Dwarf Planets

Pluto, as seen by the Hubble Space Telescope (Credit: NASA).
Pluto, as seen by the Hubble Space Telescope (Credit: NASA).

What is a planet? We’ve been asking that question at least since Greek astronomers came up with the word to describe the bright points of light that seemed to wander among fixed stars. Our solar system’s planet count has soared as high as 15 before it was decided that some discoveries were different and should be called asteroids.

Many disagreed in 1930 when Pluto was added as our solar system’s ninth planet. The debate flared again in 2005 when Eris — about the same size as Pluto — was found deep in a zone beyond Neptune called the Kuiper Belt. Was it the 10th planet? Or are Eris and Pluto examples of an intriguing, new kind of world?

The International Astronomical Union decided in 2006 that a new system of classification was needed to describe these new worlds, which are more developed than asteroids, but different than the known planets. Pluto, Eris and the asteroid Ceres became the first dwarf planets. Unlike planets, dwarf planets lack the gravitational muscle to sweep up or scatter objects near their orbits. They end up orbiting the sun in zones of similar objects such as the asteroid and Kuiper belts.

Our solar system’s planet count now stands at eight. But the lively debate continues as we continue to explore and make new discoveries.

According to the International Astronomical Union, which sets definitions for planetary science, a dwarf planet is a celestial body that:

  • Orbits the sun.
  • Has enough mass to assume a nearly round shape.
  • Has not cleared the neighborhood around its orbit.
  • Is not a moon.
  • The main distinction between a dwarf planet and a planet is that planets have cleared the path around the sun while dwarf planets tend to orbit in zones of similar objects that can cross their path around the sun, such as the asteroid and Kuiper belts. Dwarf planets also are generally smaller than the planet Mercury.

The first five recognized dwarf planets are Ceres, Pluto, Eris, Makemake and Haumea. Scientists believe there may be dozens or even more than 100 dwarf planets awaiting discovery.

The IAU recognized Pluto’s special place in our solar system by designating dwarf planets that orbit the sun beyond Neptune as plutoids. Eris, which orbits far beyond Neptune, is a plutoid while Ceres, which orbits in the main asteroid belt between Mars and Jupiter is a dwarf planet.

These artist's concepts show some of the best known objects in the Kuiper Belt in comparison to Earth. It was labeled before the objects got their official names. Image from NASA
These artist’s concepts show some of the best known objects in the Kuiper Belt in comparison to Earth. It was labeled before the objects got their official names. Image from NASA