A U.S. scientist says he has a theory why a much fainter sun 2 billion years ago didn't leave a frozen Earth unfit to develop the kind of life we see today.

Why the Earth avoided the deep freeze in what has been dubbed the Faint Young Sun Paradox is a much-debated question among scientists, but Purdue University's David Minton says he believes he might have an answer.

"If you go back in time to about 2 billion years ago, the Earth should have been frozen over," said Minton, citing geological and astrophysical observations that the sun burned at about only 70 percent of its current brightness.

"There's a lot of geological evidence that the Earth wasn't frozen over," the professor of earth, atmospheric and planetary sciences said. "So, what is not equal? That is the Faint Young Sun Paradox."

What was not equal, he proposed, was the Earth's location in relation to the sun.

"I calculated to keep the Earth from being frozen over at the beginning of its history, it would have to be 6 or 7 percent closer to the sun than it is now," Minton said. "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 proposes the Earth may have moved slightly farther from the sun over time through a process called planet-planet scattering, which occurs when one planet or more is ejected from its orbit, an increase in orbital separation occurs, or when planets collide.

"When a planet system or solar system forms there is no knowledge of how long they will be stable. They form and then they can go unstable in some time scale, and that time scale is set arbitrarily," he said.

"Most of the instabilities happen early, and the longer you go in history, the more rare instabilities become. But rare does not mean never, and rare events can happen."

In Minton's theory, two proto-Venus planets existed at one point and went into a chaotic and unstable phase, crossing the Earth's path and boosting it to its present orbit.

The two proto-Venus planets could have then collided, forming the planet Venus that exists today, he said.