Simple questions rarely have simple answers.
For example: Where did the moon come from?
It sounds simple enough, but it has vexed scientists for years. They have considered one possibility after another.
Maybe the moon is actually a piece of the Earth, hurled away billions of years ago like a ball accidentally lobbed over a neighbor's fence.
Maybe it formed in the same way that the Earth did, slowly snowballing up from a loose cloud of dust and gas around the young sun.
Maybe it's an import, assembled elsewhere in our solar system and then snagged by Earth's gravity.
But these scenarios can't reproduce the moon's true composition, which is uncannily similar to Earth's, its small iron core, or the details of its current orbit.
So, in the 1970s, a new hypothesis surfaced: Perhaps, early in the evolution of the solar system, a mini-planet slammed into the Earth, sending a mixed-up spray of rock from both bodies out into Earth orbit. This spray would have quickly settled into a flat disk radiating out from Earth's equator, which then would have more gradually coalesced into the moon.
This scenario, called the giant impact theory (or the "big whack"), is now the leading explanation of the moon's origin.
For the past few years, planetary scientists have been using simulations to fill in the details of the big whack, trying to get their computer results to square with what we know about the Earth and the moon today.
To explain why the chemical fingerprints of the Earth and the moon are so similar, for instance, researchers have had to revise the initial story, adding more angular momentum to the system, so that the young Earth was spinning fast enough — once every two or three hours — to fling a heavy portion of Earth stuff into space along with the impactor.
But one puzzle piece stubbornly refused to fall into place: While the Earth and the other planets all orbit the sun in the same plane, the moon's orbit skews away from the plane by about five degrees.
At first, researchers thought they could gloss over the discrepancy. But in 2013, scientists at the University of California, Santa Cruz showed that, by all previous models, the tilt should have nearly vanished. Since then, researchers have been trying to figure out why it persists.
Now, a new paper by lead author Matija ĆCuk of the SETI Institute and colleagues at UC Davis, the University of Maryland and Harvard suggests a new solution.
The paper lays out a scenario in which the big whack knocked the Earth nearly onto its side, with the newly-formed moon orbiting tight around the Earth's equator. Over time, the moon's orbit grew and the sun drew it into an oval, setting up a complex gravitational drama that concluded with the Earth essentially right-side-up, and the moon almost-but-not-quite in the plane of the planets.
The researchers argue that this is the most elegant explanation currently on the table for the moon's origin. And because it predicts that the moon wouldn't have formed a rigid crust for tens of millions of years, planetary scientists should be able to test it by looking at ancient moon rocks.
It's not a simple story — but then, the good ones rarely are.