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Beyond Pluto

Week of October 20, 1997 image of Kuiper Belt object

If you thought our tour of the solar system ended with Neptune and Pluto, you are setting your sights way too low. The planets of the solar system mark time relatively close to the Sun compared to interstellar space. Pluto orbits the Sun at an average distance 40 times that of the Earth, yet the nearest star (the Alpha Centauri system) to the solar system is a mind-numbing 260,000 times the distance of the Earth to the Sun. Between us and that star is a vast, empty desert of interstellar space. Or is it?

Most comets, like the recently seen comets Hale-Bopp and Hyakutake, are spotted relatively close by. Hale-Bopp was discovered farther from the Sun than any other comet so far, when it was a barely farther away than Jupiter. But those comets must come from somewhere, someplace where they are kept very cold (or else they would quickly vaporize) and where we cannot see them. They must come from far, far from the Sun. In the 1950's Jan Oort carefully measured orbits of comets and decided they must come from a vast, spherical cloud centered on the Sun that could be as large as a light year in diameter. A comet might take millions of years to orbit the Sun once at that distance. Although never directly seen, most astronomers assume the Oort cloud exists.

But it doesn't explain a simple observation: many short term comets (that is, comets with short orbits) seem to prefer to come in not from any random direction, as they would from a spherical cloud, but in a plane that coincides with the orbital planes of the planets of our solar system. Another astronomer, named Kuiper, proposed that these objects came from much closer in to the Sun, and resided in a flat disk. For years no one ever saw any Kuiper Belt objects, but in 1992 Jewitt and Luu discovered the object 1992 QB1, the first confirmed object beyond the orbit of Pluto. The picture above is the discovery image taken by Jewitt and Luu, and I got it from Gary Kronk's excellent pages about comets and meteor showers.

Since then dozens have been found. They are terribly faint, being so far away and so small. Many also appear to be very red, which may indicate the presence of organic compounds (which are necessary for life, but do not mean these objects have life on them). Some people think that perhaps Pluto is itself the king of these objects, and may not have formed in the same way as the other planets.

For now, the game is to find as many of these objects as possible, so that we may start looking at them statistically instead of case by case. The more we know about them, the better our theories can get to explain them. I myself am marginally involved in this; I have thought about using STIS parallels to try to find some Kuiper Belt objects. The odds are low, though: I did a quick calculation and I might have a 1 in 100 chance of finding any brighter then about 26th magnitude. Oh well. I'll keep trying though!

Want to know more about Kuiper Belt objects? The Nine Planets is loaded with info. You can skip right to the Kuiper Belt pages too.

©2008 Phil Plait. All Rights Reserved.

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