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Jupiter and Saturn: rare in the cosmos?

When astronomers search for planets orbiting other stars, they often look for bodies similar to Jupiter or Saturn. After all, these behemoths of the outer solar system are both big and bright. Thus, their presence should be easier to detect than the likes of Earth.

But the typical planetary system may not resemble ours, especially in its outer-most members, cautions George W. Wetherill of the Carnegie Institution of Washington (D.C.). In fact, he says, Jupiters and Saturns could be downright rare. Last week at the annual Lunar and Planetary Science Conference in Houston, Wetherill reviewed his theory and reported new calculations about the influence of Jupiter on our planet.

According to a popular scenario, all the planets in our solar system evolved from a disk of gas and dust that encircled the young sun. The inner planets arose from dust grains in the disk that clumped into planetesimals, which collided to form the planets. Planets born in the frozen reaches of the outer solar system --including Jupiter and Saturn - probably formed from an agglomeration of ice and dust in the disk (SN: 3/20/93, p. 190).

Jupiter and Saturn have huge atmospheres of hydrogen and helium surrounding their cores. Apparently, these giant bodies gravitationally grabbed these gases from the primordial solar disk. But therein lies a problem, notes Wetherill. Observations of disks around other stars indicate that the gases disappear in about 10 million years. Thus, Jupiter and Saturn must have developed their massive cores and snared circum-stellar gas, all within a few million years. This rapid sequence of events makes it unlikely, contend Wetherill and other scientists, that planets similar to Jupiter and Saturn are produced in assembly-line fashion around other stars. At best, he says, "failed" Jupiters and Saturns that never formed an atmosphere and stayed relatively small -- the size of Neptune --might be common in planetary systems.

In addition, notes Wetherill, planets in the outer solar system are not tightly bound to the sun. This enables their mutual gravitational tug to dramatically alter their orbits. In the average planetary system, such forces may move a Jupiter-or Saturn-like body into a highly elliptical or hyperbolic orbit, or perhaps eject the planet altogether.

Wetherill emphasizes that these ideas about Jupiter and Saturn are only speculative. "There could be a natural, self-regulating process that frequently leads to planetary systems resembling our own," he notes. But "more likely, relatively small natural variations in the distribution of angular momentum, mass, and temperature in the [circumstellar] disk, and the timing of loss of gas [from the disk] will lead to a variety of outer planet configurations."

David Black, director of the Lunar and Planetary Institute in Houston, says Wetherill's recent work confirms the view now held by many scientists involved in the search for planetary systems. Future instruments, he says, should have the capability of detecting planets one-thirtieth the size of Jupiter.

Wetherill adds that Jupiter's existence in our solar system has a profound influence on Earth. Through their gravity, both Jupiter and Saturn have acted to eject comets from the solar system early in its history. In addition, Jupiter gravitationally deflects comets that would otherwise bombard Earth.

He calculates that without Jupiter, comets would have struck Earth 100 to 10,000 times more frequently than they have. Moreover, the kind of impact believed to have killed off the dinosaurs--and that could decimate terrestrial life--would have occurred about once every 100,000 years rather than every 100 million years.

Concludes Wetherill: "Perhaps it should be expected that we have a Jupiter: Otherwise we wouldn't be here."
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Author:Cowen, Ron
Publication:Science News
Date:Mar 27, 1993
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