Chapter 1 - Solar/planetary formation and evolution

Abstract

The Solar System is believed to be about 4.6 billion years old. It is thought to have arisen from an amorphous cloud of gas and dust in space. The original cloud was spinning, and this spin caused it to flatten out into a disk shape, rather than a spherical shape. The Sun and planets are believed to have formed out of this disk, which is why, today, the planets still orbit in a single plane around the Sun. Only metals and silicates having relatively higher density and melting point could exist closer to the Sun, and these would eventually form the terrestrial planets (Mercury, Venus, Earth, and Mars). Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large. Giant planets are qualitatively distinct from terrestrial planets in that they possess significant gaseous envelopes. The giant planets (Jupiter, Saturn, Uranus, and Neptune) formed beyond the Frost Line. The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium. The leftover debris that never became planets congregated in regions such as the Asteroid Belt, Kuiper Belt, and Oort Cloud. Comets and asteroids are rocky materials that originated from two different locations in the Solar system. Whereas a comet is a chunk of solid body, made of ices, dust and rock originating from the outer Solar System, an asteroid is a hydrated rock in orbit located generally between Mars and Jupiter. Comets and asteroids offer clues to the chemical mixture from which the planets formed some 4.6 billion years ago. Earth was formed from the similar material that constitutes present-day asteroids. The Earth’s surface is composed of three layers — the crust, mantle and core.