Was Earth a Migratory Planet?


By all accounts Earth should be a "snowball planet" like the frigid world Hoth in the 1980 Star Wars film "The Empire Strikes Back."

Why? Because common theories of stellar evolution predict that the sun was only 70 percent of its current brightness when it first lit its fusion engine 4.5 billion years ago. The sun has been steadily growing brighter since then and will continue so into the future, eventually evaporating away Earth's oceans.

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Once Earth amassed an ocean 4.3 billion years ago, it should have quickly frozen over and reflected so much sunlight back into space that it squelched Earth's ability to thaw out for billions of years.


The dilemma, called the "faint young sun paradox," has been know about since the 1950s and was popularized by Carl Sagan. Geochemists and solar physicists have wrestled for answers all these years.

Lowering Earth's reflectivity by reducing cloud cover doesn't work. Models also show that a greenhouse effect from dense carbon dioxide and methane can't warm the Earth enough either. In some simulations, methane and carbon dioxide combine to make a photochemical smog that would have chilled Earth even further.

Now, David Minton of Purdue University has come up with a novel solution that, by his own admission, straddles science fact and fiction. Minton proposes that Earth was closer to the sun when it formed and then migrated outward to its current orbit. To keep Earth tepid under a cooler sun, our planet would have needed to have been roughly 6 million miles (9.7 million kilometers) closer to the sun than it is today.

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"Planets don't like to stay still, they like to move," said Minton during a presentation at the Space Telescope Science Institute in Baltimore, Md., on April 10.

This is proven by the discovery of hundreds of extrasolar planets that reinforce a radical new idea that would have never even been considered in the 1950s: Planet migration seems the rule rather than the exception among the stars. This explains the estimate of billions of "hot Jupiters," which are predicted to have moved to orbits precariously close to their stars — to the point of evaporating away.

More recent discoveries find pure water planets that must have migrated in toward their sun as ice balls.

But how do you nudge Earth into a new zip code? The most plausible model, out of several other unlikely mechanisms that were only present in the very young solar system, is a gravitational billiard ball game called planet-planet scattering.

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The challenge is that this effect would have had to have dragged out over 1 or 2 billion years. Even more problematic is that for this musical chairs game to work at all, one more terrestrial planet is needed in the inner solar system. And, it would need to be a big one at that, ranging between the mass of Mars and Venus.

The unlucky "odd planet out" would have wound up falling into the sun, being ejected from the solar system, or crashing into another terrestrial planet.

This isn't too far-fetched in that the solar system is fundamentally chaotic, says Minton. "Solar systems don't know if they are going to be stable for billions of years." Minton says that the best dynamical computer simulation for relocating Earth has a rogue plant that is 75 percent Earth's mass smashing into Venus — in the ultimate planetary pinball game of "three's a crowd." This would have happened as little as 2 or 3 billion years ago, with Earth getting kicked out into its present orbit as a consequence.

This idea leaves me a little chagrined because it sounds like some of the wacky imaginary planet Nibiru predictions for the 2012 doomsday warnings. What's more, in the 1970s Sagan ridiculed Immanuel Velikovsky's ad hoc theories of a runaway Venus and other colliding planets. Velikovsky's ideas were built around comparative mythology and not dynamical modeling.

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The Venus collision model is a "plausible idea" insists Minton, but "heavy toward science fiction." It would mean that Venus didn't finish forming until 2.5 billion years ago, and that would explain Venus' appearance and a geologically young looking volcanic planet, he says.

Regardless of this scenario, we're not necessarily safe into the future. In 1 of 2,500 dynamical simulations of the evolving solar system, Mercury, which is in a quasi-stable orbit, gets ejected from the solar system within 5 billion years from now. This triggers the ultimate Armageddon: a collision between Earth and Venus. Mars is tossed farther from the sun.

In chaos theory, a very slight perturbation can trigger dramatic consequences billions of years down the road. This is called the butterfly effect, where something as innocuous as a butterfly flapping its wings ultimately triggers a hurricane in the future.

One celestial equivalent of the butterfly effect is the infinitesimal altering of a planet's orbit by spacecraft using the gravity assist method, which gives the spacecraft a kick speed by robbing momentum from the planet. But chaos theory is so unpredictable that one could never tell if such an event had good or bad consequences. Perhaps by orbiting NASA’s MESSENGER spacecraft around Mercury, we are avoiding a future "when worlds collide" doomsday scenario!

Image credit: NASA

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