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Planet Formation Remains a Puzzle

© Wesley Colley

An obvious, and seemingly simple question for astronomers is, "How did the Earth form?" While the basic scenario for planet formation is widely accepted, the details are far from understood.

The planets in the solar system presumably formed in situ, that is, in place, during the formation of the sun. Stars almost always form within an accretion disc. Accretion discs develop most of the time when matter is falling together due to gravitational attraction. The disc occurs because the matter almost always has some angular momentum. Energy is dissipated by radiation of heat and light, but radiation cannot dissipate angular momentum, and the matter is forced into orbit about the central mass concentration. Angular momentum can be pushed outward by viscous exchange and magnetic braking into lower mass regions, and allow material to reach the center.

In the disc, locallized mass concentrations begin to accumulate matter, and eddy-like accretion discs form within the large accretion disc. These eddies, like the parent accretion disc, grow, and become planets.

That very simple picture is generally agreed upon, but exactly how angular momentum, the magnetic fields, and radiation interact in such an accretion disc remain difficult to observe, and to calculate. The observational difficulty is that such discs radiate principally in the infrared and are very small. The NICMOS (near infrared) camera on the Hubble Space Telescope has helped this situation a great deal (see spectacular pictures here), as have the optical cameras. The superior resolution of the HST allows us for the first time to resolve some of the structure of planetary discs, and NICMOS, particularly allows us to see the radiation coming from deep inside some star forming "globules." However, even HST is barely adequate for the job, and even larger space telescopes with dedicated infrared instruments, such as coronagraphs (which can block the light of the central star, so the surrounding disc can be seen), are required to map the structure of planetary discs completely.

Theoretically, the challenges are more grave, because modelling gas in three dimensions, including all of the physical processes, such as viscosity, radiation, magnetic fields, discrete particle collisions, etc., is computationally rigorous, and the computer must simulate billions of years of interactions. Computers are only now becoming large enough and fast enough to do the job right. Peter Goldreich (Caltech) has long been a leader in this field, and has developed a general picture of the structure of planet forming discs, which he uses to predict the sort of radiation one may expect to observe. These models have many important features, such as "warps," "flairs" and "coronae," which change the radiative properties of the discs. The work

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