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

Planet X used to be the name for a proposed 10th planet. The "X" stands for the "unknown quantity." Now you may ask, what makes scientists think that there is a 10th planet? The reason is that there is are laws of nature (Kepler and Newton) which balance the motion of the planets with how much mass is in the solar system. So far, even with the discovery of Pluto, these laws suggest that there may be another planet still to be discovered. (By the way, it was by using this law that the planets Neptune and Pluto were discovered in the first place - that is, scientists knew that they should look for more planets because the law could not be balanced).

The Voyagers' results have now yielded more accurate measurements of the masses of the planets. It now looks like that 10th planet can be satisfied by the material which is in 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.

You might also be interested in:

How many planets orbit the sun?

I was wondering if there is a new planet? Are there planets (a tenth planet?) after Pluto belonging to our solar system? What are the names of the new planets discovered in the solar system? Are there...more

What are the retrograde motions of planets in the sky?

It depends on which type of motion you are asking about. If you take a birds-eye view from the top of the solar system all the planets orbit around the Sun in a counter-clockwise (or direct) direction....more

How do Astronauts Live in Space?

Almost everyone has a question or two about living in space. What do astronauts do in space? How do they do everyday things like eat, sleep and go to the bathroom? It's important to note that astronauts...more

How far is the Earth from the Sun, the Moon and all the other planets? How far are all of the planets from the Sun? Do you know of a software that tracks the planets in real-time?

There is a really neat internet program called Solar System Live that shows the position of all of the planets and the Sun for any given day. If you go to that page, you'll see an image similar to the...more

Is it really true that man never really walked on the Moon?

The picture of the American Flag (the one put there by the Apollo astronauts) is waving (or straight out) in the wind. How could that be possible if there is no atmosphere on the Moon? Was it some sort...more

According to Stephen Hawking, any object with an energy which equals Plank's energy has to become a black hole.

If that is so, the energy released during the Big Bang must have created many such black holes. Therefore most of the Energy of the Big bang must have disappeared in that form. Then how did the Universe...more

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