Planets {planet} are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune, and Uranus.
planet formation
Planets form after star forms. 20% of stars have planets. Multiple stars probably do not have planets.
planet formation: materials
Iron, silicates, magnesium, aluminum, and calcium oxides are 0.5% of dust-and-gas-cloud mass and are solid at planetary-cloud temperatures and pressures. Water, methane, and ammonia are 1.5% of dust-and-gas-cloud mass and freeze at temperature -120 C at pressures in dust-and-gas cloud. Hydrogen and helium are 98% of dust-and-gas-cloud mass and are gases at planetary-cloud temperatures and pressures.
planet formation: temperature
Dust-and-gas-cloud temperatures stay constant at each distance from star during planet formation. Planetary-cloud inner region is 3 x 10^9 meters from star, at temperature 1600 K. Water is liquid 1.3 x 10^11 meters from star, because temperature is less than 100 C and more than 0 C.
planet formation: process
When stars form, dust-and-gas clouds become disks. Disks are thickest at 1.5 x 10^12 meters from center.
Disc contraction leaves behind flat ring 10^9 to 10^12 meters thick, with temperature from 1800 C to -250 C, total mass equal to new star mass, and dust particles 10^-6 meters diameter.
Dust particles stick together to make one-micron-diameter chunks. Making micron-size particles takes 100,000 years. Dust particles stick together beyond distance where temperature is less than 1800 K. Small grains flow with gas.
Millimeter-size dust hits gas and slows. At distance at which water freezes {snow line}, dust does not slow, because water evaporates inside that distance and gas is faster than dust grains. Orbits are elliptical.
Dust particles larger than millimeter diameter fall toward planet and cannot stay in orbit, because gas orbits slower than them, they hit gas, and slow.
Over one million years, kilometer-size planetesimals form, removing all dust.
For 100,000 years, inside distance where water freezes, planetesimals collide to make planets one-tenth Earth mass. For one to ten million years, outside distance where water freezes, planetesimals collide to make planets four times Earth mass. Interactions make spherical orbits.
planet formation: large planets
Ten-times-Earth-mass bodies, just outside distance where water freezes, can attract gas that cools by heat transfer through translucent gas. Large planets cause waves in gas that slow planet. Gas giant planets do not form if they spiral in too fast or stay too hot. Making large planets typically requires heavy elements and medium-to-large stars. It also requires that gas does not deplete too fast.
Planets like Earth are too small to attract hydrogen gas. Planets like Uranus and Neptune are just big enough to collect gases heavier than hydrogen. Planets like Jupiter can attract all gases, no matter what temperature.
planet formation: solar wind
After planets form, solar wind and radiation blow away uncondensed dust and gases.
planet formation: gas giant
For several million years, first gas-giant planet disturbs other planetesimals and gas and spirals nearer to star, or stays near original position and more gas giants form beyond it, as matter spirals in and accumulates at border cleared by gas giant.
planet formation: rocky planet
Rocky planets gather material over fifty million years. After rocky planets reach full size, heat from radioactivity melts planet. Layers with different densities form over 10^9 years. Iron is in center, silicates and oxides are near surface, and carbon dioxide, sulfur dioxide, hydrogen sulfide, methane, ammonia, and water vapor are in atmosphere. After layering, cooling forms rocky crust on planet surface.
atmosphere
Atmospheres lose gases by Jeans escape, hydrodynamic escape, polar wind, charge exchange, and solar wind. Water breakdown can lead to hydrogen loss and excess oxygen.
Physical Sciences>Astronomy>Solar System>Planet
5-Astronomy-Solar System-Planet
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Date Modified: 2022.0224