Planetary formation

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

Overview

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

One theory suggests that the formation of a solar system begins with the explosion of a massive nearby star, called a supernova. A supernova begins with a slowly contracting cloud of gas and dust that is capable of forming many individual stars, 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. Energy released may rival that of a galaxy for a short period of time. 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 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 took place in the solar nebula was gravitational collapse, the process in which atoms attract other atoms to form molecules of considerably greater mass. This greater mass increases gravitational pull and thereby attracts even more atoms. As these localized regions became denser through the infall of matter, temperatures began 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. Scientists believe that eventually all matter will be converted into a hot gaseous state.

After the majority of gas has been incorporated into the star’s formation, residual hot gases begin to cool and reform as solids during the condensation process. Every element has a specific condensation temperature at which 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 earth’s protosun, for example, only heavy elements condensed. As matter thinned out, temperatures dropped progressively and permitted lower-temperature elements to condense. Based on existing temperatures, distance from the protostar controlled, in part, 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 protosun was pulled to the plane of the ecliptic by the force of gravity. This material tended to clump together as it condensed out of the cooling gases. Some scientists believe that in our solar system 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 combined to form larger and larger particles. Growth continued through sand-sized to pebble-sized and then to boulder-sized objects. This describes the accretion process, where collision produced a larger mass rather than fragmentation. Accretion did not stop with boulders, but continued, eventually forming planetesimal-sized objects much like present-day asteroids. These had a size range from thirty meters up to about one thousand kilometers in diameter. Finally, planetesimals collected one another and assumed the planetary dimensions familiar today.

Present-day planets are not uniform in their chemical compositions. The inner, so-called terrestrial planets (Mercury, Venus, Mars, and Earth) are rich in heavy metals, while the outer planets (Jupiter, Saturn, Uranus, and Neptune) are gaseous—hence, the so-called “gas giants.” One explanation for this may be that the solar nebula was not uniformly hot, because of nonhomogeneous distribution of material based on distance from the protosun. Only the area from the sun outward to the asteroid belt (a region of small, rocky, and metallic bodies between the orbits of Mars and Jupiter) 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 a 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 during the T-Tauri stage, the solar wind would have lost most of its effectiveness at the distance of the present asteroid belt. It may have been virtually ineffective as it reached 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, involving many variables. Collectively, the aforementioned theories are known as the nebular hypothesis. It offers reasonably logical explanations for a very difficult problem.

Methods of Study

Experimentally, several aspects of the nebular hypothesis 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.

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. 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 during which collisions were commonplace. These normally spherically 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, erasing 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.

The first extrasolar planet was discovered in 1995 using a technique that involves measuring the gravitationally induced wobble in a star produced by a planet orbiting it. Ironically, the first planet found outside the solar system did not fit the typical model envisioned for a planetary system. It was a very large gas giant in an extremely tight orbit about a pulsar. Pulsars had not been considered likely candidates for solar systems with planets. A decade after that initial exoplanet discovery, more than one hundred other planets had been detected through a small number of indirect means. Most extrasolar planets were hot Jupiters, but some were found to have mass down to a multiple of a Uranus-class mass. After the first decade of discoveries, the number of objects beyond the solar system that were accepted as planets rose dramatically. As of 2023, there were more than five thousand five hundred recognized exoplanets. The tremendous variety of these planets and the nature of the solar systems in which they were found forced a serious reexamination of the theory of planetary system formation.

Methods of Study

The nebular hypothesis for planetary formation provides good explanations for many of the dynamic 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. The possible formation of 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 also responsible for the origins of comets.

One area 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 manner similar 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 protosun 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 satellites (Io, density 3.5 and Europa, density 3.0). In contrast, the two larger outer satellites, Ganymede and Callisto, have densities of 1.9 and 1.8, respectively. This comparison between the satellites of Jupiter and the arrangement of planets by density is inescapable. The inner two Jovian satellites 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 planetesimal 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.

In the more distant regions of 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 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 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 orbits close 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 astronomers 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 examination by earthbound telescopes. Methods of observation that make use of nonvisible radiation, however, 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 confirmed 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.

The Hubble Space Telescope and Spitzer Space Telescope have since found many more protostellar objects and forming planetary systems. The indirect detection of more than three hundred extrasolar planets (a number that is continually increasing), the vast majority of which do not appear to have formed in a manner consistent with the model of our solar system condensing out of a planetary nebula, puts constraints on the acceptability of the nebular hypothesis, or at least its universality. “Hot Jupiters” close to a star, pulsars with planets, and a few other unusual star-planet relationships fall well outside the classic planetary nebula model.

The Kepler Observatory, designed to detect planets while transiting their star, deployed in March 2009; it planned to survey as many as 100,000 star systems. It is estimated that the chances for finding a terrestrial planet are on the order of 1 in 210; As of July 2013, Kepler had detected 3,277 planet candidates. Such continuing indirect and direct observations of existing and forming planetary systems will surely require major modifications to the theory of planet formation.

In mid-June 2008, European astronomers announced remarkable results found using the European Southern Observatory (ESO) La Silla Observatory’s HARP spectrograph. Surrounding the star HD40307 were found three super-Earth-scale planets. HARP was capable of detecting planets between two and ten Earth masses—thus, objects on a planetary scale, between Earth and Uranus or Neptune in mass. This technique was sensitive to stellar wobbles of only a few meters per second, something necessary to determine the tiny tug on a star by an Earth-sized extrasolar planet at significant distances from its star. These researchers also announced that they expected to refine data on forty-five candidate Earth-scale exoplanets with masses between 1 and 30 Earth masses as they continued to observe with HARP. However, an extrasolar planetary system with three Earth-class planets was unique thus far in the history of exoplanet identification. In 2013, ESO announced the discovery of a system that contains a minimum of six planets, three of which are “super-Earths” that inhabit the area around the star where liquid water possibly exists. The discovery is the first of what is termed a “fully packed habitable zone.”

The December 2021 launch of NASA’s James Webb telescope has provided a scientific windfall of new data previously unavailable to scientists. Among these was the discovery of an extrasolar planet hypothesized as capable of supporting life. This exoplanet, designated K2-18b, was reported to be 8.6 times the dimension of Earth. Scientists believed K2-18b might have an atmosphere rich in hydrogen, which would also make it conducive to a liquid-water ocean. Most importantly, K2-18b lies in the “Goldilocks” zone in relation to its parent sun. This is a distance sufficiently distant from the star where the planet is not excessively heated, but also close enough so that its frigidity makes it unlikely to support life.

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 a catastrophic approach. It was realized quickly that no comprehensive answer would be forthcoming, and the “best-fit” concept was developed to provide answers based on the best science available.

In 1644, French philosopher René 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 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 Isaac Newton’s laws of gravity to Descartes’s model and concluded 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, French naturalist Comte de Buffon proposed a catastrophic explanation that contrasted with 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 in 1917, it was revised by Sir James Hopwood Jeans. Jeans’s revision had no collision but instead envisioned matter being pulled out of the sun by the tidal forces of a passing star. This matter would then condense to form planets. Jeans’s theory answered certain questions that the nebular hypothesis could not, but it was not a complete explanation.

Technological advances made during the twentieth and twenty-first centuries gave the nebular hypothesis new credibility. From the work of Carl von Weizsäcker, Gerard Peter Kuiper, and Hannes Alfvén, a revised model of the nebular hypothesis arose, envisioning a vast interstellar cloud of gas and dust that fragmented and contracted into smaller, dense regions from which stars would 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 quickly, 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.

Theories of planetary formation have given humankind a better understanding of the cosmic origins and the intricacies that led 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. Experimentally, the biggest goal of the search for extrasolar planets is the detection and later the imaging of Earth-like planets.

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