1.4  What defines a planet?

Video 1 shows some of the key differences between planets and dwarf planets.

As you have just seen, in 2006 the International Astronomical Union (IAU) met to decide what the definition of a planet in our Solar System should be. They came up with three criteria:

1. A planet must orbit the Sun.
2. A planet must have enough mass for its own gravity to cause it to be round, rather than being an irregularly-shaped large rock.
3. A planet must have cleared the neighbourhood around its orbit.

The second and third points relate to the gravity that an object must have to be considered a planet. Planets have enough mass for their own gravity to be the strongest force acting on them, which pulls them into roughly a sphere shape. Smaller objects – rocky asteroids and icy comets – are often a variety of weird and wonderful shapes because they don’t have much mass and become distorted as a result of other forces acting on them.

The third point means that a planet must gravitationally dominate its own orbit. Large objects would either consume smaller bodies, capture them as moons or send them into different orbits. Only the most massive objects can ‘clear their neighbourhood’. This means that there shouldn’t be other independent objects of comparable size that also orbit the Sun in the same region of space as a planet. Moons don’t count because they orbit a planet rather than the Sun – they are controlled by the planet’s gravity. This definition also allows for other objects such as so-called Trojan asteroids, which are small clumps of asteroids pushed along or ‘shepherded’ by a planet’s gravity.

Point number three is where Pluto lost out. As you saw in Video 1, Pluto shares its orbit with many other objects. Pluto is now defined as a dwarf planet – an object that ticks points one and two of the planet definition, but not point three. Eris, and the largest object in the asteroid belt, Ceres, have also joined this new group.