Why Pluto Was Demoted, and What It Takes to Count as a Planet
Pluto did not change in 2006. The definition did. The real culprit was a slightly larger object called Eris, and a question astronomers could no longer dodge.
Pluto was discovered in 1930 and taught as the ninth planet for three generations. In August 2006 it was reclassified as a dwarf planet, and a great many people took it personally. Pluto itself was unaffected. What changed was that astronomers finally had to write down what a planet is.
The problem was Eris
Through the 1990s and early 2000s, surveys of the outer Solar System began turning up more and more icy bodies beyond Neptune, in what is now called the Kuiper Belt. In 2005 a team found one, later named Eris, that was comparable to Pluto in size and more massive.
That left two options. Either Eris became the tenth planet, and so did the next dozen objects of similar size that were clearly coming, or the definition had to be tightened. The International Astronomical Union chose the second.
The three tests
- It must orbit the Sun. This rules out moons.
- It must have enough mass to be roughly round, pulled into a sphere by its own gravity.
- It must have cleared its orbital neighbourhood of other comparable bodies.
Pluto passes the first two comfortably. It fails the third. Pluto shares its region with a great many Kuiper Belt objects, and it is not even gravitationally dominant among them; its orbit is shepherded by Neptune, which it crosses.
So what is it now?
Pluto is the best known of the dwarf planets, alongside Eris, Ceres, Haumea and Makemake. Ceres, incidentally, has been through this before: it was called a planet after its discovery in 1801, then reclassified as an asteroid when more of them turned up. Pluto is in good company.
The New Horizons probe flew past in 2015 and sent back images of ice mountains and a vast nitrogen glacier shaped like a heart. Whatever we call it, it turned out to be far more interesting than a label.