Science Puzzle
Why Isn’t Pluto a Planet Any More?
From its discovery in 1930 until 2006, Pluto was the ninth planet. Then the International Astronomical Union, which names things in space, voted on a proper definition of a planet, and Pluto became a dwarf planet instead.
Why isn't Pluto a planet any more?
The Answer
Because it hasn't cleared its neighbourhood. Since 2006, a planet must go round the Sun, be big enough for its gravity to pull it round and be the dominant body in its own path. Pluto passes the first two tests but fails the third.
Pluto lives in the Kuiper belt, a vast ring of icy bodies beyond Neptune. It's one of the biggest, but it shares its region with thousands of others and hasn't swept them up or flung them away. The eight planets each dominate their own paths. Earth, for example, is millions of times more massive than all the other small bodies near its orbit put together.
The vote was triggered by a discovery in 2005. Astronomers found Eris, a distant icy world about the same size as Pluto and a bit more massive. Either Eris had to become the tenth planet, with dozens more likely to follow, or the definition had to change. The astronomers chose to change it, and Pluto, Eris and Ceres became dwarf planets.
Pluto's size had nothing to do with it, although astronomers had indeed shrunk their estimates of it over the years. When NASA's New Horizons spacecraft flew past in 2015, it found mountains of water ice and a huge, heart-shaped plain of frozen nitrogen. Some scientists still argue that it deserves to be called a planet.
The principle: Defining a planet. Since 2006 a planet must orbit the Sun, be round and have cleared its path; Pluto shares its path with countless icy bodies in the Kuiper belt, so it's classed as a dwarf planet.