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How Earth survived birth
Posted: 07 January 2010

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According to standard models of planet formation, Earth shouldn't exist. Now, new simulations show how inner solar system planets escape consumption by their host stars.

Standard models of they way planets form from dust and gas in the disc surrounding a young star have always assumed locally constant temperatures within the disc, but the new simulations, designed by researchers at the American Museum of Natural History and the University of Cambridge show that local temperature variations may play a more important role than previously realised.

Local temperature variations in the proto-planetary disc may have prevented Earth careering into the Sun. Image: NASA.

“We are trying to understand how planets interact with the gas discs from which they form as the disc evolves over its lifetime,” says Mordecai-Mark Mac Low, Curator of Astrophysics and Division Chair of Physical Sciences at the Museum. “We show that the planetoids from which the Earth formed can survive their immersion in the gas disc without falling into the Sun.”

The midplane of the dusty disc is opaque and cannot cool quickly by radiating heat. But local temperature variations in the disc can lead to regions of outward and inward migration, depending on the temperature structure, and the planets get trapped between regions of inward and outward migration. When the proto-planetary disc begins to dissipate, the orbits slowly move inwards until the gas density drops low enough for the planets to no longer be influenced by the disc, and they remain in that orbit, escaping further migration into the cauldron of the central star. The radius of the orbit at which a planet is released depends on its mass.

The team used a one-dimensional model this project. “Three dimensional models are so computationally expensive that we could only follow the evolution of discs for about 100 orbits – about 1,000 years,” says Wladimir Lyra. “We want to see what happens over the entire multimillion year lifetime of a disc.”

The model was presented at the American Astronomical Society meeting held in Washington this week.