Collision With A Protoplanet?

At the same time, new findings about the formation of the solar system suggested that large numbers of massive objects, called protoplanets or planetesimals, had been orbiting the sun at the time the Earth formed. Gradually, scientists came to believe, these objects collided with one another and were broken up, or massed together to make the planets, or were flung out of the solar system.

At the second major conference on the origin of the moon, held in Kona, Hawaii, in 1984, the giant-impact theory was the center of attention. It became the prevailing view of most astronomers.

As more researchers tested the giant-impact theory, they had to contend with the fact that many of the early calculations of the theory were relatively crude. Although the calculations showed that an enormous collision could have blown enough matter out of the Earth (and from the vaporized outer layers of the impacting object itself) to make the moon, they did not trace the events in detail. So while the theory was very promising, astronomers did not know how realistic it was.