Key Ideas

The Nebular Hypothesis: The most successful model of the origin of the solar system is called the nebular hypothesis. According to this hypothesis, the solar system formed from a cloud of interstellar material called the solar nebula. This occurred 4.54 billion years ago (as determined by radioactive dating).

The Solar Nebula and Its Evolution: The chemical composition of the solar nebula, by mass, was 98% hydrogen and helium (elements that formed shortly after the beginning of the universe) and 2% heavier elements (produced much later in the centers of stars and cast into space when the stars died). The heavier elements were in the form of ice and dust particles.

• The nebula flattened into a disk in which all the material orbited the center in the same direction, just as do the present-day planets.

Formation of the Planets and Sun: The terrestrial planets, the Jovian planets, and the Sun followed different pathways to formation.

• The four terrestrial planets formed through the accretion of rock and metal dust grains into planetesimals, then into larger protoplanets.
• Closer to the Sun—inside the snow line—water could not freeze into ice to help build larger planets. With rocks and metals making up only a small fraction of mass throughout the nebula, the terrestrial planets made from these materials in the inner solar system are small.
• Jovian planets form beyond the snow line, where ices of water and methane contribute to make larger cores as these planets form. These larger cores can then attract copious amounts of hydrogen and helium gas, becoming large Jovian planets.
• Gravitational interactions between the Jovian planets and a gaseous disk, and much later with a disk of planetesimals, leads to planetary migration. These migrations can explain the small size of Mars, the composition of the asteroid belt, the locations of the Jovian planets, aspects of the Kuiper belt and Oort cloud, and even account for a Late Heavy Bombardment.
• The Sun formed by gravitational contraction of the center of the nebula. After about 10⁸ years, temperatures at the protosun’s center became high enough to ignite nuclear reactions that convert hydrogen into helium, thus forming a true star.

Extrasolar Planets: Astronomers have discovered many planets orbiting other stars.

• Many of these planets are detected by the “wobble” of the stars around which they orbit. The radial velocity method detects this wobble through Doppler shifts. The astrometric method directly observes a star’s change in position as it wobbles.
• Many extrasolar planets have been discovered by the transit method, and a lesser number by microlensing, and direct imaging.
• Most of the extrasolar planets discovered to date are quite massive and have orbits that are very different from planets in our solar system. This is expected because big planets close to stars are easier to detect. We are just beginning to detect Earth-size planets at distances that might allow liquid water on their surfaces.