It's all wrong, scientists have been saying for over half a century: The Earth in its present orbit could not exist as it does if it was formed slightly around 4.3 billion years ago. According to theory, the Sun wasn't nearly as hot as it is today and the Earth should still be a frozen planet. Instead, Earth is covered with water, which indicates that what should be a frozen or a water-less world was somehow thawed and temperatures maintained to keep the oceans liquid. A new theory suggests that the Earth actually migrated to its present orbital location from a position closer to the Sun.
According to Discovery News, David Minton of Purdue University suggested during a presentation in April at the Space Telescope Science Institute in Baltimore, Md., that the Earth was closer to the sun at one point, then somehow migrated outward to where it currently circles the Sun. Working off of findings of extra-solar planets like "hot Jupiters" and pure water worlds (that were once ice planets) that evidently moved inward toward their host star to reach their current liquid state, Minton noted that Earth is probably also one such migratory planet.
A theory known as the "faint young sun paradox" notes that the Sun was 70 percent as bright as it is today when the Earth formed, which means that for water to have formed and stayed liquid, thus providing the cradle for life, the Earth would have had to have been originally located nearer the Sun. As the Sun grew brighter, the Earth should have lost its atmosphere and oceans in a burn-off. But if the Earth had formed at its present position, as the Sun grew to its current brightness, the Earth would have been frozen for far longer than would have made life viable for the time accounted for in geological records. However, getting to that moment of water formation would have taken considerable time. "Reflectivity," the ability to reflect light, would have kept Earth's oceans -- which would have frozen over because the warmth of the Sun wouldn't have been enough to liquify the water -- frozen for billions of years until the Sun grew hot enough.
But Minton, using an admitted combination of science fiction and science fact, has hit upon an explanation of how the Earth might have actually formed closer to the Sun and migrated to its current position while maintaining its life-producing oceans.
Minton's model includes a third planet, a "rogue" roughly the mass of Mars or Venus, moving into the Solar System and slamming into Venus in a cosmic billiards game between two and three billion years ago. The Earth would then be kicked out to its present orbit. The extra planet would have had to have continued on out of the Solar System, fell into the Sun, or become one with Venus (meaning Venus wouldn't have stopped forming until around 2.5 billion years ago).
That the Solar System was a violently chaotic place in its formative years is not a new scenario. One theory even has it that the world's oceans were seeded by a barrage of comets. And a quick look at the moon -- and Mars and Mercury -- through a telescope reveals ample evidence of intense impact activity through the ages. Earth itself has been targeted by several massive asteroids in the past as well, their craters left as markers to their destructive power.
The violence has continued. At least one asteroid, the Chicxulub meteoroid, is believed to have caused the mass extinction event that saw the elimination of dinosaurs as viable inhabitants of the planet. And that occurred just 65 million years ago.
With the recent discovery of hyper-velocity planets streaking through intra-galactic space and the estimation of billions of rogue worlds free-floating throughout the Milky, the idea of a planet like Earth migrating as a result of gravitational forces and catastrophic collisions or near misses does not seem such a far-fetched idea.
As for migratory planets, scientists announced for the first time in December the discovery of planets roughly the size of Earth and smaller circling the star Kepler-22. The larger planet, measuring 1.03 of Earth's radius, is believed to have once been an Earth twin. Both planets are believed to have been further from their host star before migrating inward and eventually passing inside what would be considered the star's habitable zone, making them unable to sustain living organisms as their are currently believed to exist.
A more recent study from the Harvard-Smithsonian Center for Astrophysics suggested that rogue or "vagabond" planets adrift in the formative years of star clusters could become attached to non-parent stellar bodies through gravitational capturing. Although the study suggests that the attachment of rogue worlds to alien stars would most likely occur at a distance, it also suggested that free-floating rogues might match velocity with a star and discover a gradual gravitational relationship with its newfound host. It would appear that such "captures" could also produce a planet-planet scattering (the billiard ball effect) should the rogues find the gravitational pull of star strong enough to pull the once free-ranging planet across the orbital path or paths of one or more of its retinue.
(photo credit: Ben Holt, NASA, Wikimedia Commons)