How Earth Avoided a Fiery Premature Death

Hugh Pickens writes: "Space.com reports that the classic picture of planet migration suggests that planets like Earth should have plummeted into the sun while they were still planetesimals, asteroid-sized building blocks that eventually collide to form full-fledged planets. "Well, this contradicts basic observational evidence, like We. Are. Here," says astronomer Moredecai-Mark Mac Low. Researchers 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. One problem with the classic view of planet formation and migration is that it assumes that the temperature of the protoplanetary disk around a star is constant in temperature across its whole span. It turns out that portions of the disk are actually opaque and so cannot cool quickly by radiating heat out to space so in the new model, 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. "We are trying to understand how planets interact with the gas disks from which they form as the disk evolves over its lifetime," adds Mac Low. "We show that the planetoids from which the Earth formed can survive their immersion in the gas disk without falling into the Sun.""