Scientists said the smaller moon have only survived a few million years before it collided with the one we see today, leaving just one. Landscapes on the current moon are the remains of the smaller moon from the collision.
Professor Erik Asphaug from the University of California will unveil his ‘double moon’ theory at a Royal Society conference in September; the theory was published last year in the journal of science Nature. He said in the journal that traces of this other moon linger in a mysterious dichotomy between the Moon's visible side and its remote far-side.
"All planets except the Earth, that have a moon, have more than one. So it is unusual that we have only 'the Moon,' this itself is an oddity that would need to be explained," he said.
A theory about how our moon formed in the first place, called the giant impact hypothesis, suggests that a large object the size of Mars smashed into Earth, kicking out debris that coalesced into the moon – or maybe more than one moon. Asphaug said, "When this happens, the usual end result is not a single moon, but several moons or even dozens of them that then gobble each other up until there is one."
The scientists believe that the inner solar system at one time may have had up to 20 planet-sized bodies which collided into each other until only eight remained. 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.
Last month astronomers claimed to have discovered three planets, similar to Earth, orbiting around a single star which may be able to support life. Using new observation of Gliese 667C, astronomers found seven planets circling a star, including three super-Earths orbiting in the habitable zone. Gliese 667 is a triple-star system in the constellation of Scorpius that has masses smaller than Sun.