Question: Why is Pluto not called as a planet and why i it s small?

Emma Davies answered on 21 Jun 2019: last edited 21 Jun 2019 12:54 pm

Pluto isn’t considered a planet anymore, mostly due to the discovery of Eris in 2005. Eris is a dwarf planet which is actually more than a quarter bigger than Pluto. This meant that we had to come up with an official definition of what makes a planet: it needs to orbit the Sun (which Pluto does), is massive enough for its own gravity to make it round (which Pluto is quite round but not perfectly), and has “cleared its neighbourhood” of smaller objects around its orbit (which Pluto hasn’t). Pluto is actually part of the Kuiper Belt, filled with asteroids, so it’s quite small probably because it hasn’t managed to merge with enough asteroids yet!