Imagine looking up at the night sky and seeing two moons instead of one -- according to a pair of researchers, there actually may have been sister moons orbiting Earth about 4 billion years ago.

The theory was first published in the journal Nature in 2011, and now an upcoming Royal Society Conference on the moon's origin in September has kicked up the subject again, The Telegraph reported.

"All planets except the Earth, that have a moon, have more than one," study co-author Dr. Erik Asphaug, a planetary geology professor at Arizona State University, told The Huffington Post in an email. "So it is unusual that we have only 'the Moon,' so this itself is an oddity that would need to be explained."

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."

Tidal forces and the sun's gravity likely made the moons' orbits unstable, causing them to collide. This could explain the moon's mysterious "crustal asymmetry" -- the moon's surface facing Earth is low and flat, while the far side is cratered and mountainous.

earth two moons
This diagram shows the researchers' computer simulation of four stages of a collision between the Moon and a companion moon, four percent of the lunar mass, about 4 billion years ago.

NASA's GRAIL mission tried testing the theory using mapping satellites Ebb and Flow to investigate the distribution of mass on the moon by probing its gravity field from September 2011 to December 2012. The results were inconclusive.

"We have looked for evidence of the second moon and we have not seen any of the suggested characteristics of the internal structure of the Moon that would be consistent with the idea of a second companion," lead researcher Dr. Maria Zuber, professor of geophysics at MIT, told BBC following the investigation.

But while there was no evidence supporting the theory, the GRAIL mission did not disprove it either.

"In science we earn a living by falsifying hypotheses, not just coming up with them," Asphaug told The Huffington Post, "so I have no grudge against those who are working hard to disprove it -- that's the name of the game!"

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