Origin and Evolution of the Solar System

The solar system was formed 4.6 ± 0.1 × 10 9 years ago. Astronomers have recognized a number of observable facts about the solar system that are not otherwise the result of obvious physical laws (for example, Kepler's Laws of Planetary Motion, which are the direct result of the nature of gravity). But the foundation of science assumes that every observable property must result from some cause. These features must therefore be the direct result of how the solar system formed. The following list outlines these observable facts:

  • All planetary orbits lie nearly in a single plane; in other words, the solar system is flat (the orbit of Pluto is an exception).

  • The Sun's rotational equator is in the same flat plane.

  • Planetary orbits are nearly circular (exceptions are Mercury and Pluto).

  • The planets and Sun all revolve in the same direction, that is, a motion that is west to east across the sky as viewed from Earth (what astronomers refer to as direct motion).

  • The Sun and planets all rotate in the same direction with obliquities (the tilt between the equatorial and orbital planes) generally small (exceptions are Venus, Uranus, and Pluto).

  • Planets and most asteroids have similar rotational periods (exceptions are Mercury, Pluto, and Venus).

  • Planets are regularly spaced (this is often expressed in the form of a simple mathematical progression, known as Bode's law).

  • The major moons in planetary satellite systems resemble the solar system on a smaller scale (circular orbits, uniform direction of revolution, in a flat plane with regular spacing).

  • Most angular momentum (∼ mass × velocity × orbital radius) of the solar system is in the planets (99.8%), whereas most of the mass of the solar system is in the Sun (99.8%). This may be expressed alternatively as a question: Why does the sun rotate so slowly?

  • Differences in chemical composition exist throughout the solar system, with dense, metal‐rich (terrestrial) planets found close to the sun, but giant, hydrogen‐rich (gas) planets only in the outer part of the solar system. In addition, the chemical composition of meteorites, while similar, is not identical to all known planetary and lunar rocks.

  • Comets exist in a much larger, spherical cloud surrounding the solar system.

Throughout the years, people have come up with a variety of theories to explain the observable features of the solar system. Some of these theories include so‐called catastrophe theories, such as a near collision of the Sun with another star. Modern theory of planetary origins also explicitly rejects any idea that our solar system is unique or special, thus ruling out catastrophe theories. The solar nebula theory (also known as the planetesimal hypothesis, or condensation theory) describes the solar system as the natural result of the operation of the various laws of physics. According to this theory, before the planets and Sun were formed, the material that would become the solar system existed as part of a large, diffuse cloud of interstellar gas and dust (a nebula) composed primarily of hydrogen and helium with traces (2 percent) of other, heavier elements. Such clouds can be stable for very long periods of time with simple gas pressure (pushing outward) balancing the inward pull of the self‐gravity of the cloud. But British theoretician James Jeans showed that the smallest disturbance (perhaps an initial compression begun by a shock wave from a nearby stellar explosion) allows gravity to win the competition, and gravitational contraction begins. The fundamental inability for gas pressure to permanently balance against self‐gravity is known as the Jeans Instability. (An analogy would be a yardstick balanced on one end; the slightest displacement upsets the balances of forces and gravity causes the yardstick to fall over.)

During the nebula's gravitation collapse ( Helmholtz contraction), gravity accelerated particles inward. As each particle accelerated, the temperature rose. If no other effect were involved, the temperature rise would have increased pressure until gravity was balanced and the contraction ended. Instead the gas particles collided with each other, with those collisions converting kinetic energy (the energy of a body that is associated with its motion) into an internal energy that atoms can radiate away (in other words, a cooling mechanism). About half the gravitational energy was radiated away, and half went into heating the contracting cloud; thus, gas pressure stayed below what was needed to achieve balance against the inward pull of gravity. As a result, the contraction of the cloud continued. The contraction occurred more quickly in the center, and the density of the center mass rose much faster than the density of the outer part of the nebula. When the central temperature and density became great enough, thermonuclear reactions began to provide significant energy—in fact, enough energy to allow the central temperature to reach the point where the resulting gas pressure could again supply balance against gravitation. The central region of the nebula becomes a new Sun.

A major factor in the formation of the Sun was angular momentum, or the momentum characteristic of a rotating object. Angular momentum is the product of linear momentum and the perpendicular distance from the origin of coordinates to the path of the object (≈ mass × radius × rotational velocity). In the same manner that a spinning skater revolves faster when her arms are pulled inward, the conservation of angular momentum causes a contracting star to increase in rotational velocity as the radius is reduced. As its mass shrank in size, the Sun's rotational velocity grew.

In the absence of other factors, the new Sun would have continued rapidly rotating, but two possible mechanisms slowed this rotation significantly. One was the existence of a magnetic field. Weak magnetic fields are present in space. A magnetic field tends to lock into material (think of how iron filings sprinkled onto a sheet of paper on top of a magnet line up, mapping out the pattern of magnetic field lines). Originally the field lines would have penetrated the stationary material of the nebula, but after it contracted, the field lines would have been rapidly rotating at the central Sun, but very slowly rotating in the outer part of the nebula. By magnetically connecting the inner region to the outer region, the magnetic field sped up the movement of the outer material, but slowed the rotation ( magnetic braking) of the central solar material. Thus momentum was transferred outward to the nebular material, some of which was lost to the solar system. The second factor to slow the early Sun's rotation was most likely a powerful solar wind, which also carried away substantial rotational energy and angular momentum, again slowing the solar rotation.

Beyond the center of the nebula, angular momentum also played a significant role in the formation of the other parts of the solar system. In the absence of outside forces, angular momentum is conserved; hence, as the radius of the cloud decreased, its rotation increased. Ultimately, rotational motions balanced gravity in an equatorial plane. Above and below this plane, there was nothing to hold up the material, and it continued to fall into the plane; the solar nebula exterior to the new central Sun thus flattened into a rotating disk (see Figure 1). At this stage, the material was still gaseous, with lots of collisions occurring between the particles. Those particles in elliptical orbits had more collisions, with the net result being that all material was forced into more or less circular orbits, causing a rotating disk to be formed. No longer significantly contracting, the material of this protoplanetary disk cooled, but heating from the center by the new Sun resulted in a temperature gradient ranging from a temperature of approximately 2,000 K at the center of the nebula to a temperature of approximately 10 K at the edge of the nebula.

Figure 1

Collapse of interstellar cloud into star and protoplanetary disk.

Temperature affected which materials condensed from the gas stage to the particle ( grain) stage in the nebulae. Above 2,000 K, all elements existed in a gaseous phase; but below 1,400 K, relatively common iron and nickel began to condense into solid form. Below 1,300 K, silicates (various chemical combinations with SiO −4) started to form. At much lower temperatures, below 300 K, the most common elements, hydrogen, nitrogen, carbon, and oxygen, formed ices of H −2O, NH −3, CH −4, and CO −2. Carbonaceous chondrites (with chondrules, or spherical grains that never were melted in later events) are the direct evidence that grain formation took place in the early solar system, with a subsequent amalgamation of these small solid particles into larger and larger objects.

Given the range of temperature in the protoplanetary nebula, only heavy elements were able to condense in the inner solar system; whereas both heavy elements and the much more abundant ices condensed in the outer solar system. Gases that didn't condense into grains were swept outward by radiation pressure and the stellar wind of the new Sun.

In the inner solar system, heavy element grains slowly grew in size, successively combining into larger objects (small moon‐sized planets, or planetesimals). In the final stage, planetesimals merged to form the small handful of terrestrial planets. That smaller objects were present before the planets is shown by the leftover asteroids (too far from either Mars or Jupiter to become part of those surviving planets) and the evidence of impact cratering on the ancient surfaces of the large bodies that exist today. Detailed computations show that the formation of larger bodies in this manner produces final objects rotating in the same sense of direction as their motion about the Sun and with appropriate rotational periods. The condensation into a few objects orbiting the Sun occurred in more or less regularly spaced radial zones or annuli, with one surviving planet in each region.

In the outer solar system, protoplanets formed in the same manner as those in the inner solar system, but with two differences. First, more mass was present in the form of icy condensates; and second, the amalgamation of solid materials occurred in a region rich in hydrogen and helium gas. The gravitation of each growing planet would have affected the surrounding gas dynamics until gravo‐ thermal collapse occurred, or a sudden collapse of surrounding gas upon the rocky‐icy protoplanets, thus forming the final nature of the gas giants. In the vicinity of the largest developing gas giants, the new planet's gravity affected the motions of surrounding, smaller objects with the evolution there being like a smaller version of the whole solar system. Thus, satellite systems ended up looking like the whole solar system in miniature.