Planet Formation

The leading model of planet formation: dust in a protoplanetary disk sticks into ever-larger bodies until a solid core grows massive enough — roughly ten Earth masses — to pull in a thick envelope of gas, becoming a giant planet. It naturally explains rocky planets, ice giants, and gas giants as a sequence, though building cores fast enough before the gas disperses is a live problem.

An alternative route to giant planets in which a massive, cool region of a protoplanetary disk becomes gravitationally unstable and fragments directly into a bound clump of gas, skipping the slow core-building step. It may account for massive planets on wide orbits that are hard to make by core accretion within the disk's lifetime.

A mechanism that helps solve core accretion's timing problem: centimetre-sized 'pebbles' drifting inward through the disk are efficiently swept up by a growing embryo, letting cores reach giant-planet mass far faster than by collisions of larger bodies alone. It has become a central ingredient in modern planet-formation theory.

Planets do not necessarily stay where they form: gravitational interaction with the gas disk, or later with other bodies, can move them inward or outward over time. Migration is the standard explanation for hot Jupiters — giant planets on scorching close-in orbits where they could not have formed — and shapes the architecture of whole systems.

The distance from a young star beyond which it is cold enough for a volatile such as water to freeze into ice. Past the snow line the supply of solid material jumps, favouring the rapid growth of massive cores — one reason the Solar System's giant planets lie beyond it — and its location leaves an imprint on a planet's later atmospheric composition.