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The Question

(Submitted October 27, 1998)

Recently, in physics class we learned that the orbit of Pluto was not co-planar to the other eight planets, in fact it was 17 degrees off! I want to know why this is? If you would be so kind as to answer my question and e-mail me back I would be much obliged.

The Answer

The other important thing about Pluto's orbit is the 3:2 resonance between Pluto and Neptune; that is, in the time it takes Pluto to orbit around the Sun twice, Neptune will have orbited the Sun three times. It is clear from this that there is a special relationship between the orbits of Neptune and Pluto, and also probably the origins of Neptune and Pluto.

Starting in 1992, astronomers (such as Dr. David Jewitt at University of Hawaii) have began to discover a large number of small objects in the outer Solar System, the so-called Kuiper Belt. It turns out that a surprising number of these are also in 3:2 resonance with Neptune. These are nicknamed "Plutinos", or little Plutos:

http://www.ifa.hawaii.edu/~jewitt/kb/plutino.html

Many Plutinos share properties of the high eccentricities and significant inclinations with Pluto. It is therefore natural to associate the presence of so many Plutinos with their interesting orbital properties.

According to one theory (which Dr. Jewitt calls 'as good as any, and better than most'), Neptune migrated outwards during the accretion stage of solar system formation. During this process, Neptune trapped many planetesimals in the 3:2 resonance and carried them outwards with it. Although these planetesimals were originally in more or less circular orbits on the same plane as the planets, their eccentricities and inclinations increased as they were pushed outwards.

Clearly, this is not a settled question, and more theoretical and observational works are necessary.

Here are a selection of other web sites related to this question:
http://www.seds.org/nineplanets/nineplanets/plutodyn.html
http://www.seds.org/nineplanets/nineplanets/pluto.html
http://www.boulder.swri.edu/~buie/pluto/pluto.html

Best wishes,

Koji Mukai & Maggie Masetti
for Ask an Astrophysicist

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