How Earth Survived Its Birth
This image is a combination of three photos (using an orange, green and blue filter) taken by the OSIRIS camera onboard ESA's Rosetta spacecraft. It is part of a sequence of images taken every hour through one full rotation (24 hours). The illuminated sliver is centered around the South Pole, with South at the bottom of the image.
Credit: ESA ©2009 MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA.

WASHINGTON ? Just how Earth survived the process of its birth without suffering an early demise by falling into the sun has been something of a mystery to astronomers, but a new model has figured out what protected our planet when it was still a vulnerable, baby world.

In short, temperature differences in the space around the sun, 4.6 billion years ago, caused Earth to migrate outward as much as gravity was trying to pull it inward, and so the fledgling world found equilibrium in what we now know to be a very habitable orbit.

Planets like the Earth are thought to form from condensing clouds of gas and dust surrounding stars. The material in these disks gradually clumps together, eventually forming planetesimals ? the asteroid-sized building blocks that eventually collide to form full-fledged planets.

As the planets are forming, they are also thought to migrate within the surrounding dust disk. The classic picture of this planet migration suggests that planets like (and including) the Earth should have plummeted into the sun while they were still planetesimals.

"Well, this contradicts basic observational evidence, like We. Are. Here," said astronomer Moredecai-Mark Mac Low of the American Museum of Natural History in New York.

Mac Low and his colleagues investigated this apparent paradox and came up with a new model that explains how planets can migrate as they're forming and still avoid a fiery premature death. He presented these findings here today at the 215th meeting of the American Astronomical Society.

One problem with the classic view of planet formation and migration is that it assumed that the temperature of the protoplanetary disk around a star is constant in temperature across its whole span, Mac Low explained.

It turns out that portions of the disk are actually opaque and so cannot cool quickly by radiating heat out to space. This creates temperature differences across the disk, and these differences have not been accounted for before in models. So Mac Low and his colleagues created new model simulations of planet migration that include a disk with variations in temperature.

What happens when you change the temperatures in the disk is this: The temperature changes can completely alter the nature of the planet migrations, causing planets to migrate outward instead of inward.

"Well, that is a major development," Mac Low said, because you can put it in the model and see if outward migration cancels inward migration "and allows us to survive, or at least our progenitors."

Sure enough, that seems to be the case. Within the disk, zones of inward and outward migration develop that meet at equilibrium zones; once planets reach these, "they more or less sit there," Mac Low said.

And eventually the disk dissipates to a point where its gravity can no longer influence the planets to pull or push them into new orbits.

So the model suggests that outward migration "allows planetoids to survive," which explain how planets in our solar system and others that we see in galaxy survive, Mac Low said.