A large number of stars are not solitary, but belong to systems formed by two or more stars, in which the formation of planets can be difficult due to the absence of stable orbits: the protoplanets would be dragged in either direction by the gravitational influences of the different stars. In these systems it is likely that the only thing that is formed are pieces of cosmic debris like those that exist in our asteroid belt.
The process of planet formation is very efficient. Initially, collisions between the planetésimos occur at low speed, so they collide objects that tend to merge and grow. At a typical Earth-Sun distance, an object of 1 km takes only about 1000 years to grow up to 100 km. Another 10,000 years produce protoplanets of almost 1000 km in diameter, which grow by 10,000 years to protoplanets of almost 2000 km in diameter. Thus, objects the size of the Moon can be formed in as little time as 20,000 years.
As the protoplanets become larger and more massive, their severity grows. When some objects reach a size of about 1000 km, they begin to attract the rest of smaller objects. Gravity attracts rock clusters the size of asteroids, at increasingly higher speeds. They go so fast that when they collide, they don't merge, but they pulverize. While the larger protoplanets continue to grow, the rest turn each other into dust.
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