The dwarf planets

• What is a dwarf planet?
• Eris
• Pluto
• Haumea
• Ceres
• Makemake
• Dwarf planet sizes

What is a dwarf planet?

In 2006 the International Astronomical Union (IAU) agreed on the official definition of a planet.

According to the definition, in order to be designated as a planet in our Solar System an object must:

• orbit the Sun
• be ‘gravitationally rounded’ - in other words, be large enough to have a rounded shape due to its own gravity
• be ‘gravitationally dominant’ – in other words, the planet needs to have cleared its orbit around the Sun of other objects of comparable size, also known as ‘clearing the neighbourhood’.

As a result of this new definition, another classification was created for objects that fall one short of these criteria: a dwarf planet. 

Objects are called dwarf planets if they orbit the Sun and are gravitationally rounded, but are not gravitationally dominant.

The IAU's definition is the reason why Pluto is no longer considered a planet. 

Pluto, which had been classed as a planet since its discovery in 1930, was downgraded to a dwarf planet in 2006 because it's possible for other objects to cross its orbit: it is not gravitationally dominant.

Currently there are only five dwarf planets confirmed and recognised by the IAU: Pluto, Ceres, Eris, Haumea and Makemake.

However, there are several other identified astronomical bodies that could be dwarf planets and might move into this category in the future, including outer Solar System objects Gonggong, Quaour and Sedna, which are found in a region of space called the Kuiper belt.

What’s more, according to the IAU there may be more than 100 dwarf planets waiting to be discovered.

Discover more about dwarf planets with astronomers from Royal Observatory Greenwich.

Are dwarf planets asteroids?

Asteroids are irregularly shaped lumps of rock which orbit the Sun independently (meaning that they don’t orbit a planet, which in turn orbits the Sun).  

If an asteroid is massive enough to be rounded by gravity, and so be roughly spherical, it would be considered a dwarf planet rather than an asteroid.  

However, there’s not a clear distinction between a dwarf planet and an asteroid. For example, Ceres is referred to as the largest asteroid in the asteroid belt between Mars and Jupiter, as well as the only dwarf planet in the asteroid belt.

Are dwarf planets smaller than Earth?

All the dwarf planets are smaller than Earth, but the definition of a dwarf planet isn’t based on size alone. The distinction between planet and dwarf planet instead depends on whether the object has its own clear orbit in the Solar System.  

However, the more massive an object is, the more gravitational influence it has – and therefore it’s more likely to clear its orbit around the Sun.

This means the dwarf planets are all relatively small. Eris and Pluto are the biggest dwarf planets, but they’re both smaller than Mercury (the smallest planet in the Solar System).

Do dwarf planets have moons?

It’s possible for a dwarf planet to have a moon, but not all dwarf planets have moons. So far moons have been discovered around Makemake, Haumea, Eris and Pluto.

Where are the dwarf planets?

Four of the dwarf planets – Eris, Haumea, Makemake and Pluto – reside in the region beyond Neptune, usually referred to as the Kuiper belt. As these dwarf planets are farther out in the Solar System than Neptune they are also called ‘plutoids’, a subcategory of dwarf planets.

Around 3,500 small bodies, called Trans-Neptunian Objects (TNOs), have been found in this area but until Eris all the TNOs discovered were significantly smaller than Pluto. Many of these TNOs are possible candidates for dwarf planet status.  

Ceres is unlike the other dwarf planets, as it's located in the asteroid belt in the inner Solar System, between the orbits of Mars and Jupiter. 

What’s the difference between a dwarf planet and a minor planet?

A minor planet is defined as any small object in the Solar System which independently orbits the Sun but is neither a planet nor a comet.  

A dwarf planet meanwhile is an object which orbits the Sun independently and is massive enough to be gravitationally rounded, but is not gravitationally dominant.

However, definitions can blur when it comes to classifying objects, and objects don't always fall neatly into one category or another.

Pluto facts

Pluto is about 2,380km across and takes 248 Earth years to orbit the Sun once, while a day on Pluto is 153 Earth hours long. At an average of almost 6 billion km from the Sun, the surface temperature is about -230°C.

This means that nitrogen, methane and carbon monoxide – all gases here on Earth – become solids on the surface of Pluto.

One of its most famous features, a giant heart-shaped glacier called Sputnik Planitia, is made of solid nitrogen. Pluto’s thin atmosphere is also mainly made of nitrogen.

It’s thought that in the past Pluto may have had liquid on the surface, and it still might have an ocean underneath the surface.

Pluto’s name comes from the Roman god of the underworld, and was originally suggested by an 11-year-old girl. Pluto's five discovered moons – Charon, Nix, Hydra, Styx, and Kerberos – also get their names from characters associated with the underworld in both Greek and Roman mythology.

Pluto's moon Charon is around half the diameter of Pluto and one eighth of its mass, making this the closest parent-to-moon ratio of a planet or dwarf planet in the Solar System. Because of this, Charon pulls on Pluto and makes Pluto do a small orbit around their combined centre of gravity. Many consider them to be a binary system in which two objects orbit one another. 

Exploring Pluto: the New Horizons spacecraft

The New Horizons mission to Pluto launched on 19 January 2006 and flew past Pluto in July 2015, investigating and collecting data on the dwarf planet and its moons.

This was the first spacecraft to visit Pluto and the first time astronomers had been able to image the surface of Pluto in any detail, as it’s so far away it looks like a small dot from Earth-based telescopes.

The entire data set was about 6.25 gigabytes, but it took 15 months to transfer it all back to Earth due to the huge distances involved and the slow data rate possible. For comparison, this is equivalent to a speed about 26,000 times slower than what is considered a ‘good’ internet connection today on Earth.

On board the New Horizons mission were some of the ashes of Clyde Tombaugh (1906-1997), meaning the astronomer who discovered Pluto went to the dwarf planet almost 85 years later!

Take a look back at the New Horizons mission with Royal Observatory astronomer Patricia Skelton. To watch more free astronomy talks and explainers, subscribe to the Royal Observatory on YouTube.