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From an Earth‑based perspective, the planets appear to wander across the night sky — a fact that gives the word "planet" its ancient Greek meaning. This apparent motion is the result of the planets orbiting the Sun on nearly circular paths, with their orbital radii remaining constant over human history. Yet on geological timescales, their orbits have shifted due to planetary migration.
The dominant force shaping planetary motion is the Sun’s gravity, which keeps each planet in its orbit. In reality, smaller perturbing forces — such as the gravitational pull of massive neighbors like Jupiter and Saturn, as well as cumulative encounters with asteroids and comets — gradually alter the paths over millions of years.
When the solar system formed ~4.6 billion years ago, a massive disk of gas and dust surrounded the young Sun. This protoplanetary disk exerted a strong drag on the nascent planets, pulling the inner rocky bodies (Mercury, Venus, Earth, Mars) inward toward the Sun.
Jupiter, the giant, also migrated inward until it stalled near its present distance from the Sun, likely halted by Saturn’s gravitational influence. Both gas giants then drifted outward, approaching the current orbits of Uranus and Neptune. By this stage, most of the gas and dust had dissipated, slowing further migration.
Approximately 3.8 billion years ago, before life first emerged on Earth, a second migration phase occurred. Jupiter and Saturn briefly locked into a 1:2 mean‑motion resonance—Saturn’s orbital period was twice that of Jupiter. This resonance destabilized the entire outer system, forcing rapid realignment: Jupiter moved slightly inward, while Saturn, Uranus, and Neptune migrated outward. Within a few million years, the four planets settled into the nearly stable configuration that persists today.
These dynamical shifts are supported by numerical simulations and studies of the Kuiper Belt, and they explain why the current spacing of the planets differs from their initial arrangement after formation.
