Pluto is the ninth planet in our solar system.
Pluto was reclassified as a dwarf planet by the IAU in 2006. The solar system now has 8 recognized planets.
What changed?
When Clyde Tombaugh spotted a faint dot moving against the background stars in 1930, the world needed a ninth planet. Astronomers had long suspected something was tugging on Neptune's orbit, and Pluto, named by an 11-year-old girl from Oxford, fit the bill. For 76 years it held its place in textbooks, mnemonics, and the collective imagination of every schoolchild.
Then Mike Brown showed up.
In 2005, Brown and his team at Caltech discovered Eris, a distant object in the scattered disc beyond Neptune. The problem: Eris appeared to be larger than Pluto. If Pluto was a planet, Eris would have to be one too. And Sedna. And Makemake. And probably dozens of icy objects yet to be found.
The International Astronomical Union faced a choice: expand the planet count to ten, twenty, fifty, or draw a cleaner line. In August 2006, astronomers voted. To qualify as a planet, an object now needs to do three things: orbit the Sun, have enough mass to pull itself into a roughly spherical shape, and, this is the one Pluto fails, "clear the neighbourhood" around its orbit.
Pluto shares its orbital zone with thousands of other Kuiper Belt objects. It hasn't gravitationally dominated its region the way Earth, Jupiter, or Saturn have. By the new definition, that makes it a dwarf planet, and the prototype of a whole new class of solar system bodies.
It doesn't make Pluto less interesting. NASA's New Horizons probe flew past it in 2015 and revealed a world with towering water-ice mountains, a heart-shaped nitrogen plain called Tombaugh Regio, and a surprisingly complex atmosphere. Pluto is fascinating. It's just not a planet.
