Dwarf planets in the Kuiper Belt

Just outside of Neptune’s orbit is a ring of icy bodies. This is called the Kuiper Belt. Pluto is the most famous Kuiper Belt Object.

The Sun is at the center of our solar system. It is orbited by eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. But what’s past Neptune? Just outside of Neptune’s orbit is a ring of icy bodies.

This is where you’ll find dwarf planet Pluto. It’s the most famous of the objects floating in the Kuiper Belt, which are also called Kuiper Belt Objects, or KBO’s.

Pluto and other dwarf planets are a lot like regular planets. So what’s the big difference? The International Astronomical Union (IAU), a world organization of astronomers, came up with the definition of a planet in 2006. According to the IAU, a planet must do three things:

Dwarf planets like Pluto were defined as objects that orbit the Sun, and are nearly round, but have not been able to clear their orbit of debris.

So far, the IAU has only recognized five dwarf planets. In order of distance from the Sun they are: Ceres, Pluto, Haumea, Makemake, and Eris. But the IAU says there may be many more dwarf planets – perhaps more than a hundred – waiting to be discovered.

Pluto is by far the most famous dwarf planet. Discovered by Clyde Tombaugh in 1930, Pluto was long considered our solar system’s ninth planet.

But after other astronomers found similar intriguing worlds deeper in the distant Kuiper Belt – the IAU reclassified Pluto as a dwarf planet in 2006.

There was widespread outrage on behalf of the demoted planet. Textbooks were updated, and the internet spawned memes with Pluto going through a range of emotions, from anger to loneliness.

On July 14, 2015, NASA’s New Horizons spacecraft made its historic flight through the Pluto system – providing the first close-up images of Pluto and its moons and collecting other data that has transformed our understanding of these mysterious worlds on the solar system’s outer frontier.

Dwarf planet Ceres is closer to home. Ceres is the largest object in the asteroid belt between Mars and Jupiter, and it’s the only dwarf planet located in the inner solar system.

Like Pluto, Ceres also was once classified as a planet. Ceres was the first dwarf planet to be visited by a spacecraft – NASA’s Dawn mission.

An exoplanet is any planet beyond our solar system. Most of them orbit other stars, but some free-floating exoplanets, called rogue planets, are untethered to any star. We’ve confirmed more than 5,800 exoplanets out of the billions that we believe exist.

Most of the exoplanets discovered so far are in a relatively small region of our galaxy, the Milky Way. (“Small” meaning within thousands of light-years of our solar system; one light-year equals 5.88 trillion miles, or 9.46 trillion kilometers.)

Even the closest known exoplanet to Earth, Proxima Centauri b, is still about 4 light-years away. We know there are more planets than stars in the galaxy.

By measuring exoplanets’ sizes (diameters) and masses (weights), we can see compositions ranging from rocky (like Earth and Venus) to gas-rich (like Jupiter and Saturn).

Some planets may be dominated by water or ice, while others are dominated by iron or carbon. We’ve identified lava worlds covered in molten seas, puffy planets the density of Styrofoam and dense cores of planets still orbiting their stars.

The Target Star Catalog is a guide to intriguing nearby stars that astronomers want to study with future missions, such as the Habitable Worlds Observatory, which will be built specifically to find and observe Earth-like exoplanets, to search for signs of life.

So far scientists have categorized exoplanets into the following types: Gas giant, Neptunian, super-Earth, and terrestrial with subcategories like mini-Neptunes within those groups.

NASA welcomes the public to assist with projects and sift through data. Collaborating with NASA scientists, volunteers known as citizen scientists have contributed to thousands of important discoveries.

NASA / ABC Flash Point News 2025.