The definition of a planet continues to be debated. The debate about whether Pluto is a planet was generated by recent detections of hundreds of planetary objects in the outer solar system. The International Astronomical Union classifies Pluto as a dwarf planet. Most people call Pluto a planet because it orbits the Sun and it is large enough that its own gravity has pulled it into a spherical shape.

The Pluto Debate

Recent discoveries of hundreds of objects in the outer part of the solar system has opened a planetary debate. These objects are thought to be part of a large collection of Pluto-sized and smaller bodies, that form a disk-shaped cloud beyond Pluto's orbit called the Kuiper Belt.

There may be more planets than the usual nine, if we count the larger asteroids and the possibly thousands more Pluto-sized objects in the distant Kuiper Belt that have not been discovered. But just because Pluto is small that does not mean it is not fundamentally a planetary body.

News stories in the mid-1990s associated with a movement to demote Pluto from planethood drew attention to this issue. Singer Christine Lavin even wrote a song about it!

To learn more about the solar system classification and the discussions of Pluto as a planet, visit Yes, Pluto really is a Planet and Is Pluto a Planet?

What is a Plutino?

These objects have fallen under the influence of Neptune, and orbit in resonance with that planet. So-called 3:2 resonant objects, which include Pluto itself, make two orbits for every three of Neptune's.  These are sometimes called "Plutinos," "little Plutos," or "resonant KBOs." So-called 2:1 objects, which are farther from the Sun, orbit once for every two orbits of Neptune, and so on.

What is an Ice Dwarf?

Scientists classify the planets into groups. For example, Earth, Mars, Venus and Mercury comprise the "terrestrial planets," which are mostly rocky objects. In contrast, the "gas giant" planets, which include Jupiter, Saturn, Uranus and Neptune, are dominated by thick, molecular hydrogen atmospheres. Pluto and Charon seem to be part of a third category called "ice dwarfs" — they have solid surfaces but, unlike the terrestrial planets, a significant portion of their mass is icy material (such as frozen water, carbon dioxide, nitrogen, methane or carbon monoxide).

Destination: Sedna or Quaoar?

Sharp-eyed astronomers have recently discovered larger KBOs - named Quaoar and Sedna - though these objects are still much smaller than Pluto.

Unfortunately, New Horizons can't reach either object. Quaoar is located far away from the trajectory of any spacecraft that travels toward Pluto during the next several decades. The outer solar system is a big place with a lot of volume! New Horizons is just our first attempt to probe this region, but scientists are sure Quaoar and Sedna will be high on the list of candidate targets as they contemplate other missions to explore the outer solar system during the next several decades.