From a Seed article on scientists searching for earth-like planets:

No one yet knows for certain precisely how planets form, but the process seems to be a complex chain reaction that is highly dependent upon initial conditions. It begins with the creation of a star, which forms from a gravitationally collapsing cloud of gas and dust. The leftovers flatten out, due to the conservation of angular momentum, forming a spinning disk of material. To create a rocky world like Earth, dust must condense in the disk to form grains, grains must settle to form pebbles and rocks, and rocks must collide to form planetesimals, kilometer-sized objects that can gravitationally attract each other. These planetesimals must collide to form embryos, Moon-sized objects that collide in turn to finally form a planet.

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