Why Is Pluto No Longer a Planet?

With the New Horizons space probe giving the world the best images of Pluto ever captured, more and more people are looking into Pluto’s history. Two questions almost always come up. Wasn’t Pluto a planet once? And how come it isn’t now?

To understand that, two definitions need to be given. A planet is an object orbiting the sun that is rounded by gravity, isn’t big enough to induce thermonuclear fusion, and has cleared its orbit of small objects called plantesimals. A dwarf planet is a planet that hasn’t cleared its orbit of those objects. Pluto was once considered a planet, but is now considered a dwarf planet.

Pluto itself didn’t change from the time it was found to now, at least not in a significant way. Before sophisticated astronomy equipment made space open up, a planet was any of the wandering objects in the sky that orbited the Sun. However, in 2005, an object found three times farther away from the Sun than Pluto, Eris, was discovered. The debate began. Was Eris a planet or wasn’t it? The International Astronomy Union decided in 2006 that “planet” needed a more specific definition. After voting on the subject, they defined a planet according to the terms above, and Eris was deemed a dwarf planet. Pluto met the criteria for a dwarf planet and was reclassified as such. Three other objects – Ceres, Haumea, and Makemake – were also classified as dwarf planets. Haumea and Makemake were discovered about the same times as Eris, while Ceres had been found in the asteroid belt in 1801. If an alternate definition would have been approved, the number of planets in the solar system would have jumped to 12, including Pluto.

Naturally, there were howls from the public who were used to Pluto as a planet, but also among scientists. Despite the vote, the man who coined the term “dwarf planet” disagreed with Pluto not being a planet, saying that if the definition was strictly applied Earth, Jupiter, Mars, and Neptune would not be planets since they share their orbit with asteroids and the man who helped discover Eris, Haumea, and Makemake criticized the fact no more dwarf planets were added to the classification. A third thought that the term “dwarf planet” should be discarded entirely. Almost 3,000 astronomers were present for the IAU assembly held in 2006 in Prague, but only about a tenth of them voted on the final definition.

For anyone looking for more pictures of Pluto, no matter what it’s officially called, go to the New Horizons site.

[Photo by NASA via Getty Images]