By rights, the much fainter sun two billion years ago should have left Earth as a giant snowball, incapable of developing life.
But it did, and scientists have for a long time puzzled as to why. Now, though, Purdue University's David Minton believes he might have the answer: the Earth may have been closer to the sun.
"I calculated to keep the Earth from being frozen over at the beginning of its history, it would have to be six or seven percent closer to the sun than it is now," he says.
"It's a few million miles, but from an orbital mechanics standpoint, it's not that far. The question is what could make a planet move from one location to another?"
Minton suggests that the Earth may have migrated further out through a process called planet-planet scattering. This takes place when one planet or more is ejected from its orbit, there's an increase in orbital separation or when planets collide. Other possibilities, he says, can be ruled out because of the timeline involved.
He speculates that two proto-Venus planets existed at one point and went into a chaotic and unstable phase, crossing Earth's path and boosting us to our present orbit. The two proto-Venus planets then collided, forming the Venus that we see today.
The theory fits with current understanding of Venus's composition and history.
"One way we could have ruled this out would be if Venus had a geological history older than two billion years ago. We know, though, Venus is a relatively young planet," he says.
"Venus looks like it became one age all at once. Venus could look like it does because at some point in the last billion years it was two planets that collided and had this catastrophic event. This hypothesis of the Faint Young Sun Paradox fits the evolution of Venus."
If his theory's right, it was only these events that made the earth habitable.
"Depending on when it happened, it could have had a major effect on the Earth's biosphere," says Minton. "You're basically shifting the Earth's orbit from one area to another pretty dramatically."