Existing comment:
TRADING PLACES: Shortly after their formation, the giant gas planets follow nearly circular orbits close to the Sun (lower left). About 3.8 billion years ago, Neptune and Uranus switch places and trigger major disruptions (center & top left). Then the giant planets settle into their current orbits, the Kuiper Belt and Oort Cloud comet reservoirs form, and the major bombardment ends (top right). Lower right: Detected Kuiper Belt objects. |