Jupiter and Saturn: Rare in the Cosmos?
Cowen, Ron, Science News
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. …