We don't have a 10th planet... yet
Q: A classmate told the class that a 10th planet had been discovered behind Pluto but I can't find anything about it on the internet. I can only find
that the scientists believe that there is one but not that they actually found it. IS there a 10th planet and where can I find out more?
A: NASA says no. "There is no known Planet X or 10th planet in our solar system." We don't need a 10th planet to explain the orbits of the outer planets.
You can find out more by checking the web sites in "Further Surfing" below.
Below: [NASA] Hubble Space Telescope picture of Pluto, 1996. 
But, what's a planet? Is Pluto? Certainly it is because it meets the two criteria:
- 1. Orbits the Sun.
- 2. Is big enough to maintain a spherical shape under its own gravity.
What a strange icy, little dwarf of a planet it is, though. Pluto's orbit slants severely from the ecliptic (plane of Earth's orbit
about the Sun) and is so far from circular that it crosses Neptune's orbit. Pluto doesn't hit Neptune because, where the orbits
cross, Pluto is high above Neptune. And that is because Pluto completes two orbits around the Sun for every three of
Neptune's orbits. This is called 3:2 mean-motion resonance.
"Pluto (smaller than our Moon) is still under construction," says Richard Teske, professor emeritus of the University of
Michigan. The solar system is still incomplete, five billion years later. Way out, between Neptune and Pluto and
beyond-zillions of small, icy bodies zing around the Sun in a belt named after the Dutch-American astronomer, Gerard P. Kuiper, who in 1951 theorized such a
distant family.
"The bigger ones are built up from collections of the smaller ones," says Teske. These are like the bodies which gathered together to build Saturn, Uranus,
Neptune, and are still building Pluto. The farther out you go, the younger the planets are because orbital speeds slow and collision times for sticking little
bodies to big get incredibly long.
Pluto lies in the belt and "...now appears special only because it is larger than any other member of the Kuiper belt," say Jane Luu of Harvard University and
David Jewitt of the University of Hawaii, writing in Scientific American, May 1996.
We have discovered over 300 Kuiper Belt Objects since Jewitt and Luu discovered the first in 1992. Up to half the bodies have the same 3:2 mean motion
resonance as Pluto does--another similarity. Recently, in 1998 and 1999, astronomers Lynne Allen, Gary Bernstein, and Renu Malhotra found two dozen
more, nine of which are 100 miles or more across (about 5% the Moon's diameter) and most lie within Pluto's orbit. This trio of astronomers think yet more
distant objects may well be out there-small, to be sure, but planets in the making, and beyond Pluto.
Is there a 10th planet beyond Pluto? Sure, scads of them if you relax Rule 2 above: the Kuiper Belt Objects. They may not have enough gravity to be spherical
yet. But, give them time.
(Answered by April Holladay, science correspondent, August 1, 2001)
Further Surfing:
USATODAY.com, Asteroid found beyond Pluto
Starchild/NASA: Is there a planet X or 10th planet?
U of Hawaii: Kuiper Belt
NASA: Pluto-Kuiper 2020 mission
Royal Observatory Greenwich: The planets
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