New model shows how Earth survived its birth

Astronomers have proposed a new model for planet formation that has a distinct advantage over previous ones: it predicts that the Earth should exist.

Researchers at the American Museum of Natural History and the University of Cambridge, U.K., say their simulation shows how variations in temperature can keep nascent planets from spiralling into the sun.

Under current planet formation models, a spinning disk of gas and dust forms around a newly born star. Larger clumps of matter attract more matter, forming asteroid-like protoplanets.

The model suggests that the protoplanets coalesce into small, rocky planets like Earth and Mars, and huge gas giants like Jupiter.

Unfortunately, the calculations also predict that the protoplanets should fall into the sun within a million years, well before the planets have a chance to take shape.

Clearly, this is a problem with the model, since the Earth and Mars and all the other planets are still here.

The researchers came up with a new computational model that takes into account variations in temperature within the disk of dust.

"We are trying to understand how planets interact with the gas disks from which they form as the disk evolves over its lifetime," said Mordecai-Mark Mac Low of the American Museum of Natural History.

The model predicts the changes in temperature and density in the disk over time, as the protoplanets form and the disk thins out.

It shows that planets form in orbits between regions where dust and planetoids are pulled into the sun and regions where they move away from the sun.

Eventually, the disk of dust dissipates and only the planets remain, stable in their orbits.

"We show that the planetoids from which the Earth formed can survive their immersion in the gas disk without falling into the sun," said Mac Low.

The model is so mathematically complex that only one dimension — distance from the sun — is used to predict the behaviour of a three-dimensional system.

"Three-dimensional models are so computationally expensive that we could only follow the evolution of disks for about 100 orbits — about 1,000 years," said study co-author Wladimir Lyra.

"We want to see what happens over the entire multimillion-year lifetime of a disk," he said.

Mac Low presented the research at the American Astronomical Society meetings in Washington, D.C., on Jan. 6. The research has been submitted for publication in the Astrophysical Journal.