Origins of the Solar System

The solar system formed about 4.6 billion years ago from a cloud of gas and dust that contracted due to its own gravity. Most of the matter went to form the Sun at the center. The planets, their satellites, and the asteroids and comets formed through condensation and accretion in an equatorial disk that developed around the contracting proto-Sun.

Overview

Although many models and theories of the solar system have been generated from ancient times, most scientists accept a version of the nebular hypothesis, which states that the solar system formed from a cloud of gas and dust. This nebula probably was part of a much larger interstellar cloud of gas and dust that contained enough matter to form several hundred to several thousand stars like our Sun. Density irregularities probably caused the larger cloud to fragment into many smaller clouds that produced stars and accompanying planetary systems. One of these smaller clouds was the solar nebula, the protosolar system.

Although the solar nebula was only a small fragment of the much larger parent cloud, it probably was at least one light-year in diameter and contained at least as much matter (and maybe up to twice as much) as the solar system today. Its composition probably was similar to what we determine spectroscopically today for the Sun’s atmosphere: about 71 percent hydrogen, 27 percent helium, and 2 percent all the other known chemical elements. The hydrogen and helium were formed in the aftermath of the Big Bang, which occurred about thirteen to fourteen billion years ago. The heavier chemical elements (those up to iron in atomic mass) were synthesized by nuclear fusion reactions in the cores of massive stars, and those heavier than iron were formed when massive stars exploded as supernovae. The explosive deaths of these massive stars dispersed the heavier elements through interstellar space, where they enriched the clouds of gas and dust from which new generations of stars and their systems of planets would form.

The trigger for the gravitational contraction of the solar nebula could have been its passage through the density wave associated with one of our Milky Way galaxy’s spiral arms. As the cloud orbited the center of our galaxy, passing through a spiral arm density wave would have slowed the cloud a bit, compressing the gas and starting its gravitational contraction. Support for this idea comes from observing that the spiral arms of galaxies are highlighted by groups of very young stars that have just recently formed and glowing clouds of gas that have been excited by nearby hot young stars. Another possible trigger to compress the gas and start gravitational contraction could have been a shock wave from the explosion of a nearby supernova. In fact, there is evidence based on the abundance of certain isotopes that such a nearby supernova exploded not long before the solar system formed.

At first, the cloud of gas and dust was cold and rotated slowly. Gravity pulled the material toward the center, and as the cloud contracted, it rotated faster to conserve angular momentum. It continued to contract and consequently rotate even faster, and after about 100,000 years, it spun off a disk of material in its equatorial plane. Much of the matter of the solar nebula collected at the center of the disk, becoming the proto-Sun. As the proto-Sun contracted, gravitational potential energy was converted into thermal energy, heating the proto-Sun to the point where it began to shine at infrared wavelengths.

The proto-Sun continued to contract, increasing in temperature and luminosity. This heated the inner part of the equatorial disk close to the proto-Sun, but not the part farther out. The radial difference in temperature across the disk led to differential condensation into small, solid grains. At the high temperatures of the inner part of the disk, only metals and Silicateminerals could condense, but farther out, where it was cooler, more abundant materials such as water, ammonia, and methane could condense into ices. The solid grains that formed by condensation collided with each other as they orbited the proto-Sun; if the collisions were not too violent, they stuck together in a process called accretion, gradually building objects up to several kilometers in size called planetesimals. Exactly how the grains stuck together is not certain; perhaps they acquired electrical charges and were held together by static electricity, or if the grains were near their melting points, they might have been somewhat “sticky.”

As the planetesimals grew in size, their increased mass gravitationally attracted additional material until they had swept clear the area around their orbits. The planetesimals continued to grow through collisions, becoming protoplanets. The planetesimals and protoplanets that grew in the inner part of the disk were mostly rocky and metallic in composition, while those that grew farther out in the disk were composed mostly of ices, such as water, ammonia, and methane. Because water, ammonia, and methane were so much more abundant than metals and silicate minerals, the outer protoplanets grew much larger than the inner ones; with their increased mass, they were able to capture hydrogen and helium gases, which were even more abundant in the disk and hence grew larger still. The hydrogen and helium formed thick envelopes around the planets' cores, with the material in the outer part of the envelope remaining gaseous but liquefying with depth due to increasing pressure. Eventually, only a few planet-sized bodies remained: four small, rocky, and metallic inner planets (the terrestrial planets) and four large gas/liquid/ice outer planets (the so-called gas giants).

The asteroid belt, a region of small rocky and metallic bodies between the orbits of Mars and Jupiter, probably is leftover material from an early stage of solar system development that failed to coalesce into a planet, perhaps because of the gravitational influence of Jupiter. The Kuiper Belt, a region of icy and rocky bodies beyond the orbit of Neptune, like the asteroid belt is probably also leftover material that never coalesced into a major planet; Pluto is one of the largest members of the Kuiper Belt. Ices in the vicinity of the Jovian planets accreted into small cometary nuclei that, through gravitational interactions with giant planets, were tossed randomly into the far outer reaches of the solar system to become the Oort Comet cloud.

Most of the larger moons of the planets probably formed in a process similar to that of the planets themselves: by accretion in an equatorial disk around the protoplanet. Smaller moons may have formed separately from the protoplanet and were later captured gravitationally. The Earth’s Moon probably formed as the result of the collision of a large protoplanet with the early Earth, the impact blasting material from the Earth’s crust and mantle, as well as the impacting object itself into orbit around the Earth, there to accrete into our Moon.

As the proto-Sun continued to contract gravitationally, the density and temperature in its core eventually became high enough to initiate the fusion of hydrogen into helium. In this thermonuclear reaction, four hydrogen nuclei fuse to make one helium nucleus. The combined mass of the four hydrogen nuclei that go into the reaction slightly exceeds the mass of the single helium nucleus that results, and this small excess in mass gets converted into energy. Once hydrogen fusion was initiated in its core, the Sun became a full-fledged main sequence star.

The young Sun was much more active than it is today, shedding gases profusely from its surface into space. This became the solar wind, streams of electrically charged particles ejected from the atmosphere of the Sun. With a speed of at least several hundred kilometers per second, the early energetic Solar wind blew any gas and dust remaining in the equatorial disk out into space.

The solar system took a few hundred million years to stabilize. Accretion ended with a period of heavy bombardment, when the remaining planetesimals and protoplanets, as well as icy cometary nuclei, smashed into the planets and moons; the scars left by thousands of impacts remain today on the surfaces of many of the planets and moons that are geologically inactive. It may be that much of the water on Earth came from icy cometary impacts early in Earth’s history.

Methods of Study

To deduce how the Sun and planets formed, scientists are constrained by what the solar system is like at the time of study. In a sense, investigating the origins of the solar system is like a detective mystery in which ambiguous—and sometimes misleading—clues must be put together to assemble a plausible sequence of past events. Unfortunately, some clues are still missing, and still others are poorly understood.

Meteorites, asteroids, and cometary nuclei are among the smallest bodies of the solar system. These objects should have changed the least since their formation, and their composition should reflect the original material from which the solar system formed. The age of the solar system is determined by dating meteorites using the decay of radioactive isotopes they contain. Most meteorites have very nearly the same age, 4.6 billion years. The most abundant type of meteorite, the chondritic stony meteorites, also appear to be the most primitive and unprocessed. Their internal structure of small mineral grains, along with somewhat larger glassy chondrules, is taken to be evidence of the early period of condensation and accretion. Asteroids seem to be the parent bodies of many meteorites, based on comparison of the spectra of the light both reflect. Asteroids are thought to be examples of the planetesimals that formed through accretion, and the compositions of different types of asteroids and meteorites probably represent the processing of solar nebular material that occurred as the planetesimals grew in size. Cometary nuclei probably are unprocessed samples of the ices that condensed in the outer part of the solar nebula.

Spectra of the light emitted by the Sun and reflected by other solar-system objects provides information about the composition of their atmospheres (if any) and their surface materials. Actual samples of surface material include Earth rocks and minerals, Moon rocks brought back by the Apollo lunar missions, and a few meteorites that are thought to have been blasted off the Moon and Mars by large impacts. In addition, landers on the Moon, Mars, and Venus have sent back information on surface composition. Clues about the overall composition of solar-system objects are provided by their average density, found by dividing their mass by their volume.

Context

Just about every culture has its creation myths—stories about the origin of the world. Naturalistic explanations for the origin of stars generally and the solar system, in particular, can be traced back to the late 1600s, after scientists came to accept the heliocentric model of the solar system and Sir Isaac Newton published his theory of gravitation. Newton himself suggested that the Sun and stars could have formed by gravitational contraction of initially diffuse matter evenly dispersed through an infinite space. At about the same time, the French philosopher René Descartes introduced perhaps the first description of what has come to be called the nebular theory. He proposed that a large cloud of gas (a nebula) had contracted under its own gravity, with the Sun forming at the center and the planets forming in the cooler outer parts. This idea was developed by the German philosopher Immanuel Kant in the mid-1700s. The French mathematicianPierre-Simon Laplace, in 1796, added conservation of angular momentum to the model, concluding that, as the nebula contracted, it would spin faster.

Alternative models for the origin of the solar system were also proposed. Many involved a second star in addition to the Sun. One hypothesis was that another star passed near our planetless Sun and gravitationally pulled matter out of it to become the planets. Another hypothesis was that our Sun was a member of a binary star system; the companion star exploded, and its debris formed the planets. However, by the early to mid-twentieth century, these alternative models fell into disfavor because of difficulties in getting them to work the way they were supposed to when physics was applied to them to try to make them more rigorous. Meanwhile, the nebular model continued to be refined to the point where it is the generally accepted explanation.

Any successful model for the formation of our solar system must explain several patterns and regularities observed today. First, the planets all orbit the Sun in nearly the Sun’s equatorial plane and in the same direction that the Sun rotates. The orbits of the planets are nearly circular. The four inner planets (the terrestrial planets) are all small, dense, and composed mainly of rocky and metallic material. The four outer planets (the Jovian planets) are large, low in density, and composed mainly of gases, liquids, and ices. The Sun, the planets, and other solar system objects all have about the same age—about 4.6 billion years. Accepted versions of the nebular model account for all these points.

Solar system oddities and irregularities also need to be explained. In the basic nebular model, one would expect that the planets would all rotate in the same direction as they revolve around the Sun, and their rotational axes should be perpendicular to their orbital planes. Indeed, this is the case for a few planets, like Jupiter. However, most have their rotation axes tilted at moderate angles of 20° to 30° away from the perpendicular to the orbital planes; for example, Earth’s rotational axis is tilted about 23.5°. The most glaring exceptions are Uranus, which is tilted over so much that its rotational axis lies almost in its orbital plane, and Venus, which rotates very slowly and opposite to its direction of revolution. These departures are explained by impacts during the final accretion of the planets. Off-center impacts by moderately sized protoplanets could account for the moderate axial tilts of planets like Earth, and off-center impacts by large protoplanets are invoked to explain the unusual rotations of Uranus and Venus.

A feature not fully understood is the spacing of the planetary orbits; the orbits of the four inner terrestrial planets are much more closely spaced than the orbits of the four outer Jovian planets. Of course, the Jovian planets are larger, which means that they formed from more material spread over a greater range of distances. Did they just happen to form this way by chance, or was it because of gravitational interactions among the planets leading to long-term orbital stability?

A major problem with early nebular models concerned the distribution of angular momentum in the solar system. The Sun, with more than 99 percent of the mass of the solar system, accounts for only 2 percent of the total angular momentum; the planets, with less than 1 percent of the mass, together possess 98 percent of the angular momentum. There had to be some mechanism for the early Sun to transfer most of its angular momentum to the equatorial disk in which the planets formed. It is generally assumed that magnetic braking between the magnetic field of the early Sun and the ionized gas in the inner part of the surrounding disk transferred the angular momentum. Also, much of the original angular momentum could have been carried out of the system entirely by the energetic early solar wind.

Until 1995, most scientists agreed that there was only one system of planets orbiting a star: our own. Then, in 1995, the first planets orbiting other stars were detected; in the twenty-first century several hundred such systems are known. Many of these systems are quite different from our own in terms of their planets’ masses and the distances of those planets from their parent stars. Comparing the properties of our own solar system with these others can help refine our models for the formation of planetary systems generally.

Bibliography

Chaisson, Eric, and Steve McMillan. Astronomy Today. 6th ed. New York: Addison-Wesley, 2008.

Dermott, S. F., ed. The Origin of the Solar System. New York: John Wiley & Sons, 1978.

Fraknoi, Andrew, David Morrison, and Sidney Wolff. Voyages to the Stars and Galaxies. Belmont, Calif.: Brooks/Cole-Thomson Learning, 2006.

Frazier, Kendrick. Solar Systems. Alexandria, Va.: Time-Life Books, 1985.

Freedman, Roger A., and William J. Kaufmann III. Universe. 8th ed. New York: W. H. Freeman, 2008.

Hartmann, William K. Moons and Planets. 5th ed. Belmont, Calif.: Thomson Brooks/Cole, 2005.

Henbest, Nigel. Mysteries of the Universe. New York: Van Nostrand Reinhold, 1981.

“In Depth | Our Solar System – NASA Solar System Exploration.” NASA Solar System Exploration, 18 Sept. 2023, solarsystem.nasa.gov/solar-system/our-solar-system/in-depth. Accessed 19 Sept. 2023.

Sagan, Carl. Cosmos. New York: Random House, 1980.

Schneider, Stephen E., and Thomas T. Arny. Pathways to Astronomy. 2d ed. New York: McGraw-Hill, 2008. .