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Our Glaciers: Then and Now activity kit helps you see the changes taking place in glaciers around the world. See all our activity kits and classroom activities.

What can you tell me about Planet X, the unknown planet?

Planet X used to be the name for a suggested 10th planet. The "X" means "unknown." Now you may ask, what makes scientists think that there is a 10th planet? Scientists can tell by the motion of the planets where other planets might be. So far, even with the discovery of Pluto, this motion suggests that there may be another planet still to be discovered. (By the way, it was this way that the planets Neptune and Pluto were discovered in the first place.)

However,it now looks like that the idea of the 10th planet can be satisfied by the Kuiper Belt, an asteroid belt outside of Neptune and Pluto. So there probably is no planet X.


Submitted by Kayla (North Dakota, USA)
Submitted by Eric (California, USA)
Submitted by Dirk (Wisconsin, USA)
Submitted by Amanda (New Jersey, USA)
Submitted by Austin (Texas, USA)
(December 29, 1997)



Last modified January 29, 2001 by Jennifer Bergman.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA