$$\require{cancel}$$

# 7.37 Dwarf Planets

The Solar Nebula model for Solar System formation leaves us with a picture of the Sun surrounded by planets, with left over rock and ice debris mixed in with the planets, and forming a sphere of icy bits surrounding them. While there were theories predicting a belt of icy objects around and extending beyond Neptune's orbit, it was unknown how large these objects might be. Pluto, once a planet, was found in 1930 and was, until the 1990s, the only icy object known in this region. When David Jewitt and Jane Luu and other members of the astronomy community began discovery new icy object in the 1990s, many began to wonder if anything larger than Pluto would be found. Beginning in the 2000s, this possibility started to look more and more likely.

Objects of the?Kuiper belt?(blue). Plot displays the known positions of objects in the outer?Solar System?within 60 astronomical units?(AU) from the Sun.Epoch?as of January 1, 2015. Click here for original source URL

Most Kuiper Belt Objects (KBOs) are probably only about 100 km across. The exact sizes of these objects are uncertain because they are so far away. Most telescopes can't resolve them, so estimates on their sizes have to be made from measurements of how much light the bodies reflect and how much heat they emit. These distant bodies are composed of some of the most primordial material in the Solar System. They have barely been heated by the Sun, and they have probably escaped the impact processing that bodies in more heavily populated regions of the Solar System experience. Scientists hope that these cold, icy bodies will contain clues to the early Solar System and the formation of planets.

In 2002, an object half the size of Pluto was discovered in the Kuiper Belt. Named Quaoar, this 900 km diameter object is made of ice and rock in roughly equal proportions. Water ice has been detected on the surface, and other ices are probably also present, such as frozen methane and frozen carbon dioxide (dry ice).

New, and larger, discoveries began to come quickly in succession. Sedna, another icy and rocky object, was found in 2003 on an orbit that carried it in from the Oort cloud. With a radius of between 1200 and 1600 km, this object raised questions about when something comes to be called a planet. This question became more and more important as new and larger objects were found, with Haemea (diameter about 1150 km), Make make (diameter about 1500 km), and Eris (diameter about 2300 km) all being announced in 2005. This last icy world is actually larger than Pluto! For a brief period of time, many people organizations (including NASA) referred to Eris as the 10th planet. Work on all these objects was pioneered by teams including Michael E. Brown, Chad Trujillo, and David Rabinowitz. Michael Brown informally named his newly discovered planet Xena, and its small orbiting moon Gabrielle, in honor of the TV show heroine, but the International Astronomical Union vetoed that idea and settled for the more traditional mythical name Eris.

Without a clear way to define what is and isn't a planet, a naming controversy broke out. When the International Astronomical Union (IAU), the governing body for astronomy and the official naming authority of astronomical objects, met in Prague in the fall of 2006, committees met to discuss not just what proper names to give these objects, but also how to classify them. A set of three rules, written by committee and voted on by the broad membership of the IAU, were developed, and as a consequence of these rules none of the icy objects got to be planets. Even Pluto was demoted, becoming a "Dwarf Planet."

The three rules of planet hood state that to be a planet an object must:

• Orbit the Sun

• Be of sufficient mass that its self-gravity pulls the mass into a round shape.

• It must have cleared its orbital path of all (significant) materials.

It is this last detail that got Pluto into trouble. It is at most a few percent of the total mass the Kuiper Belt. For comparison, all the mass crossing Earth's orbit adds up to less than a percent of the Earth's mass. This makes Pluto nothing more than an above average chunk of ice orbiting in a belt of otherwise like objects. In creating the definition for "Planet" they also set aside a new class — dwarf planets — for objects that meet the first and second criteria. This allows the asteroid Ceres to also become dwarf planet