The Kuiper Belt and dwarf planets

In short: The Kuiper Belt is a ring-shaped region beyond Neptune’s orbit where numerous icy small bodies and several dwarf planets orbit. Pluto, once known as the ninth planet, is the best-known object in this region.

In more detail: In 2006, the International Astronomical Union reclassified Pluto as a dwarf planet because it doesn’t fully meet the definition of a planet.

In detail

• Dwarf planet definition: A dwarf planet orbits the sun and is nearly round due to its own gravity, but hasn’t cleared its surroundings of other objects.
• Pluto: At about 2,400 kilometers in diameter, Pluto is smaller than Earth’s moon and has five known moons, including its large companion Charon.
• Other dwarf planets: Eris, Makemake, and Haumea are also among the well-known objects in the Kuiper Belt.
• Composition: Kuiper Belt objects consist mainly of frozen water, methane, and ammonia, similar to comet nuclei (kometen-und-asteroiden).
• Discovery: Astronomer Gerard Kuiper theoretically predicted this region as early as 1951, long before objects there were actually discovered.
• New Horizons mission: The NASA probe delivered the first detailed close-up images of Pluto in 2015 and later flew past further Kuiper Belt objects.
• Distinction from the Oort Cloud: The Kuiper Belt lies significantly closer to the sun than the even more distant Oort Cloud, from which many long-period comets originate.