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"Exotic Earths: Forming Habitable Worlds with Giant Planet Migration," S. Raymond, A. Mandell & S. Sigurdsson 2006 Science, 313, 1413

Close-in giant planets (e.g. "Hot Jupiters") are thought to form far from their host stars and migrate inward, through the terrestrial planet zone, via torques with a massive gas disk. Here we simulate terrestrial planet growth during and after giant planet migration. Several- Earth mass planets also form interior to the migrating Jovian planet, analogous to recently-discovered "Hot Earths." Very water-rich, Earth-mass planets form from surviving material outside the giant planet's orbit, often in the Habitable Zone and with low orbital eccentricities. More than a third of the known systems of giant planets may harbor Earth-like planets.



Mixing: A Strict Limit on Infall in the Chemical Evolution of Galaxies

Galactic chemical evolution models assume infalling gas mixes instantaneously with disk gas. We investigate whether physically-realistic models for mixing of infalling gas are consistent with the small present-day scatter in metallicity logZ, ~ 0.04 dex at most. The observational limit is well-established for oxygen from many FUSE/HST lines of sight, and iron and other elements in star clusters. Unlike previous work, we solve a kinetic equation for the evolution of the Z probability distribution. Mixing only occurs in nature by microscopic diffusion, which must be amplified by a complex velocity field to bring Z gradients to very small scales. The two mixing models are instabilities in ISM-sweeping supernova shells, and interstellar turbulence. Both processes have time scales that are well-constrained, and we show that either can account for the observed scatter in the absence of infall. With infall, mixing is constantly counteracted by the addition of gas with Z much different than the disk mean, and the resulting Z scatter is a factor of 5 to 10 larger than observed. Infall will not resolve the so-called G-dwarf problem, the original motivation for its suggested existence. Our results are the first quantitative support for qualitative arguments by Larson (1998) and Haywood (2006) that infall is not required to account for any major chemical evolution constraint, and in fact contradicts them.


2 November 2006
Astronomy Program · The University of Texas at Austin · Austin, Texas 78712
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