't necessarily answer the question "what's the big deal?" Here are some facts about the former planet/interstellar media sensation that should help clear things up.





1. The ninth planet

Pluto, named for the Roman god of the underworld, was first discovered in 1930 by a 22-year-old named Clyde Tombaugh. Until Tombaugh's discovery, our solar system, as we knew it, contained eight planets. Pluto made nine.

In 1978, scientists discovered Pluto's largest moon, Charon, and were then able to calculate Pluto's mass and size. Pluto is tiny—only about 1,500 miles (2,400 km) across—but, to be fair, Mercury is only 3,032 miles (4,880 km) across, and it's still considered a planet.

So what gives?





2. Discovery of Eris

In 2005, scientist Mike Brown and his team made a new discovery that would change the way we looked at Pluto forever—they'd discovered an object (later named Eris) further out than the orbit of Pluto that was bigger than Pluto. Turns out, Eris is approximately 1,600 miles (2,600 km) across and has approximately 25% more mass than Pluto.

This is significant because it means that, instead of being the only planet in its region (like the rest of our solar system's planets), Pluto is just one object among many in the Kuiper Belt in space. If Eris is larger and made of the same ice/rock mixture as Pluto, does that make it a planet? What then, exactly, makes something a planet?

3. Tough break, Pluto

According to the International Astronomical Union, an object needs to meet three requirements in order to be a planet:

It needs to be in orbit around the sun

It needs to have enough gravity to pull itself into a spherical shape

It needs to have "cleared the neighborhood" of its orbit

Pluto satisfies the first two requirements, but it fails #3, which means it is considered a dwarf planet. There are many objects with similar size and mass to Pluto jostling around in its orbit, and until Pluto crashes into a bunch of them and gains mass—thus "clearing the neighborhood" around itself—it will remain a dwarf planet.

4. We meet at last, old friend

Like any underdog, Pluto holds a special place in the hearts of Earthlings. But that’s not what makes the latest images from the New Horizons spacecraft so incredible.

For one thing, it's the first time we've ever seen Pluto's face. We first saw Venus and Mars in the 1960s, Mercury, Jupiter, and Saturn in the 1970s, and Neptune and Uranus in the 1980s. That means it's taken us over 30 years (!) to see Pluto. That's an entire generation, millions of people who are experiencing new spacecraft discoveries for the first time. If you weren't impressed already, you should be now.

New Horizons was launched back in 2006, and it's taken the spacecraft nearly a decade to travel over 3 billion miles, past each planet's orbit, from Mars to Neptune. For some more perspective, Pluto takes 248 Earth years to orbit the sun—so when Pluto was last in the position it's currently in, humans didn't even know it existed. (!!)

The new images have also showed us that Pluto has icy mountains, and its moon Charon has canyons much like Earth's Grand Canyon. (!!!)

5. Pluto's moons are totally different from ours

On July 15, 2015, New Horizons discovered that Charon (left), Pluto's largest moon, has a surprisingly smooth (and "youthful") surface. Very few craters are visible (unlike our own moon), and this indicates a "relatively young surface that has been reshaped by geologic activity."

On July 21, 2015, NASA released new images of two of Pluto's smaller moons, Nix (middle, false-colored pink) and Hydra (right). The lopsided, wonky shapes of these moons stand in stark contrast to the moons that dominate our solar system, which are mostly spherical.

Pluto has five moons total, all of which are believed to have chaotic orbits around the dwarf planet. We've known about Charon since 1978, but Nix and Hydra were discovered in 2005, Kerberos was discovered in 2011, and Styx wasn't discovered until 2012. New Horizons' images of Kerberos and Styx won't be available until October.

6. The final frontier?

But why did we bother flying a spacecraft across the solar system to get pictures of Pluto?

The main reason scientists wanted to learn more about Pluto is because it is believed to have formed at the same time as the rest of our solar system and from the same materials as Earth. By studying the information from New Horizons about Pluto's geology, atmosphere, and moons, scientists hope to better understand how the solar system formed billions of years ago.

Whether or not Pluto is officially deemed a planet, this mission completes humanity's first look at the traditional set of nine planets in our solar system. As one reporter commented in a New York Times column, "None of us alive today will see a new planet up close for the first time again."

For more info, check out NASA's New Horizons page and this comprehensive article from Universe Today.