Chapter 8: Interplanetary Bodies

Portrait of Tycho Brahe

In ancient times no one knew how far away comets were. Many people thought that they were phenomena in our own atmosphere. Seneca, the Roman contemporary of Jesus, wrote: "Some day there will arise a man who will demonstrate in what regions of the heavens the comets take their way." That man was Tycho Brahe. He arranged observations of a bright comet in 1577 from two different locations. The comet showed a parallax shift with respect to the stars as seen from the two places. Brahe proved by triangulation that the comet was more distant than the Moon. This ruled out the old theory that comets were terrestrial. He used direct observations and simple geometry to sweep away centuries of mystery and speculation. This is the way that science works as we try to make sense of the universe.

Gerard Kuiper

Jan Oort

Where do comets go when they are not visible to the eye or the telescope? The solution to this puzzle began with the work of Dutch astronomer Jan Oort and the Dutch-American astronomer Gerard Kuiper in the 1950s. Oort was a remarkable astronomer who worked on everything from comets to cosmology; he was an active researcher well into his 90s. He also established a strong tradition of astronomy in Holland that continues to this day. Kuiper was also a visionary. He spent a number of years searching for the best places to do observational astronomy — the ideal sites are high, dry, and dark. Kuiper was convinced of the merits of Mauna Kea in Hawaii, but at the time it was a barren, extinct volcano at the inhospitable altitude of 14,000 feet. Today it is host to one of the largest collections of telescopes in the world.

Oort and Kuiper studied a large collection of data on comet orbits with particular attention to how long comets spent at large distances from the Sun. They also thought carefully about the way the Solar System might have formed. They deduced that there must be two major sources for comets, both in the extreme outer edges of the Solar System. According to Kepler's third law, more distant objects have longer orbital periods. Kepler's second law says that an object travels more slowly when it is far from the Sun. If the reservoir of comets is far from the Sun, then comets will have very long orbits and they will spend most of their time far from the Sun.

Several distinct groups of comets have been recognized. Oort proposed that there is a spherical swarm of comet nuclei far beyond Pluto that orbits around the Solar System like bees around a hive. The comets are 50,000 to 150,000 A.U. from the Sun, much too far from Earth to be seen. They take 10 million to 60 million years to go around the Sun. What led Oort to this striking hypothesis? He first noted that most comets are one-time visitors. Unlike Halley's comet, they have not repeated their orbits within human history. He also knew that comets could arrive from any direction; this suggests that they occupy a spherical region rather than the flat plane defined by the planet orbits. From Kepler's law, Oort knew that each comet must spend most of its orbit far from the Sun. So for every comet seen in the inner Solar System, there must be many more lurking in the depths of space beyond Pluto. From statistics of comet orbits and their rate of appearance, Oort calculated that roughly 100 billion inactive comet nuclei, invisible from Earth, lie in this frigid, distant region!

Diagram of the Oort Cloud

Many comets that we see in the inner Solar System are temporarily "dropping in" from the Oort cloud. They travel on huge elliptical orbits that bring them thousands of A.U. to the inner Solar System, where they loop around the Sun at a distance of only a few A.U. or less and then return to the deep freeze of the Oort cloud.

Kuiper discovered a second group of comets just beyond Neptune and Pluto, lying roughly in the plane of the Solar System. Called the Kuiper belt, this group is concentrated at a distance of 30 to 100 A.U. from the sun. Comet nuclei located in this group are mostly too faint to be seen even with large telescopes and are too cold to give off gas or form tails. Nonetheless, sensitive searches with electronic detectors began to turn up comets in this region in 1992. Astronomers David Jewitt and Jane Luu, working in Hawaii, discovered more than a dozen such objects in the mid-1990s. These are the largest and closest comets in the Kuiper belt; most are located at distances from 35 to 45 A.U. from the Sun. Jewitt and Luu estimated that the Kuiper belt contains 35,000 comet nuclei bigger than 100 kilometers and even more small ones.

Observations of Sun-like stars sparked renewed interest in the Kuiper belt. By carefully blocking out the light from the star, astronomers revealed that a number of these stars have disks of dust — possibly comet debris — surrounding them. The disks reach out to several hundred A.U. from the star. If stars like the Sun have Kuiper belts of cometary debris, it could be a sign that they have planets too. Perhaps planets and comets are a natural consequence of star formation. Perhaps our rocky vantage point in space is not unique. This speculation spurs astronomers in their search for debris around stars.

How do comets get from the Oort cloud or the Kuiper belt into the region of Earth, where we can see them? We do not know for sure but we have a plausible scenario. Comets are probably deflected from their initial, distant orbits by the gravity of passing stars. Some may move directly into orbits that pass inside Earth's orbit; others may get deflected only into the region of the giant planets. The set of comets that come close to the outer planets are called Centaurs, after the half-man, half-horse creatures of Greek mythology. Centaurs pass among the giant planets and some of them get deflected toward Earth too. All of these orbits are complex and difficult to predict.