Birth of our Solar System
What we know about the formation of the solar system comes from two
types of studies.
First, using powerful instruments such as the Hubble Space Telescope we can peer out into the galaxy and look for stars like the sun that appear to be in the process of formation. Although we cannot watch an individual star evolve from a nebula (cloud of gas) we can study several stars that appear to be at different stages in the process.
Second, we know a lot about the present composition of the solar system, including the composition, size, mass, and density of the planets. This information comes from physics calculations based on the orbits of the planets and the laws of gravity, from Earth-based telescopic and spectroscopic observations, and from measurements made by robotic space probes sent into the solar system.
Currently, planetary scientists believe that the initial formation of the solar system took a relatively short amount of time, possibly about 100 million years. The initial formation and contraction of the original solar nebula was probably due to shock waves from a nearby supernova (exploding star). With time, the solar nebula collapsed into a rotating disk with the majority of its mass in the form of hydrogen gas concentrated in the center, forming the protosun (pre-sun). Fusion ignition of the Sun was followed by a one million year period of violent solar activity. Solar winds sweep lighter materials (H, He, H2O, Ammonia, etc) outward from the Sun, leaving the inner solar system enriched in refractory materials such as silica and iron.
The rotating disk contained dust-sized particles which began to clumping into particles of larger and larger sizes, forming a range of objects from meteoroids to planetesimals. Jupiter’s large mass and high gravity attracted much of the material available from its region of the solar nebula. This left the asteroid belt too depleted in mass to form a planet and resulted in a relatively small mass for the planet Mars. It has been estimated that without Jupiter the frequency of impact between asteroids and comets and the Earth would have been 1000 times greater.
Accretion of the remaining planets continued from the earlier planetesimals. Most material was swept up by the four inner planets. Major collisions between planets and large planetesimals result in formation of the Earth’s moon (collision with a Mars-sized impactor), loss of much of Mercury’s mantle, and reversal of Venus’ direction of rotation.