How does accretion explain planet formation?

1 Answer
Jul 20, 2017

Accretion IS planetary formation, at least for rocky planets.


Accretion, meaning the process of growth or increase by gradual accumulation of matter is how rocky planets form.

Obviously some initial conditions have to exist such as having enough matter that could form a planetary body in that orbit.

What happens is that gravity slowly gathers material together, even stuff as small as dust. as each of these grow, they each start to exert more and more pull on the others so they clump together, eventually forming meteorites, then meteors. If there is more material the meteors will start to clump together and form planetoids, and reach the size of a dwarf planet. That is when it reaches a mass that is big enough for gravity to exert enough pressure to shape it like a ball.

At this point, if there is more matter in its orbit it will likely attract and colide with that matter and grow even more, making a planet. It could colide with other planetoids in its path and may join or be destroyed by those collisions. One of the prevailing hypothesis on the formation of the Earth/Moon system is that the primordial Earth was hit by another planetary body which added some material to our planet and left an accretion disk around us that eventually coalesced into the Moon.

Now, a major factor that may prevent all this from happening is a big outside source of gravity that prevents the large body formation, such as what happens in the asteroid field between Mars and Jupiter. Jupiter has such powerful gravity that it prevents, or at least slows down, the asteroids from clumping together into a planet. I say slow down, because there is a dwarf planet within the asteroid belt, and it compromises about 1/3 of the mass of the belt. See link here: Ceres

For a bit more on the formation of the Earth and Moon see here:When was the moon formed in relation to the earth?