Scientists have found evidence that the moon was formed after the collision of Earth with another planet-sized object that occurred 4.5 billion years ago. Analysis of the lunar rocks that were brought back by Apollo astronauts showed that the earth was hit by large object called 'Theia'.

The name 'Theia' is derived from a goddess in Greek mythology who was said to be the mother of Selene, goddess of the Moon.

The German researchers claim that the discovery proves the previous theory that the moon was created by collision between the Earth and Theia. Until now, no-one has found definitive evidence for the collision theory, according to the lead researcher, Dr Daniel Herwartz, from the University of Goettingen.

According to the scientists, the collision between the Earth and Theia was a huge one. While the latter one was destroyed due to the impact, its remnants along with some Earth debris eventually led to the formation of the moon.

"It was getting to the stage where some people were suggesting that the collision had not taken place. But we have now discovered small differences between the Earth and the Moon. This confirms the giant impact hypothesis," BBC News quoted him as saying.

"I think that most people believe in the Giant Impact Hypothesis. And now that we have solved one of the last gaps in explaining it, those who criticize it will have a hard time."

The German researchers used more specified techniques to evaluate the isotope ratios in lunar samples, with those from Earth. Initially, the team used lunar samples from meteorite that had fallen on to the Earth and later studied Apollo moon rocks but in both the cases, failed to find any major difference in isotope ratios between Earth and the moon.

However, Herwartz and his team used a new approach to find for differences in the ratio of two oxygen isotopes, oxygen-17 and oxygen-16 between both the moon and Earth rocks. For this, the scientists extracted samples of oxygen from all the moon rock samples and discovered that for every million oxygen-16 isotopes, the lunar rocks had more of oxygen-17 isotopes than rocks obtained from Earth's mantle.

This difference "supports the view that the Moon formed by a giant collision of the proto-Earth with [an impactor]," the scientists said in a statement, according to Science.

The new findings suggest that moon is composed of nearly 40 percent of Theia remnants while the Earth is responsible for the rest, said the scientists.

"Now that a difference has been found, many will work to confirm or deny it and do battle over what it means," Science quoted David Stevenson of the California Institute of Technology in Pasadena.

The study was published in the journal Science.