Kids ask: Why is Pluto not a planet?
"Why is Pluto not a planet?," asked Nicole Johnson, 10, a fourth-grader at Diamond Lake School in Mundelein.
Pluto got its name from an 11-year-old girl who suggested that our solar system's smallest planet be named after Pluto, the Roman God of the underworld. That was back in the 1930s when the planet was first discovered, and now scientists aren't sure if Pluto really is a planet.
Is it a planet, or merely a space object? Scientists are still trying to decide. Part of the problem is that there's no checklist that tells us what is a planet and what isn't.
Two guidelines - mass and orbit - help decide what's a planet.
Mass is the amount of material in an object. "The more mass an object has, the stronger its gravity is, and the more likely it will be spherical," said Dr. Geza Gyuk, astronomy director at the Adler Planetarium. "This means everything on the planet's surface is the same distance from the center of the planet. Large objects in the solar system tend to be spherical."
An orbit is the journey a planet makes as it revolves around the sun. The gas and dust ball that became our solar system spun off planetary objects which are still in motion as they orbit the sun.
Gravity also works like a frenzied housekeeper, sweeping away space objects in the path of the orbit. The more massive an object, the stronger the gravitational force, the cleaner the space is around the planet.
"Pluto is very much smaller in mass than the eight planets," Dr. Gyuk said. "It's large enough to be spherical, but not large enough to clear its orbit. Hundreds of other objects share that part of the solar system, including tens that are approximately the same size as Pluto."
For the eight major planets, that orbit is circular and on the same plane. Pluto's orbit is a mess - sometimes closer to the Sun than the planet Neptune, sometimes a billon miles farther away. Comets and asteroids can share these characteristics.
So what is Pluto? Scientists call it a trans-Neptunian object. That's another word for space material that lies outside Neptune's orbit. It is one of the largest icy balls in the Kuiper belt, a portion of the solar system that lies just beyond Neptune and contains about 70,000 space objects.
A compromise is to call Pluto a dwarf planet. In the future, astronomers and scientists will be able to analyze close-up images of Pluto and the Kuiper Belt as NASA's New Horizons spacecraft arrives for a visit beginning in July 2015.
For more information about our universe, visit the Adler Planetarium Web site at adlerplanetarium.org.
Check these out
The Vernon Area Library in Lincolnshire suggests these titles on Pluto:
• "Pluto: From Planet to Dwarf," by Elaine Landau
• "When is a Planet not a Planet? The Story of Pluto," by Elaine Scott
• "Why Isn't Pluto a Planet?," by Steve Kortenkamp
• "A Look at Pluto and Other Dwarf Planets," by Anna Kaspar
• "The Planet Hunter: The Story Behind What Happened to Pluto," by Elizabeth Rusch