Why was poor Pluto kicked out of the Solar System?

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Why was poor Pluto kicked out of the Solar System?

11:17 AM Fri, Sep 12, 2008 | Permalink
Avis Gunther-Rosenberg

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.

When I was a third grader at Cranston's Eden Park Elementary School, studying space was just plain easy:

You make a wish on the first star that twinkles. They say the moon isn't made of green cheese, but no one has been there yet to prove it isn't. And there are nine planets which are simple to remember from this quote, "Mary's Violet Eyes Make Jack Stay Up Nights Period."

So, whatever became of the Period, er, Pluto?

NASA's Space Place offers the answer:

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 Space Place 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.

Tags NASA planets Pluto space

Comments

Laurel Kornfeld said:

Actually, astronomers do not agree on a lot of the above, including the statement by the International Astronomical Union (IAU) that dwarf planets are not planets at all. Only four percent of the IAU members voted on this definition, and most of them are not planetary scientists. Most planetary scientists, those who specialize in studying planets, are not IAU members. A total of 300 professional astronomers, led by Dr. Alan Stern, Principal Investigator of NASA's New Horizons mission to Pluto, signed a petition saying they will not use the new planet definition and that a better definition is needed.

Pluto, Ceres, MakeMake and Eris do fit into the overall broad category of planets, just into one particular subcategory, the dwarf planets.

Link | September 12, 2008 4:34 PM
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Avis Gunther-Rosenberg said:

Thank you for your reply. Hopefully, other teachers will comment on what they are teaching about dwarf planets in their classrooms.

Link | September 15, 2008 1:45 PM
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