Where do comets come from?
Comet Hale-Bopp, like most comets, has an orbit that is very different from the planets. The planets are all circling the Sun in roughly the same plane, as if they were all traveling on the same dinner plate. But Comet Hale-Bopp came swooping in from above the solar system, passing through the northern skies for a few months, then passing through to the southern part of the skies where only people who live in the southern hemisphere could see it. While a comet is looping in toward the Sun, the Earth is circling in its own orbit. When, the Earth is on the opposite side of the Sun from the comet, the comet is hard to see. Comet Hale-Bopp passed closest to the Earth on March 22 and closest to the Sun on March 31, so this was when the comet looked its brightest. Usually the best display is for a few weeks after a comet passes closest to the Sun.
I’ve been thinking about comets a lot lately, trying to image C/2009 G1, reading “The Hunt for Planet X”, wondering why Galileo was so wrong about them and recently reading a Young Earth creationist blog post on them. The latter referred to a very interesting pre-publication article. And I’d like to discuss this article, as this illuminates not only the origins of comets but also how science is done.
Mathematical theory suggests that most comets may come to the solar system from very far away, as far away as 100,000 AU. An AU is the distance from the earth to the sun and is roughly equivalent to 100,000 miles. Mars is 1.5 AU from the sun, Jupiter is 5 AU from the sun, and Pluto is 39 AU from the sun. So comets come from very far away indeed. Comets are observed to come to the solar system from all directions, therefore the place where the comets come from is thought to be a giant sphere surrounding the solar system. This sphere is called the Oort cloud. Thus comets are said to come from the Oort cloud. But some comets may come to the solar system from closer in. The place where these comets come from is called the Kuiper Belt, which is located past the orbit of Pluto. It is natural to ask How did comets get so far away?
Comets are believed to have two sources. Long-period comets (those which take more than 200 years to complete an orbit around the Sun) originate from the Oort Cloud. Short-period comets (those which take less than 200 years to complete an orbit around the Sun) originate from the Kuiper Belt. Danish astronomer Jan Oort proposed that comets reside in a huge cloud at the outer reaches of the solar system, far beyond the orbit of Pluto. This has come to be known as the Oort Cloud. Statistics imply that it may contain as many as a trillion comets and may account for a significant fraction of the mass of the solar system. However, since the individual comets are so small and so far away, we have no direct evidence about the actual existence of the Oort Cloud.
