planet migration

We used to assume planets simply stay where they were born. Then we found giant planets hugging their stars closer than Mercury hugs the Sun — places far too warm to have built such worlds. Something must have moved them. Planet migration is the idea that a planet does not stay put after forming: it can spiral inward (or sometimes outward) as it pushes and pulls on the gas and debris around it, drifting to a new orbit, the way a swimmer who shoves off a pool wall ends up somewhere other than where they started.

While the gas disk is still present, a young planet's gravity stirs up spiral wakes in the surrounding gas. Those wakes tug back on the planet, and the tug is usually slightly unbalanced, draining the planet's orbital energy so it slowly sinks toward the star — this is the gas-driven migration that can carry a giant from beyond the snow line in to a scorching close-in orbit. Migration can also happen later and more abruptly, when planets gravitationally scatter off each other or off leftover debris, flinging some inward, some outward, and some out of the system entirely. The amount of drift can be huge — many times the planet's birth distance.

Migration reshaped our whole picture of planetary systems. It explains hot Jupiters (giants that could not have formed where we find them), tightly packed chains of planets locked in resonance, and the surprising diversity of orbits we see. It also complicates everything: a planet's current address may say little about where it was assembled, so reading a system's history now means untangling not just how planets formed but how far and how violently they wandered afterward.