The moon could have been born from a blob of the Earth's mantle, blasted outward by the explosion of a runaway nuclear geo-reactor, a new study claims.

If correct, it would replace the scientifically accepted model for the origins of the moon, known as the "giant impact theory."

That theory suggests a Mars-sized planet called Theia slammed into the early proto-Earth about 62 million years after the solar system's birth — 4.6 billion years ago.

The iron and nickel core of both planets coalesced to form the Earth, while the lighter silicate mantle and crust was ejected into space to form the moon — explaining why the moon isn't as dense as Earth.

Computer simulations suggest about 80 per cent of the moon came from the impactor and 20 per cent from the proto-Earth.

But Rob de Meijer at the University of the Western Cape in South Africa and Wim van Westrenen of VU University in Amsterdam believe the theory has a problem.

"Both moon and Earth rocks have almost the same isotopic content, which is put down to debris mixing in orbit as the moon formed," they write in a paper published on the electronic preprint website arXiv.

"While that works for lighter elements, it can't easily account for the identical ratio of heavier elements such as chromium, neodymium and tungsten."

According to the paper, the Earth was once a rapidly spinning mass of molten rock with gravity only just preventing it from spinning apart.

"Just a slight kick would have been enough to eject a blob into orbit, eventually forming the moon," they write.

Nuclear kickstart

The idea has been around for ages, but rejected because nobody could work out where the energy to kick a lunar-sized blob into orbit could have come from.

De Meijer and van Westrenen believe the energy could have come from a nuclear explosion deep in the Earth's mantle.

"High concentrations of radioactive elements like uranium and thorium near the Earth's surface could have caused a runaway nuclear chain reaction going supercritical," they write. "The explosion blasted material into orbit eventually forming the moon."

Natural nuclear fission reactors are known to have existed on Earth. In 1972, French physicist Francis Perrin discovered evidence of nuclear chain reactions having taken place two billion years ago in uranium deposits in Gabon, in West Africa.

But planetary scientist Simon O'Toole of the Anglo Australian Observatory said that "while it's an interesting idea, the science isn't strong."

"It's based around the disputed hypothesis of a naturally occurring georeactor going super critical."

O'Toole said, "the assumptions required for their idea to work are so large compared to the generally accepted theory, that if you apply Ockham's Razor and ask what's most likely, well, this isn't it!"

"The idea of the Earth spewing out the moon is great for science fiction, but a bit hairy for science fact."