Dwarf Planets

Planets are big, round and bossy. Dwarf planets are just big and round.

The dwarf planets are; Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, Ceres, and Orcus. You can find out more at NASA Science.

Pluto by the New Horizons spacecraft in 2015.

Eris has never been imaged up close.

What do I mean by big, round and bossy?

Big

You probably need to have a diameter of more than 1000km.

Round

When objects in space form, they gather together from smaller objects. If enough small objects gather and stick, they start to crush each other towards their centre of mass. This is due to gravity.

Collect a huge number of small objects and the crushing force due to gravity is also huge. It starts to shape the object into a sphere. It can even melt the interior of the object and causing its composition to change and differentiate.

Rounded, melted, differentiated? Congratulations you are a dwarf planet.

Bossy

Very big, very round? Want to be a planet in the Solar System?

You need to be the biggest thing at your orbital distance from the Sun. In other words, you need to have cleared your neighbourhood of other large objects.