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Artist's depiction of a collision between two planetary bodies. Such an impact between Earth and a Mars-sized object likely formed the Moon.. The giant-impact hypothesis, sometimes called the Theia Impact, is an astrogeology hypothesis for the formation of the Moon first proposed in 1946 by Canadian geologist Reginald Daly.
One classical thermal escape mechanism is Jeans escape, [1] named after British astronomer Sir James Jeans, who first described this process of atmospheric loss. [2] In a quantity of gas, the average velocity of any one molecule is measured by the gas's temperature, but the velocities of individual molecules change as they collide with one another, gaining and losing kinetic energy.
By 10 million years, gas in the protoplanetary disc has been blown away, and outer planet formation is likely complete. [38] 10 million – 100 million years 4.5–4.6 bya: Terrestrial planets and the Moon form. Giant impacts occur. Water delivered to Earth. [2] Main sequence 50 million years 4.5 bya: Sun becomes a main-sequence star. [32] 200 ...
If the mergers happen after the gas disk dissipates terrestrial planets can form, if in a transition disk a super-Earth with a gas envelope containing a few percent of its mass may form. If the mergers happen too early runaway gas accretion may occur leading to the formation of a gas giant.
Observations suggest that gas in protoplanetary disks orbiting young stars have lifetimes of a few to several million years. [1] If planets with masses of around an Earth mass or greater form while the gas is still present, the planets can exchange angular momentum with the surrounding gas in the protoplanetary disk so that their orbits change gradually.
The North Polar Basin on Mars is speculated by some to be evidence for a planet-sized impact on the surface of Mars between 3.8 and 3.9 billion years ago, while Utopia Planitia is the largest confirmed impact and Hellas Planitia is the largest visible crater in the Solar System.
For the very hottest gas giants, with temperatures above 1400 K (2100 °F, 1100 °C) or cooler planets with lower gravity than Jupiter, the silicate and iron cloud decks are predicted to lie high up in the atmosphere. The predicted Bond albedo of a class V planet around a Sun-like star is 0.55, due to reflection by the cloud decks.
The "Mars problem" is a conflict between some simulations of the formation of the terrestrial planets which end with a 0.5–1.0 M E planet in its region, much larger than the actual mass of Mars: 0.107 M E, when begun with planetesimals distributed throughout the inner Solar System. Jupiter's grand tack resolves the Mars problem by limiting ...