Dwarf planets

The universe contains unique objects that don't always fit into neat categories like stars, planets and galaxies. Dwarf planets share particular characteristics that have traditionally made them a bit harder to classify.

What are dwarf planets?

Dwarf planets are celestial objects that:

• orbit a star
• are roughly spherical
• often have many other large bodies such as comets, asteroids, or other dwarf planets near them

As their name suggests, the main difference between a dwarf planet and a planet is size. Because they are smaller, dwarf planets lack the gravitational forces needed to pull in and accumulate all of the material found in their orbits. Each known dwarf planet in our solar system is actually smaller than Earth's Moon!

Dwarf planets in our solar system

As the authority on the naming and classification of celestial objects, the International Astronomical Union officially recognizes five dwarf planets in the solar system:

• Pluto
• Eris
• Ceres
• Makemake
• Haumea

Several dozens more are being considered for the category, and scientists estimate that hundreds or even thousands of dwarf planets may exist in the solar system.

Pluto

When Pluto was discovered in 1930, it was called the ninth planet in our solar system, but its status as a fully fledged planet came into question in the 1990s. Pluto was officially reclassified as a dwarf planet in 2006.

The best-known dwarf planet, Pluto is also the largest in size and the second largest in mass. Pluto has five moons. The largest, Charon, is over half the size of its host. Pluto's orbit is not circular like those of the other planets and it actually crosses Neptune's orbit, which means that Pluto is sometimes closer to the Sun than Neptune is. It takes Pluto nearly 250 years to complete one trip around the Sun.

Not much was known about Pluto before NASA's New Horizons mission. Launched in 2006, the spacecraft took nearly nine years to reach its target. The mission revealed that Pluto's surface features plains and mountains made of nitrogen ice and water ice.

Eris

Located beyond the orbit of Neptune, Eris completes one trip around the Sun every 557 years. It is slightly smaller than Pluto but actually contains over 25% more matter. The discovery of this denser dwarf planet in 2005 may have been the turning point that forced astronomers to reconsider Pluto's classification as a planet. To illustrate this disruption, Eris was named after the Greek goddess of discord. Since Eris is so far away, no surface details can be seen with current instruments, but astronomers have detected the presence of methane ice and believe Eris's surface is similar to that of Pluto.

Ceres

Ceres is the largest object in the asteroid belt between the orbits of Mars and Jupiter. This dwarf planet alone contains about a third of all the matter found in the belt! Its near-spherical shape means that this rocky, icy body is not considered an asteroid.

While most dwarf planets orbit the Sun at the very outer edges of our solar system, Ceres is the only one located inside the orbit of Neptune. It takes Ceres 4.6 years to complete one trip around the Sun. Scientists suspect that this unique dwarf planet may even have an ocean of liquid water hidden under a layer of ice.

NASA's Dawn spacecraft was launched in 2007 and entered into orbit around Ceres after first observing nearby asteroid Vesta. Dawn allowed astronomers to see detailed images of the dwarf planet for the first time and find out more about its composition and surface.

Makemake

Makemake was discovered in 2005, just a few months after Eris was found, and by the same team of astronomers. It is located in the Kuiper Belt, a ring of icy debris beyond the orbit of Neptune – around 30 to 50 times farther from the Sun than Earth is. Astronomers say that Makemake is likely reddish in colour, similar to Pluto. In 2015, a moon nicknamed MK2 was discovered orbiting the dwarf planet. Makemake takes over 300 years to complete a trip around the Sun.

Haumea

Haumea was discovered in 2004 in the Kuiper Belt beyond the orbit of Neptune. Although it takes the dwarf planet 285 Earth years to complete a trip around the Sun, Haumea spins on itself in under four hours. Astronomers believe that this fast rotation has deformed Haumea into an ellipsoid (egg-shaped). This dwarf planet has two moons: Hi'iaka and Namaka. It may even be the only Kuiper Belt object to have its own ring.

Before the discovery of a celestial body is officially announced, research teams often give their finds temporary code names. Interestingly, since Makemake was discovered just after Easter, and Haumea was discovered in the days following Christmas, scientists informally codenamed the rocky bodies "Easterbunny" and "Santa," respectively.

However, in keeping with the naming guidelines established by the International Astronomical Union, Makemake was named after a creator god from the myths of the Rapa Nui, the native people of Easter Island. Haumea shares its name with the Hawaiian goddess of fertility and childbirth, and the dwarf planet's moons are named after her offspring.

Explore further

• Comets and asteroids
• Neptune
• The Moon