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Project Progress

Dr. Lufkin, in collaboration with Drs. Richardson and Mundy, has investigated the effect of giant planet migration on disks of planetesimals, to estimate the feasibility of finding terrestrial planets exterior to known Hot Jupiter systems. Theories of planet formation suggest that giant planets should be forming and migrating at the same time as terrestrial planets are forming in and around the habitable zone. The giant planet can strongly affect the orbits of planetesimals that might otherwise be forming Earth-like planets (Figure 1).

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The effect is strongly dependent on the mass and migration rate of the migrating giant planet. This dependence is the key result of this work, and is shown in Figure 2. A low mass or rapidly migrating giant will have a small impact on a disk of planetesimals. Conversely, a massive or slowly migrating planet will excite all the planetesimals, ejecting several percent. Given that a migrating giant excites a disk of planetesimals, can those planetesimals cool and resume growth via accretion? We estimate that half of the planetesimals will be able to cool onto circular, coplanar orbits within the lifetime of the circumstellar gas disk. This result naturally leads to the prediction that terrestrial planets can exist, even in solar systems with a Hot Jupiter close in.

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All the work described above has been performed on the computing cluster of the University of Maryland Astronomy Department. This work was presented at the Protostars and Planets V meeting in October 2005 and at the Goddard Extrasolar Planets Group in May 2006. This work is directly relevant to Objective 1, “Habitable Planets”, of the Astrobiology Roadmap. CURRENTLY THE PAPER DESCRIBING THIS RESEARCH IS IN SECOND REVISION FOLLOWING POSITIVE REVIEWER COMMENTS.

Dr. Richardson, in addition to working with Graeme Lufkin on the simulations described above, led the effort to merge together various versions of the numerical code used for these simulations and related work, including simulations of planet formation. In particular, code developed in the previous year with former graduate student Zoë Leinhardt (ApJ 625:427, 2005) has been reconciled with the official production code. The Leinhardt code uses a rubble pile model for resolving the outcomes of planetesimal collisions with mass loss, appropriate for the early and middle stages of planetary growth. Richardson and Leinhardt are currently running a very long-term planet formation simulation (it has been going since May 2005) that uses, for the first time, realistic planetesimal sizes together with the new collision model. They will meet over the summer to analyze the results and prepare a paper for publication. Meanwhile, Richardson is supervising an Astrobiology summer intern, Jessica Hasseltine, who is working with Richardson and Lufkin to implement, test, and run a new algorithm for quantifying and tracking the radial movement of planetesimals in a planet-forming disk that takes into account simultaneously the incorporation of planetesimals into larger bodies and their disruption into smaller bodies. This effort will use the Leinhardt collision model, thereby making it the most sophisticated direct simulation of planetesimal evolution attempted to date. This work is directly relevant to Objective 1, “Habitable Planets”, of the Astrobiology Roadmap.