Orbital Evolution and Migration of Extrasolar Planets

D. E. Trilling (LPL), W. Benz (LPL & Steward Observatory), T. Guillot (Univ. of Reading), J. I. Lunine, W. B. Hubbard (LPL), A. Burrows (Steward Observatory)

Giant planets in circumstellar disks can migrate inward from their initial (formation) positions. Migration is caused by inward torques between the planet and the disk; by outward torques between the planet and the spinning star; and by outward torques due to Roche lobe overflow and mass loss from the planet. Summing torques on planets in disks with various physical parameters, we find that Jupiter-mass planets can stably arrive and survive at small heliocentric distances. Inward migration timescales can be approximately equal to or less than disk lifetimes and star spindown timescales. Therefore, the range of fates of Jupiter-mass planets is broad, and generally comprises three classes: (I) planets which migrate inward too rapidly and lose all their mass due to Roche lobe overflow; (II) planets which migrate inward and survive in very small orbits; and (III) planets which do not migrate very far. Some, but not all, of the planets in Class II lose mass during their evolution and migration times, resulting in planets with final masses smaller than their initial masses. For example, in our model, we produce planets similar to 51 Peg b which have lost tex2html_wrap_inline11 75% of their initial mass.

The observed extrasolar planets, both those with extremely small semi-major axes (51 Peg b at 0.05 AU, tex2html_wrap_inline13  Boo b (0.046 AU), tex2html_wrap_inline15  And b (0.057 AU), and 55 Cnc b (0.11 AU)) and those with more moderate semi-major axes ( tex2html_wrap_inline17  Cor Bor b (0.23 AU), 47 UMa b (2.1 AU)) form a subset of the potential outcomes of the system, in that Jupiter-mass objects can stably survive in orbits with a wide range of semi-major axes. Our numerical model produces planets which have similar characteristics to the observed planets, as well as planets similar to Jupiter, and many intermediate cases. Since Jupiters can stably migrate to various orbital separations, we predict that, as planetary detection techniques improve, Jupiter-mass planets will be found in a wide range of orbits, from much less than 1 AU to several AU or more.