Planet Formation

Type of physical science: Astronomy; Astrophysics

Field of study: Planetary systems

In the densest portion of a contracting solar nebula, a star is forming. Assuming that conditions are suitable, a family of planets can also form. The processes that influence planetary formation are extremely complicated, but reasonably well understood. Planet formation may be a very common occurrence throughout the galaxy.

Overview

Planet formation is the general term used to describe how various planetary bodies formed from the residual material surrounding a forming star. The two most discussed theories suggest either a catastrophic or evolutionary origin for the planets. In either case, the planets would have formed in a similar manner to the star, but on a much smaller scale and with considerably less mass.

One theory in particular suggests that the formation of a solar system begins with the supernova explosion of a massive nearby star. It begins with a slowly contracting cloud of gas and dust that is capable of forming tens of individual stars. This is called a cocoon nebula. Where the concentrations of matter are densest, giant stars will form. If they are massive enough, they will progress rapidly through their evolutionary stages and explode with gigantic force. The energy released may rival that of a galaxy for a short period of time. The compressional shock waves generated by the explosion, along with the excess matter thrown off, can initiate the formation of smaller nearby stars such as the sun. Evidence for such an event is suggested by certain isotopic ratios for various elements present in meteorite compositions and even the sun itself.

The majority of mass in a solar nebula will go into the formation of individual stars.

Some estimates indicate that a star will account for greater than 99 percent of the total mass. The remaining 1 percent will represent the matter available for planetary formation. Increasing pressures and temperatures at the gravitational center of a solar nebula will eventually form a protostar. This is the initial stage that precedes the beginning of nuclear processes in a star. The protostar is more representative of a hot glowing ball of gas than a true star. It is believed that planets will commence their earliest formation processes at this time as well.

Planet formation is characterized by three distinct processes: gravitational collapse, condensation, and accretion. The initial process that takes place in the solar nebula is gravitational collapse. This is where atom attracts atom to form molecules of considerably greater mass. This greater mass increases gravitational pull and thereby attracts more atoms. As these localized regions become more dense through the infall of matter, temperatures begin to increase. Gases and solid dust grains that were originally cold now begin to heat up and glow as more and more matter becomes concentrated in a rapidly shrinking volume. Eventually, all matter will be converted into a hot gaseous state.

After the majority of gas has been incorporated into the star's formation, the residual hot gases begin to cool and re-form as solids. This is the condensation process. Every element has a specific condensation temperature where a phase change takes place. Metallic elements such as aluminum and magnesium condense from a gaseous state at very high temperatures, while gases such as methane or water vapor condense at low temperatures. Where concentrations of matter were greatest near the proto-sun, only heavy elements would condense. As matter thinned out, temperatures dropped progressively and permitted lower-temperature elements to condense. Based on existing temperatures, distance from the protostar would, in part, control the abundance ratios of the various elements found in those regions. This is one reason that the inner planets are high in iron content relative to the outer gaseous planets.

Material in the residual gas rings that surrounded the protostar was pulled to the plane of the ecliptic by the force of gravity. This material would tend to clump together as it condensed out of the cooling gases. Some scientists believe that this condensation phase was rapid enough to trap some of the lighter gases within the forming clumps of heavier elements. This may account for the large amounts of volatiles present during the final stages of planetary formation.

As condensation progressed, only microscopic grains were in evidence. Later, these would combine to form larger and larger particles. Growth would continue on through sand-sized proportions, to pebble-sized, and then boulder-sized objects. This describes the accretion process, where collision produces a larger mass rather than fragmentation. Accretion would not stop with boulders, but would continue on eventually to form planetesimal-sized objects much like present-day asteroids. They would have had a size range from 30 meters up to about 1,000 kilometers in diameter. Finally, planetesimals would collect one another and assume the planetary dimensions familiar today.

The present-day planets are not uniform in their chemical compositions. The inner planets (Mercury through Mars) are rich in heavy metals, while the outer planets (Jupiter through Neptune) are gaseous. One explanation for this may be that the solar nebula was not uniformly hot, because of unequal distribution of material based on distance from the protostar. Only the area from the sun outward to the asteroid belt was sufficiently hot to experience heating of the original nebular material and subsequent condensation. Material that was present at the distance of Jupiter and beyond remained in its unaltered state until it became concentrated in the protoplanetary stage of formation.

An additional factor to be considered in planetary formation is the T-Tauri stage in the development of a main sequence star like the sun. This is a hyperactive stage in the young star's life, as fusion reactions begin to dominate. As a result, large quantities of matter are ejected at a very rapid rate. It has been suggested that the early sun may have lost as much as one-half its original mass in as short a period as 1 million years. This would be evident as an intense solar wind many times greater than it is today.

Distance would once again be an influencing factor in planet formation. As the solar wind came in contact with the inner protoplanets it would have stripped away the lighter elements, thereby concentrating the heavier ones. Yet, even with its great intensity, it would lose most of its effectiveness at the distance of the present asteroid belt. It may have been virtually ineffective by the vicinity of Jupiter, thereby leaving the outer planets with most of their original mass. In fact, the Jovian planets may have increased their mass as gravity collected additional matter flowing past from the inner solar system.

In the end, the planets as they are known today were formed. The processes at work were very complex with many variables present. Collectively, the above theories are known as the nebular hypothesis. It offers reasonably logical explanations for a very difficult problem.

Experimentally, several aspects of this theory can be examined in the laboratory through high temperature and pressure studies on various materials believed to have been present in the nebula. Nevertheless, in the final analysis, no experiment can reproduce an actual solar system or a planet. It is possible, however, to compile all available data into a computer model and develop a graphic representation of what may have taken place. Undoubtedly, its accuracy will remain a topic of much discussion.

Scientists do not have to rely totally on computer modeling for answers; there are actual clues. Evidence for the condensation and accretion phases of planet formation can be found in the chemistry and mineralogy of stone meteorites. The chemical compositions of minerals found in meteoritic objects called chondrules bear witness to an early high temperature condensation history. The physical condition of these chondrules--as they are found in the meteorite--also exhibits evidence of a very violent phase where collisions were commonplace.

These normally spherical-shaped objects can be found as fragments or in a partially melted and deformed state. They were incorporated later into a reforming object that eventually would reach planetesimal proportions. Although scientists do not totally agree about the mechanisms for the origin of chondrules, they are certain of their primitive nature.

The bulk chemistry of meteorites also provides additional information about planetary formation. One type of meteorite, called a type one carbonaceous chondrite, contains the most primordial chemistry of all materials in the solar system. Its chemical composition most closely resembles the "condensable" part of the sun's chemical makeup. Also present are low-temperature, water-bearing minerals; carbonate and sulfate minerals; and 8 percent to 22 percent water. High-temperature minerals are rare, and chondrules are notably absent. This strongly suggests that this type of meteorite may be very close in chemical composition to the original unaltered material of the solar system.

The unique primordial nature of meteorites was destroyed as they were incorporated into the growing planetesimals. As the planetesimals accreted further into actual planet-sized masses, evolutionary processes took over to erase all evidence of their protoplanetary stage. For the inner planets, intense bombardment from infalling planetesimals turned their surfaces into a crater-dominated world. Those planets that had sufficient mass would heat up internally, with volcanism, plate tectonics, and erosion eventually reshaping their surfaces. In contrast, the outer planets never got beyond their protoplanet stage. Their great masses kept them hot, and they never cooled off. As they are seen in the 1990's, these planets may offer scientists the best evidence for what all planets may have been like in the very beginning.

Applications

The nebular hypothesis for planetary formation provides good explanations for many of the dynamical and chemical properties found in the solar system, but it is not all-inclusive. Some inconsistencies do exist, such as the axial tilt and the rotational direction of Uranus. This planet is more representative of a catastrophic event that may have literally knocked it on its side.

Comets are another example. Although much is known about their physical and chemical characteristics, little is known about how they fit into the nebular hypothesis. They apparently come from the outer fringe of the solar system, but their highly elliptical orbits are not consistent with protoplanetary theory. A likely explanation would suggest that a catastrophic event is somehow responsible here, too.

One place scientists can look for possible confirmation of the nebular hypothesis is in the satellite systems of the gaseous planets. At their formation these protoplanetary bodies resembled small stars rather than planets. It is possible that they acted in a similar manner to a forming star with respect to their satellite families.

One can envision a protoplanetary Jupiter gravitationally gathering in the largest amount of mass and perhaps radiating a small amount of energy just as the early proto-sun did.

Its mass would not permit it to progress further, but it may have remained hot long enough to influence the chemical compositions of its inner higher density moons (Io, density 3.5 and Europa, density 3.0). In contrast, the two larger outer moons, Ganymede and Callisto, have densities of 1.9 and 1.8, respectively. This comparison between the moons of Jupiter and the arrangement of planets by density is inescapable. The inner two Jovian moons are composed of rock, ice, and small amounts of metal, while the outer two are dominated by ice and dust.

Astronomers are investigating if they truly formed according to protoplanetary theory. Only future spacecraft exploration will tell.

The ring systems of the outer planets may well answer some of the questions concerning accretion and planetismals formation. For example, are the rings representative of a fragment satellite, or one just coming together? What about orbital dynamics, and the influence of shepherding satellites on the orbital motion of the particles in the rings? These are all valid questions whose answers will undoubtedly shed light on planetary origin.

As one moves outward through the solar system, the density comparison between satellites in a given system is not so apparent as it is in the Jovian system. These satellites all appear to be relatively uniform mixtures of various ices and dust. Even large Titan, with a density of 1.9, does not match up to those of Io and Europa. It is then apparent that temperatures in these far-removed regions of the solar nebula were merely too cold to permit any large concentration of heavier elements regardless of the influence of the planet they orbit.

Perhaps the most striking evidence for accretion can be found in the physical characteristics of the small Uranian moon called Miranda. Apparent in the Voyager 2 photographs were two distinctly different surfaces. One was a densely cratered terrain so typical of the low-density ice satellites of the outer planets. The other had no comparison. There were circular patterns of angular landforms surrounded by alternating bright and dark bands, along with fault scraps and ridges. These were clearly of a much younger age than the ancient cratered surface. Scientists are searching for a possible explanation for such a unique satellite. One theory suggests that Miranda may have experienced an early partial internal melting. This would have been followed by a low-velocity impact that broke it in several pieces. These large pieces remained in close orbits to one another, and in a relatively short period of time reaccreted into a single large mass. The patchwork nature of Miranda's surface makes this a strong possibility and would thus provide science with an example of an actual accretionary body.

Theory, and the examples from the solar system can provide valuable evidence for planet formation, but scientists would like to see the actual event as it happens. Considering the vast number of young stars in the Milky Way, this should be possible, but distance prevents direct telescopic examination. Yet, other indirect methods of observation may provide the information necessary to confirm the nebular hypothesis.

In 1983, the Infrared Astronomical Satellite (IRAS) began gathering data on various sources of infrared energy in space. This included energy emitted from newly forming planetary systems. One of the first candidate sources was the young star Vega. IRAS showed it to be surrounded by a spherical cloud of cool dust just as theory predicted for a forming planetary system. A second observation showed the star Beta Pictoris to have a dust cloud surrounding it in a typical flattened plane. Optical observations confirmed this disk to have a radius ten times larger than that of Pluto's orbit and that the dust was orbiting within 5 degrees of that star's plane of the elliptic. Evidence such as this does not necessarily confirm the nebular hypothesis, but it strongly supports it.

Context

Prior to the mid-seventeenth century, most theories for the origin of Earth and the solar system were based on either myth or religious interpretation. Later, science would tend to take either an evolutionary or catastrophic approach. It was realized quickly that no comprehensive answer would be forthcoming, and the "best-fit" model concept was developed to provide answers based on the best science available.

In 1644, the French philosopher Rene Descartes presented the first scientific theory for the origin of the solar system. His model was based on an evolutionary process. Descartes envisioned space as being filled with swirling gases. Large whirlpools developed that would form stars eventually, with smaller vortices forming planets and their satellites. In 1755, Immanuel Kant applied Sir Issac Newton's laws of gravity to Descartes' model and concluded further that the swirling gases would assume a disk shape. In 1796, the French mathematician Pierre-Simon Laplace proposed further that the disk would separate eventually into rings. The planets would condense from these rings.

Collectively, the theories of Descartes, Kant, and Laplace became known as the nebular hypothesis. It offered reasonable explanations for the flattened appearance of the solar system, the nearly circular orbits of the planets, and their similar orbital directions around the sun.

Nevertheless, it left many questions unanswered and by the late nineteenth century had fallen from popularity.

In 1745, the French naturalist Comte de Buffon proposed a catastrophic explanation that contrasted the nebular hypothesis. His theory suggested that the solar system formed after a collision between the sun and another massive object. The material thrown out by that collision eventually formed the planets. It was a relatively simple theory that provided answers to questions left unanswered by the nebular hypothesis. The Buffon theory was resurrected and modified by American geologist Thomas Chronder Chamberlin and American astronomer Forest Ray Moulton in 1900, and later revised by Sir James Hopwood Jeans in 1917. In Jeans' revision, he had no collision but had matter being pulled out of the sun by the tidal forces of a passing star.

This matter would then condense to form planets. It answered certain questions that the nebular hypothesis could not, but it was not a complete explanation.

Technological advances made during the twentieth century have given the nebular hypothesis new credibility. From the work of Carl von Weizsacker, Gerard Peter Kuiper, and Hannes Alfven, a new revised model of the nebular hypothesis arose. It envisions a vast interstellar cloud of gas and dust that will fragment and contract into smaller dense regions from which stars will form. It is from within one of these smaller regions that the sun and its planets formed.

In astronomical terms, the formation of the sun and planets happened relatively quick, perhaps in only 100 million years. It is suggested that zones of turbulent eddies once existed in the gas and dust cloud. Through a combination of processes of regional gravitational collapse, condensation, and subsequent accretion of solid particles, large protoplanetary bodies approximated the chemical compositions and were probably the precursors of the present-day planets.

The theories of planetary formation have given humankind a better understanding of the cosmic origins and the intricacies that lead to Earth's existence in the solar system. In seeking answers to such questions, both mental and technological capabilities have been expanded and this has overflowed into everyday lives.

Principal terms

ACCRETION: the gravitational accumulation of matter that can result eventually in the formation of a planet or smaller-sized body

ASTEROIDS: group of relatively small solid objects that occupy mainly the region between the orbits of Mars and Jupiter

CHONDRULES: small, spherical-shaped inclusions found in certain types of stony meteorites; are of a primitive origin

CONDENSATION: condition in the early solar nebula when hot gases cooled to form solids; a preliminary stage to accretion

METEORITES: generally small solid masses consisting of varying ratios of metal to silicate minerals; classified accordingly as stones, stony-irons, and irons

PLANETESIMALS: theoretical objects that formed in the early solar nebula through condensation and later accretion; existed as an intermediate phase between tiny grains and larger planetary bodies

PROTOPLANET: a preliminary stage to planet formation that exists as a hot, gaseous body much like present-day Jupiter

SOLAR NEBULA: cloud of dense gas and dust with the potential to produce a star with a family of planets

SOLAR WIND: radial flow of charged particles emitted from the sun, with its effects being detected as far out as the furthest planets

T-TAURI STAGE: temporary stage of instability in the early life of a main sequence star when it experiences great mass loss and intense solar wind

Bibliography

Baugher, Joseph F. THE SPACE-AGE SOLAR SYSTEM. New York: John Wiley & Sons, 1988. A very well-written introduction to the solar system; well complemented with good and interesting illustrations. Baugher presents a good overview of the planets. Chapter 20 offers a concise discussion of the theories of planetary formation.

Dermott, S. F., ed. THE ORIGIN OF THE SOLAR SYSTEM. New York: John Wiley & Sons, 1978. Offers twenty-nine separate articles that cover virtually every aspect of planetary formation. Written for the graduate student or professional science reader. A good bibliography is provided for further reference.

Hartmann, William K. MOONS AND PLANETS. 2d ed. Belmont, Calif.: Wadsworth, 1983. This work represents one of the better books that discusses the solar system's origin and planetary formation. Follows a technical style, but it is very readable and well illustrated. The bibliography is excellent.

Kerridge, John F., and Mildred Shapley Matthews, eds. METEORITES AND THE EARLY SOLAR SYSTEM. Tucson: University of Arizona Press, 1988. In this comprehensive work of fifty articles, the relationship between meteorites and planetary formation is well defined. This book is of a technical nature and serves as an excellent resource for the graduate student and professional researcher.

Moore, Patrick, and Gary Hunt. ATLAS OF THE SOLAR SYSTEM. Chicago: Rand McNally, 1983. A comprehensive work that attempts to present relevant information in an easily readable form. Excellent illustrations accompany this well-written text. The discussion on the origin of the solar system offers a concise but very informative overview. Suitable for the general reader.

Morrison, David, and Owen Tobias. THE PLANETARY SYSTEM. Reading, Mass.: Addison-Wesley, 1988. A comparative approach is followed in this examination of the planets in the solar system. Non-mathematical and very understandable for a nonscience reader. Chapter 15 discusses planetary formation and serves as a good introduction to the topic.

Short, Nicholas M. PLANETARY GEOLOGY. Englewood Cliffs, N.J.: Prentice-Hall, 1975. Although this text is somewhat dated, it still serves as an excellent overview for planetary studies. Chapter 4 discusses the origin of planets and presents a very good description for the nonscience reader.

Wood, John A. THE SOLAR SYSTEM. Englewood Cliffs, N.J.: Prentice-Hall, 1979. This book was written to provide the general reader with an excellent introduction to the solar system. Although this book was written before the spectacular results of the space probes of the 1980's, it is still valuable.

Presolar materials in meteorites

Physical properties of the sun, the planets, and Earth's moon

The planets can be divided into "rocky dwarfs" and "gas giants"

The Behavior of Gases

Nuclear Synthesis in Stars

Protostars and Brown Dwarfs