Astronomy & Celestial Mechanics
the sky, computed
Concept

Radial drift — where it appears

The inward spiral of solid particles in a gas disc, caused by gas that orbits slightly slower than a lone particle would because its own pressure partly supports it. Pebbles of a millimetre to a metre feel the headwind most and can drift to the star in a few hundred orbits.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

The ice line sweeps from 6.4 to 2.7 AU while the disc drains. The radius at which a disc around a 1 solar-mass star reaches 170 K, against the disc's age, both axes logarithmic. The temperature is the fourth root of the sum of two fluxes: starlight, which does not change, and the disc's own accretion, which releases gravitational energy at a rate set by how fast material is flowing inward. The accretion rate decays as the disc drains — taken here as 5e-5 solar masses a year falling off as t^(−3/2) beyond 0.1 million years — so the viscous term fades and the line sweeps in. It starts at 6.38 AU and ends at 2.73, the passive value the closed form gives. A body at three astronomical units formed dry if it formed early and icy if it formed late, so a composition dates a formation rather than locating one, and the dating is only as good as the accretion history assumed.

A composition that dates a formation rather than placing it

The ice line in a young disc starts six astronomical units out and sweeps inward to under three as the disc drains. A body at four astronomical units therefore formed dry or wet depending only on when — so what it is made of is a clock, and reading it as a map is the mistake the moving line makes easy.

exoplanets · The snow line The mass at which a growing core cuts off the pebbles drifting past it. The pebble-isolation mass — the mass at which a core perturbs the gas around it enough to raise a pressure maximum outside its orbit, where drifting solids stop — against distance from the Sun, for a passively heated disc whose thickness grows as the distance to the 2/7, with α = 10⁻³. The mass goes as the cube of the disc's aspect ratio, because a planet has to push the gas by a fraction of its own pressure, and so it grows outward: 7 Earth masses at 1 AU, 30 at 5 AU where Jupiter's core formed, 138 at 30. A core reaching this mass stops growing from pebbles, starts accreting gas, and becomes a dam: everything drifting inward from outside its orbit is held there.

A core that shut the door on the pebbles

Solids in a gas disc drift inward, and a growing planet eats them as they pass until it is heavy enough to bend the gas around it. Then a pressure maximum forms outside its orbit, the drift stops there, and everything beyond is cut off from everything within. Jupiter's core reached that mass early, and the solar system's meteorites still fall into two families that never mixed.

exoplanets · The snow line

Named alongside it

The objects these essays reach for when they reach for this one.

Protoplanetary discThe snow lineAccretion rateCarbonaceous chondriteCondensation sequenceDisc evolutionEquilibrium temperatureFormation timeJupiterPebble accretionPebble isolation massPlanetesimal

All concepts