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Why Pluto Was “Kicked Out” of the Solar System


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Space Place Newspaper article September 2008

Why Pluto Was “Kicked Out” of the Solar System

Poor Pluto. Just how did he get “kicked out” of our family of planets?


Alas, Pluto just didn’t fit in. He was like a stray puppy adopted by a pack of wolves. Now, his real family has been found.
Who are his other family members? Astronomers have already named three other objects in the solar system that are about the same small size as Pluto. They are Ceres [SEAR-ees], Makemake (MAH-kee-MAH-kee], and Eris (AIR-iss]. These objects, along with Pluto, are much smaller than the “other” planets.
Ceres orbits in the Asteroid Belt between Mars and Jupiter. Makemake, like Pluto, is part of the “Kuiper [KI-per] Belt,” which is a region of trillions of icy objects orbiting beyond Neptune. Eris’ orbit is even farther out.
Astronomers have put these objects into a new family called dwarf planets. The astronomers agreed that (1) a planet is a round object that orbits the Sun. It is large enough for its gravity to have cleared out all the rocks and debris in and near its orbital path. (2) A dwarf planet is also a round object that orbits the Sun. However, it is too small for its gravity to be strong enough to clear the debris in its path, so there will be lots of other objects sharing its orbit. And (3), a plutoid is any dwarf planet that orbits farther out than Neptune.
Ceres is an example of a dwarf planet. It is round. It orbits the Sun in the Asteroid Belt, along with over a million smaller objects. Pluto and Makemake are also dwarf planets, since they orbit near lots of other objects in the icy Kuiper Belt. But they are also plutoids, as is Eris. They are plutoids because they orbit farther from the Sun than the farthest planet, Neptune.
Take a look through the “I See Ice” viewer at spaceplace.nasa.gov/en/kids/ice to see and learn about Pluto, Makemake, and other icy places in the solar system.
This article was written by Diane K. Fisher. It was provided by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.


Caption:


This image shows the eight planets and the four dwarf planets named so far: Pluto, Makemake, Eris, and Ceres. Note that the orbits in this image are not to scale.
Note to editors: this image can be downloaded from http://spaceplace.nasa.gov/news_images/dwarf-planets.jpg


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