"How Earth Survived Its Birth"
Space.com (January 7, 2010)
"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.
So far, so good: that's pretty much what astronomers and planetary scientists have figured for some time now. There's just one catch to this established model, and it's a big one.
The math says that as planets form, they also migrate inside the disk of dust. Specifically, the math says they migrate towards the star. Then they fall in. According to the mathematical models.
Obviously, since I've written this post, and you're reading it, there's something wrong with this picture. Earth is rather definitely here, philosophy 101 freshman-baiting notwithstanding.
Or, as one of the astronomers put it:
"...'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...."
The existing mathematical models didn't take temperature differences within the dust disk into account. Turns out, the disk is opaque - and quite a bit hotter near the star. Factor that into the equations, and you don't have Earth falling into the sun a bit over 4,000,000,000 years back.
The new mathematical models predict that planets are pulled in - just like in the earlier models - and that temperature differences push them away from the star. Each planet, again in the newer model, reaches an equilibrium point where its orbit stays stable - and eventually the dust dissipates.
Wait a few billion years, and you've got trilobites, dinosaurs - and us.
Since the model that Mac Low and his colleagues came up with doesn't have Earth plunging into the sun long before life got started, it would seem that it's a closer match with reality than previous ones.
Particularly since it's becoming quite obvious that stars with planets orbiting them aren't anywhere near the 'chicken teeth' rarities they were supposed to be, not all that many decades back. (But aren't as common as they might be, either - More about that, tomorrow morning.)
Exciting times, these that we live in.
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