Yu. A. Shchekinov, M. Safonova, J. Murthy
Several planets have recently been discovered around old and metal-poor stars, implying that the planets are also old, formed in the early universe. The canonical theory suggests that the conditions for their formation could not have existed at such early epochs. The required conditions such as sufficiently high dust-to-gas ratio, could in fact have existed in the early universe immediately following the first episode of metal production. Metal-rich regions may have existed in multiple isolated pockets of enriched and weakly-mixed gas close to the massive stars. Observations of quasars and gamma-ray bursts show a very wide spread of metals in absorption from $\rm [X/H] \simeq -3$ to $\simeq -0.5$. This suggests that physical conditions in the metal-abundant clumps could have been similar to where protoplanets form today. However, planets could have formed even in low-metallicity environments, where formation of stars is expected to proceed at higher densities. In such cases, the circumstellar accretion disks are expected to rotate faster than their high-metallicity analogues. This in turn can result in the enhancement of dust particles at the disk periphery. Radiation from the central protostar can also act to drive small-scale instabilities with masses in the earth to jupiter mass range. Discoveries of planets with low-metallicity hosts show that planets did indeed form in the early universe, which may require modification of our understanding of the physical processes that produce them. This work is an attempt to provide one such heuristic scenario for the physical basis for their existence.
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http://arxiv.org/abs/1212.0519
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