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.docTHE SEARCH FOR FORMING PLANETARY SYSTEMS
Planet formation used, to be a subject for speculative natural history. Now it is an active field of observational astronomy, and the evidence is that planetary systems are abundant.
Anneila I. Sargent and.Steven V. W. Beckwirh
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.
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. (The astronomical unit is the mean distance between the Earth and the Sun; the radius of Pluto's orbit is 40 AU.) 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. Therefore one needs very sensitive instruments both to resolve these protoplanetary disk structures and to discriminate their emission from, the strong background radiation of Galactic star-forming clouds.
New technology
The impetus for the present generation of searches for forming planetary systems comes largely from recent advances in infrared and millimeter-wave instrumentation. In 1983 a consortium of American, Dutch and British scientists launched the Infrared Astronomical Satellite, designed to survey the sky at wavelengths of 12,25,60 and 100 μm. Liquid helium cooled the 60-cm telescope to a few kelvin. This cooling reduced background emission from the telescope structures and improved the sensitivity to long-wavelength radiation a thousandfold. Most important, IRAs mapped the entire sky during the year its coolant lasted. One of its first electrifying discoveries was the detection of systems of solid particles in 'of bit around more than a dozen mature nearby stars. The most reasonable interpretation was that these particles were the debris left over after the formation of planetary systems in our local neighborhood.
At about the same time, major advances in the instrumentation and technology of millimeter-wave observation markedly improved both sensitivity and angular resolution in this regime. The largest millimeter-wave instruments then in use were 12-meter telescopes limited by diffraction to resolutions no better than about 1 arcminute. But by connecting several such telescopes together into interferometric arrays, groups at Caltech and Berkeley were able to achieve angular resolutions of a few arcseconds. The resolution of these arrays is determined by the separation of the telescopes — up to 200 meters — rather than by the size of an individual telescope. At the distance to the nearest young stars, 1 second of arc corresponds to about 150 AU, about twice the diameter of the solar system. These interferometric telescope arrays made it possible to detect the gas in a planet-forming disk. In 1986 one of the first maps produced by these arrays showed just such a gaseous disk.
To improve sensitivity at millimeter wavelengths the IRAM consortium of French, German and Spanish radio-astronomers built a 30-meter telescope at Pic6 Veleta in Spain with great surface accuracy. This sixfold increase in collecting area over the largest existing millimeter-wave telescopes yielded a corresponding increase in sensitivity to long-wavelength radiation. It was now possible to detect the thermal emission from solid particles embedded in a distant protoplanetary nebula fast enough so that many nearby stars could be searched. In its first two years the IRAM 30-meter telescope looked at more than 100 stars and discovered nebulae circling almost half of them!
It is common for great technological advances to uncover new cosmic phenomena. It is less common for three new technologies simultaneously to revolutionize the inquiry into an outstanding problem. The IRAs satellite, the millimeter-wave arrays and the 30-meter millimeter-wave telescope represent the cornerstones of the triad that is offering us a very good look at young planetary systems. Far-infrared emission implies the presence of solid particles near the stars; the temperature of this orbiting material determines the shape of the far-infrared spectrum. High-resolution images of interstellar gas reveal the size and velocity fields of the circumstellar disks. And the millimeter-wave continuum emission is sensitive to the mass of the material: It can tell us whether there is enough matter circling the star to accumulate into large planetary bodies. Taken together, these three methods have stimulated rapid advances in our understanding of protoplanetary nebulae. They have reconciled a lot of earlier data and unified several subfields of astronomy.
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.
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