Section 1.3.1: What happened to Pluto?
Pluto, originally called a planet, is now considered to be a dwarf planet. This was the result of the International Astronomy Union clarifying the definition of a planet on 24th August 2006 (see the bottom of this page of the IAU definition). There are four reasons why Pluto is very different to the planets:
1 - Our Solar System is very flat - like a large flat pancake! - with the exception of Pluto, whose orbit around the Sun is tilted by 17 degrees.
2 - The orbits of all the planets are very close to being circular (they are actually slightly elliptical, or oval shaped), but Pluto's orbit is very elliptical, varying from 30 to 50 times further away from the Sun than we are during the course of an orbit.
3 - Also, Pluto is tiny compare to the other planets - it's 30% smaller than our Moon.
4 - As our telescopes improve, astronomers have now discovered lots of similar objects at the edge of our solar system. And if we call Pluto a planet, we would have to call all those other objects (and there could be millions of them!) planets as well.
Similarly, when Ceres was discovered in 1801 orbiting the Sun between Mars and Jupiter, it was also originally called a planet. But once they had discovered more similar objects they called them all asteroids (or minor planets) instead.
The reason was that there are a number of other rocky bodies orbiting in the same area as Pluto and it therefore has not 'cleared the neighbourhood around it'. A number of other bodies have been deemed dwarf planets including Ceres, in the asteroid belt, and Eris, near Pluto.
International Astronomy Union rules for planets
A celestial body that:
- is in orbit around the Sun
- has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
- has cleared the neighbourhood around its orbit.