The scientists noted that their proposition differed from the current leading theory, which holds that the Moon was created from material from a giant body that struck the Earth.
The smaller ‘twin’ moon is believed to have only survived a few million years before it collided with the one we see today, leaving just one.
The theory will be explained by Professor Erik Asphaug, from the University of California at Santa Cruz at a conference about the moon to be held at the Royal Society this September.
He said: “The second moon would have lasted for only a few million years; then it would have collided with the moon to leave the one large body we see today.
“It would have orbited Earth at the same speed and distance and just got slowly sucked in until they hit and then coalesced.”
Prof Asphaug told the Sunday Times he believes the landscape of the moon, which appears to have mountains, are the remains of Earth’s smaller moon when the pair collided. The moon’s smaller twin is believed to have been about onethirtieth of the size.
The Earth and its moon are thought to have been formed between 30 million and 130 million years after the birth of the solar system, about 4.6 billion years ago.
A total of nine ‘super-Earths’, planets between one and 10 times the mass of Earth. have previously been found.
Scientists from Harvard put forward a theory last year that suggested the Moon was once part of Earth that spun off after they collided with another body. The study was published in journal Science.
Last month astronomers reported discovered three planets, similar to Earth, orbiting around a single star which may be able to support life.
Researchers estimate there could be as many as 100 billion planets similar to the Earth in our galaxy, the Milky Way.