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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 and 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?

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.

Other planetary systems can be detected indirectly by the back-and-forth movement of stars reacting to the orbiting planets, although stellar orbits around a planet-star center of momentum are extremely small and slow. Viewed from a nearby star 30 light-years away, the Sun's periodic motion in response to Jupiter subtends a maxi­mum angle of 0.5 milliarcseconds. In the 12-year period the velocity variation is at most 13 m/sec. Such indirect identification of other planetary systems should now be possible, because one can get angular precision of about 0.1 milliarcseconds and velocity precision of 5 m/sec with modern astrometric and Doppler-tracking techniques and the new large-diameter telescopes. Position and velocity measurements so obtained are sufficient in principle to let the observer deduce the number of orbiting bodies and their masses as a function of the mass of the parent star. Nevertheless, owing to the painfully long orbital periods, it will probably be a decade before the existence and properties of nearby planetary systems can be determined with confidence.

Planetary systems in the making are much easier to detect and image than planets already formed. In the primitive solar nebula, the material now buried in planets was spread in a thin disk approximately 100 astronomical units in diameter and a few AU thick at its largest extent. Elements heavier than helium resided primarily in solid particles, many of submicron size, whose combined surface area was very much greater than the surface area of a planet. Before it coalesced into the planets, the solid material's cross section for emission and absorption of radiation was greater by more than 12 orders of magni­tude. At wavelengths longer than 10 microns, where cool nebular dust emits most strongly, the radiation from a protoplanetary disk actually exceeds that from the star by several orders of magnitude.

The disk gas, which is almost all in molecular form, also radiates strongly at millimeter wavelengths, where the lowest rotational transitions of the heavy polyatomic molecules are found. This radiation provides a separate channel for investigating protoplanetary disks. Unfortunately, suitable very young candidate stars are at least 100 times farther away than the closest stars that might have fully developed planets.