Researchers say theories for how Earth was formed may be wrong

Contributed by
Dec 17, 2012

First we learn that the moon isn't from where we thought it was from, and now the Earth's origins are being called into question. After years of following the same model for how our planet was formed, some researchers now think they have a new explanation for how Earth got here, and why it's drier than it should be.

It might seem odd to think of the Earth as a "dry" planet, but based on the prevailing theory for how and where the planet was formed it should actually be much wetter. Water only makes up about 1 percent of our total mass, you see, and if you look at the previous models for the planet's formation, that just doesn't add up.

Why? Well, it all has to do with the "snow line," the line beyond which ice could condense back when the planets were first forming. According to new research, it was never where we thought it was.

About 4.5 billion years ago our sun was still surrounded by a mass of gases and other materials. In the old model for how Earth and other planets formed, the material in the disk was fully ionized, causing bits of it to fall into the sun, heating the disk until it ran out of stuff to burn and cooled down. While the disk was still in place, the snow line was about 1 billion miles away from the sun, but as the protoplanetary disk came apart the line moved inward, past where Earth orbits now, and the planet formed.

But there's a problem with that, according to new research: The sun was too young to ionize the disk in the first place.

"Very hot objects such as white dwarfs and X-ray sources release enough energy to ionize their accretion disks," said Rebecca Martin of the Space Telescope Science Institute (STScI) in Baltimore, who co-authored a paper on a new snow line theory. "But young stars don't have enough radiation or enough infalling material to provide the necessary energetic punch to ionize the disks."

If the old model is correct, Martin and her co-author Mario Livio argue, the planet should be much wetter and icier than it is now. Their theory? Earth's orbit was never outside the snow line.

"If the snow line was inside Earth's orbit when our planet formed, then it should have been an icy body," Martin said. "Planets such as Uranus and Neptune that formed beyond the snow line are composed of tens of percents of water. But Earth doesn't have much water, and that has always been a puzzle."

In Martin and Livio's theory, the disk around the sun would not have ionized, but would instead have continued to orbit around the star to create a "dead zone," which would have blocked anything migrating toward the sun. Nothing would have melted into the sun's surface. Instead, the blockage caused by the disk would have grown denser and hotter, creating a warmer zone outside the disk. This, Martin and Livio say, is where Earth formed, well inside the snow line.

Though this theory would explain Earth's water content, it can't explain the formation of every planet, as Livio notes.

"Conditions within the disk will vary from star to star, and chance, as much as anything else, determined the precise end results for our Earth," he said.

Check out a diagram showing the differences between the old theory and the new theory below.

(Via Huffington Post)

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