Word: proto
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...fierce Viking warlord, Stoick (300's very own Gerard Butler), whose tribe has been battling dragons for centuries. When Hiccup wounds an elusive creature called the Night Fury, no one believes him. Soon he tames, trains and learns to ride the beast, thus schooling his clan in the proto-eco message that the wilder forces of nature should not be fought but instead cultivated. (See TIME's photo-essay "Animated Movies: Not Just for Kids...
Here's the basic difference: when the planets in our solar system first formed, they were swimming through a disk-shaped cloud of gas. Their passage roiled and compressed the gas, and the gravity of the compressed gas in turn pulled on the proto-planets. The original models suggested that the net effect would have been to drag the proto-planets inward - and while the drag would have stopped as the gas eventually dissipated, it would have been too late. They would long since have fallen into...
...those early models didn't take into account the fact that compressed gas heats up, which limits how dense it can become, and in turn limits how hard its gravity can pull on the proto-planets. Beyond that, the planets' own gravity would fling gas around - the same sort of phenomenon NASA counts on, say, when a spacecraft en route to Saturn gets a slingshot velocity boost from Jupiter on the way. By adding in both effects, Mac Low's collaborator Sijme-Jan Paardekooper, now at Cambridge, found that there are places where the net force pushes a planet inward...
...much more sensible location, like Jupiter did, and then migrated inward, establishing a stable orbit there. It all made sense, except for one tiny problem: this same model also suggested that a little world like Earth shouldn't exist at all; it (or more precisely, the moon-size proto-planets that eventually assembled into Earth) should have spiraled into the sun more than 4 billion years ago. A star might not gobble a Jupiter whole when it moves close enough, but it could surely swallow a canapé like proto-Earth...
Eventually, the disk of gas dissipates, and the proto-planets are fixed in their permanent orbits. Exactly where those orbits lie depends on all sorts of factors peculiar to a given planetary system - how much material there is in the original proto-planetary disk, how much of that is dust and how much is gas, how big the dust grains are, how hot the star is and more. That's one reason we should expect each solar system to look a little bit different. Which, as it turns out, they...