What Is Pluto?
In 1905, an American astronomer named Percival Lowell noticed some odd deviations in the orbits of Neptune and Uranus. These irregularities seemed to suggest that the gravity from an unknown world could be tugging at these two planets. Lowell was able to predict the location of this undiscovered object in 1915, but would die before actually finding it.
In 1930, however, a scientist named Clyde Tombaugh was able to make a breakthrough. He was working at the Lowell Observatory (founded by Percival Lowell) in Arizona, and was able to find the object using predictions from Lowell and others. The name for this celestial body came from 11-year-old Venetia Burney of Oxford, England. She suggested the name Pluto, after the Roman god of the underworld, to her grandfather, who in turn passed the suggestion along to the Lowell Observatory.
Pluto, which is roughly two-thirds rock and one-third ice, is pretty small compared to the other planets in the Solar System. It’s surface area of 17,646,012 square kilometers is roughly the same size as the surface area of Russia. Pluto is even smaller than Earth’s moon.
Pluto is also very far away and very cold. It is about 40 times farther from the sun than Earth is, and it takes about 248 Earth years to make one trip around the sun. A day on Pluto is about 6.5 days on Earth.
Pluto has five moons. Its largest moon is named Charon, which is about half the size of Pluto. Pluto’s four other moons are named Kerberos, Styx, Nix and Hydra.
Is Pluto a Planet?
The short answer: no! Pluto is more accurately described as a dwarf planet. Dwarf planets orbit stars just like other planets, but are much smaller.
If this status of Pluto is news to you, don’t feel too bad. For more than 70 years, Pluto was in fact recognized as one of the nine planets in our Solar System. When Tombaugh discovered Pluto in 1930, textbooks were quick to update their planetary lists with this ninth member. However, in the decades that would follow after its discovery, astronomers and others would begin to question whether Pluto might better fit with the other small, icy bodies that scientists believed were found beyond Neptune.
This region, known as the Kuiper Belt, had long been hypothesized, but it wasn’t until 1992 that the first resident was discovered. Soon after, other Kuiper Belt Objects (KBOs) were discovered, including some with masses comparable to Pluto. One of these KBOs, Eris, appeared to be even larger than Pluto.
This led to even more debate in the scientific community. In 2000, Hayden Planetarium in New York, which is directed by astrophysicist Neil deGrasse Tyson, caused a stir by unveiling an exhibit featuring only eight planets. Finally, in 2006, Pluto was relegated to the status of dwarf planet by the International Astronomical Union (IAU).
Why Was Pluto Demoted?
At a 2006 IAU General Assembly in Prague, debates raged on how to deal with planets and KBOs. Under one plan, the number of planets would have increased from nine to twelve, with a few KBOs being upgraded to planet status. However, there was worry that as more KBOs were discovered, the list of planets might jump into the tens and even hundreds. Over the course of discussion, however, it was decided to adopt a new resolution outlining the criteria for naming a planet:
1. A Planet is a celestial body that:
a. is in orbit around the sun,
b. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes hydrostatic equilibrium (nearly round) shape, and
c. has cleared the neighborhood around its orbit.
Pluto orbits the sun, and has a nearly round shape, so it meets the first two planetary criteria. However, it is the last point that proved to be Pluto’s undoing.
“Clearing the neighborhood” means that the planet has achieved gravitational dominance by either “sucking up” or “ejecting” other large objects around it. Pluto shares its neighborhood with other icy KBOs. So under this new resolution, Pluto was stripped of its planetary designation, and was instead relegated to the category of dwarf planet, along with other large KBOs and Ceres, the largest body in the asteroid belt.
The Future of Pluto
Pluto’s reclassification was not the end of the matter, however. The whole “cleared the neighborhood” criteria caused some debate. There are nearly 12,000 near-Earth asteroids, begging the question, has Earth cleared its neighborhood? Is Earth a planet?
Additionally, the vote to adopt this resolution came at the end of the IAU General Assembly, when many scientists who had attended the meeting had already left. As such, the debate over Pluto’s status continues to this day.
Is Pluto a planet? Are you #TeamPlanet or #TeamKuiperBelt? Let us know in the comment section.
How well do you know your planets? Test yourself in the quiz below!
Mark Heald is an Associate Product Manager and Sporcle Admin. He enjoys spending time with his family, traveling, and bemoaning the fact the Sonics left Seattle.