When it was discovered in 1930, Pluto was considered to be the ninth planet of the Solar System. It’s quite tiny, though: about half the size of our own Moon.
(Pluto is bottom left)
But over time, astronomers began to realise just how many similar objects there are out there beyond Neptune, in what is called the Kuiper Belt (which is kind of similar to the Asteroid Belt between Mars and Jupiter). Dozens, maybe hundreds or even thousands of them.
So if we are calling Pluto a planet, putting in the same category as Earth, Mars, etc., then there’s potentially a vast number more which should be added to the traditional list of planets. That’s not very convenient or clear.
So in 2006 the International Astronomical Union decided to define exactly what we mean by the word ‘planet’. An upshot of that was that Pluto didn’t fully qualify and came to be redefined as a ‘dwarf planet’. That is,
- it’s a round object and it’s orbiting the Sun just like a planet,
- however, it’s very small, more like the size of a moon or asteroid,
- and as such, it’s not big enough (gravitationally) to ever carve out an orbit all to itself like any big planet would.
Pluto is now just one of several dwarf planets that have been identified, including Haumea, Makemake, Eris and Ceres.
A couple of interesting facts about Pluto:
Pluto’s 248-year elliptical orbit around the Sun sometimes brings it closer to the Sun than Neptune’s orbit.
Pluto has a surprisingly big satellite (moon), called Charon. In effect, it’s a binary system.