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THE SEARCH FOR FORMING PLANETARY SYSTEMS

For the first few million years of its life, the Sun was a large red star surrounded by a disk of gas arid dust-the primitive solar nebula-that reached out beyond the boundaries of our present planetary system. Like all stars, the Sun was created by the collapse of a cold, dense core within a rotating cloud of interstellar material, debris from supernovae and other evolved stars of a previous generation. Angular momentum in the cloud allowed collapse along only one axis, thus leaving the material in a thin orbital plane around the new Sun. During the next hundred thousand years, the dust began to coagulate into larger particles, which eventually became planetesimals, the seeds for the nine planets. Over a further 10 million years, these planetesimals grew larger, clearing the residual gas and dust by dynamical sweeping and gravitational capture. Collisions led to further growth and the creation of a system of planets, comets and asteroids orbiting the young Sun. Within a billion years, the first life came into existence on one of those planets. Now, 5 billion years later, that life has developed to a degree of sophistication that seemingly defies the second law of thermodynamics.
In general outline, this picture is the same as that put forward in the 18th century by Pierre Laplace and
Immanuel Kant. Two hundred years have added substantial refinements and quantitative predictions, but our current understanding of how the solar system formed remains largely theoretical, informed by the meteoritic record and by extrapolation from contemporary observations of the planets. We cannot verify the sequence of events by direct observation of the early solar system. But because it relates very much to our own origins, understanding this sequence is one of the central aims of modern astronomy and planetary science.
Technological innovations in the last decade have provided us with examples of other stars in some of the gestational stages of planetary formation. At one extreme, we have evidence of planetary systems in the making; at the other we see mature stars surrounded by what may be the detritus left over from a planet-forming epoch. These discoveries give us new means for estimating the number of planetary systems in the Galaxy.
Are planetary systems common or rare? If they are common, are they conducive to intelligent life? Are we alone in the Galaxy? Most important, can we clarify the pathway between the extreme gestational states we have observed so as to understand the sequence of events that lead to the formation of a mature planetary system?
Searching for planetary systems
The greatest impediment to direct observation of fully developed planetary systems is the enormous contrast in brightness and size between a star and even its largest satellites. At visible wavelengths, planetary radiation is predominantly scattered starlight. Jupiter can serve as the best-case example for assessing the probability of detecting planets outside the solar system. The total sunlight reflected off Jupiter is ten orders of magnitude less than the direct solar output. Such glaring contrast is too great to be detected with existing telescopes, even if one were looking for a Jovian planet orbiting our nearest neighbor star, only 4 light-years away. At longer wavelengths, where the solar radiation decreases and planetary thermal emission increases, the contrast is less. But because the diffraction-limited angular resolution of a telescope decreases with increasing wavelength, current instruments could not resolve a nearby Sun-Jupiter-type system at wavelengths longer than about 5 microns.
Evolution of planetary systems
After the gravitational collapse of a protostellar core out of a cloud of molecular gas and dust (mostly submicron bits of silicate and carbon compounds), the young star is surrounded by a disk whose diameter is typically between 100 and 1000 AU. While the protostar and its disk are embedded in a still-infalling envelope of gas and dust, stellar winds force a characteristic "bipolar" outward flow of material through the poles of the spinning envelope, as shown in figurej2b. For a star of modest size like the Sun this-bipolar outflow phase lasts for about a million years.
The planets of our solar system revolve in nearly circular orbits coincident with the plane of the Sun's rotational equator, strongly suggesting that planet formation took place within a flattened dissipative disk around the proto-Sun. Thus planetary* system formation in general probably begins at some point during the short-lived bipolar outflow phase and continues within the surrounding nebula as mass loss and turbulence subside. Micron-size grains settle toward the disk's mi4plane, where they collide and coagulate over a few million years to become centimeter-size particles and, eventually, kilometer-size planetesimals. The rocky "terrestrial" planets like the Earth, all of which lie within 4 AU of the Sun, probably formed from the planetesimals within 100 million years of the cloud collapse. In the colder precincts farther from the Sun, ice grains composed of volatile compounds condensed. As a result, the more massive Jovian planets accreted volatile material from the proto-solar nebula onto their rocky cores. This accretion of volatiles implies that Jupiter and the other giant planets must have formed before the solar nebula was completely dissipated.
Figure 2c suggests how dissipation of a preplanetary nebula proceeds. Probably the first sign of planet formation is a central void adjacent to the star or annular gaps in the extended nebula. Over time the newly formed planets effectively "shepherd" the intervening gas into discrete rings, just as Saturn's moons shepherd its rings. Eventually the material in the rings is accreted by the forming planets or dissipated by the remaining stellar wind (which is not to be confused with the far more energetic bipolar outflow), leaving a system like the one shown schematically in figure 2d, where fully formed planets in a relatively rarefied medium orbit a star that is more than 100 million year old.