Meet the dwarf planets

What is a dwarf planet?

There are three rules something has to follow to be called a planet:

1. It must orbit the Sun.
2. It must be round.
3. It must have “cleared the neighbourhood around its orbit”. This means that there can’t be any other objects sharing the orbit, unless they are controlled by the planet’s gravity (for example, moons).

A dwarf planet is something that obeys these first two rules, but not the last one. Officially, our solar system has five of these: Pluto, Eris, Ceres, Makemake and Haumea.

Pluto

Until recently Pluto was thought of as the ninth planet, but now it’s the most famous dwarf planet. That’s because the three rules didn’t exist when Pluto was discovered in 1930. When they were written in 2006, Pluto had to be downgraded.

This is because Pluto shares its oval-shaped orbit with hundreds of other space rocks. For part of its journey, this path crosses over Neptune’s orbit, meaning Pluto is sometimes closer to the Sun.

Eris

Eris is the largest of all the dwarf planets, but only just. Pluto’s width is only twenty kilometres shorter. It’s also the most distant. Eris is further from the Sun than almost any other object in the Solar System (that we know of). It takes sunlight nine hours to reach Eris, compared to only eight minutes to get to Earth.

Like Pluto, Eris has an oval-shaped orbit. As it travels outwards, it gets colder. This makes its atmosphere freeze and fall to the surface like snow. This process reverses as it returns closer to the Sun.

Ceres

Ceres is found in the asteroid belt between Mars and Jupiter, far closer to the Sun than every other dwarf planet. Despite being the smallest of the five, it still makes up a third of the entire mass of the asteroid belt.

Ceres was discovered by Giuseppe Piazzi in 1801. He first thought he’d found a comet, then decided it was a planet. Fifty years later astronomers started to discover the asteroids, and thought Ceres was one of them. However, it’s big enough and different enough from its neighbours to now be called a dwarf planet.

Makemake

Makemake is the most recently discovered out of the dwarf planets. It was first found just after Easter in 2005. This led to its first nickname, ‘Easterbunny’, as well as its current name; Makemake is a goddess in Easter Island mythology.

Makemake is the second-brightest object in the Kuiper Belt after Pluto. The Kuiper Belt is a donut-shaped region surrounding the solar system, filled with icy objects like comets. It’s also the home of every dwarf planet except Ceres.

Haumea

The most noticeable feature of Haumea is its shape. Unlike most planets and dwarf planets, it’s not spherical. Instead, it’s shaped like a slightly squashed football. This is the result of its incredibly fast rotation – a full day on Haumea only takes four hours.

Haumea has two moons, Namaka and Hi’iaka. It’s also the only dwarf planet to have a ring (like Saturn’s, except less impressive).

This article was published in Issue 40 of Double Helix magazine (https://www.csiro.au/en/Education/Double-Helix). Copyright for this article is held by CSIRO.