Planets and our Sun

Abstract

The formation of planets is closely related to the formation of stars, but the physics is intrinsically different. Planets form in the final phase of a star’s creation, using the material that is left over. As the Sun formed from an interstellar cloud some 5 billion years ago, the remaining dust and ice began to coalesce into billions of objects, each only several kilometers in size. These objects would have progressively collided, to create ever larger objects by the process of accretion. Mercury, Venus, Earth and Mars are rocky bodies that formed in the warm environment close to the Sun, and Jupiter, Saturn, Uranus and Neptune are gas giants formed in the cooler realm farther out. It seems that the gas giants formed before the rocky planets, and the asteroids are the small fragments that were left over. It is believed that whereas the gas giants were formed in several million years, the rocky planets required tens of millions of years. The gas giants are primarily hydrogen and helium, with only atrace of what astronomers call metals. The time available for the formation of gas giants was so limited because the radiation pressure from the new star would have blown the remaining gas away. In a gas giant, the heavier elements are located at the center.