Night Sky Gaze
Why Pluto Is No Longer a Planet

Why Pluto Is No Longer a Planet

Poor Pluto. Discovered in 1930, it held the honor of being the 9th planet in the solar system for just 76 years. Now it’s considered to be one of at least 2 dwarf planets that exist beyond Neptune. Here’s the real reason why Pluto is no longer a planet.

In Search of Planet X

The discovery of Pluto actually begins in the mid-18th century. Astronomers used their current understanding of gravity and mechanics to predict that there must be another planet beyond Uranus. Even though we couldn’t see this planet, the orbit of Uranus did not make mathematical sense without it.

That logic led to the discovery of Neptune. And then the same math led astronomers to hypothesize that a 9th planet existed. Philanthropist Percival Lowell funded a search for this mysterious “planet X.” After his death in 1916, the search died down.

Then, in 1930, a 23-year-old junior astronomer named Clyde Tombaugh finally spotted Planet X. Although many names for the planet were suggested–including Percival and Constance, after Lowell and his wife–11-year-old Venetia Burney came up with the name Pluto. She was awarded £5 for her contribution.

Pluto Gets Demoted

In 2006, the International Astronomical Union decided to reclassify the term “planet,” officially demoting Pluto. The reason the IAU made the decision involves, appropriately enough, a trans-Neptunian object called Eris. In Greek mythology, Eris was the goddess of chaos. She certainly created chaos in 2005. That’s when a new object, more massive than Pluto though roughly the same diameter, was discovered.

Instead of adding Eris as the 10th planet in the solar system, astronomers decided it made more sense to redefine the term “planet.” Several more dwarf planets have been discovered in the Kuiper belt–a field of frozen debris beyond Neptune–and it’s possible that we’ll find more as spacecraft like New Horizons explore the distant reaches of the solar system.

New Horizons

A day on Pluto lasts 153 hours, and a year is the equivalent of 248 years on Earth! It’s smaller than our Moon and composed mostly of ice and rock.

Astronomers have discovered 5 moons orbiting Pluto. The largest is Charon (named for the boatman who helped souls cross into the underworld. In fact, Charon is slightly more than half the size of Pluto, leading some astronomers to argue that the two bodies are actually a binary system. Imagine looking up and seeing a moon that blotted out most of the sky!

Thanks to images and data sent from New Horizons, we know that Pluto hosts massive mountains that put the Rockies to shame, blood-red snow, and a glacier bigger than Texas. That glacier isn’t made out of frozen water, by the way–it’s frozen nitrogen. A thin, hazy atmosphere of nitrogen shrouds the planet, too.

Deana Adams

When Deana Adams was a kid, she begged her parents for a backyard telescope every birthday and Christmas until they finally caved. As an avid amateur stargazer, she memorized the constellations and hunted for the planets, collecting facts about astronomy the way other kids collected baseball cards or Barbies.

Deana is a contributing writer at NightSkyGaze. She still has her very first telescope.