Planets are cool.
With Wu (Toronto) and Udalski (Warsaw), Hoekstra developed and tested an algorithm to detect blends with eclipsing binaries in planet transit searches. They demsonstrated that this technique can provide a useful culling of candidates from the original imaging data (OGLE in this case). One case, TR33 was shown to be a blend with an eclipsing binary, whereas another (confirmed) planet transit showed no signal.
W. Kley (Tubingen), M. Lee (UCSB), N. Murray, and Stan Peale (UCSB) have investigated the GJ 876 planetary system. The two outer planets appear to have undergone extensive migration from their point of origin in the protoplanetary disk---both because of their close proximity to the star (30 and 60 day orbital periods) and because of their occupying three stable orbital resonances at the 2:1 mean-motion commensurability. The resonances were most likely established by converging differential migration of the planets leading to capture into the resonances. A problem with this scenario is that continued migration of the system while it is trapped in the resonances leads to orbital eccentricities that rapidly exceed the observational upper limits of e1 = 0.31 and e2 = 0.05. As seen in forced 3-body simulations, these lower eccentricities would persist during migration only for an eccentricity damping rate exceeding about 40 times the inward migration rate. Previous theoretical and numerical analyses have found an eccentricity damping rate comparable to the migration rate, or even eccentricity growth through disk-planet interactions. In an attempt to find effects that could relax the excessive eccentricity damping requirement, Murray and his coworkers explore the evolution of the GJ 876 system using two-dimensional hydrodynamical simulations that include viscous heating and radiative cooling in some cases. Before they evolve the whole system, the disk with just the outer planet embedded is brought into equilibrium. With a range of parameter values, they find that a hydrodynamic evolution within the resonance, where only the outer planet interacts with the disk, always rapidly leads to large values of eccentricities that exceed those observed. The resonance corresponding to the resonant angle involving the inner planet's periapse longitude, is always captured first. There is no additional delay in capturing the other resonant angle (involving the outer planet's periapse) that is attributable to the secular prograde contribution to the precession of the outer planet from the interaction with the disk, but an eccentric disk can induce a large outer planet eccentricity e2 before capture and thereby further delay capture of the second angle for larger planetary masses. Only if mass is removed from the disk on a time scale of the order of the migration time scale (before there has been extensive migration after capture), as might occur for photoevaporation in the late phases of planet formation, can the system end up with eccentricities that are consistent with the observations.
Future surveys for transiting extrasolar planets are expected to detect hundreds of jovian-mass planets and tens of terrestrial-mass planets. For many of these newly discovered planets, the intervals between successive transits will be measured with an accuracy of 0.1 to 100 minutes. M. Holman (CfA/Harvard) and N. Murray show that these timing measurements will allow for the detection of additional planets in the system (not necessarily transiting) by their gravitational interaction with the transiting planet. The transit-time variations depend on the mass of the additional planet, and in some cases terrestrial-mass planets will produce a measurable effect. In systems where two planets are seen to transit, the density of both planets can be determined without radial-velocity observations.
Sievers, Mainzer (UCLA,JPL), McLean (UCLA), and Young (Arizona) developed a new technique for efficiently searching for low-mass, isolated brown dwarfs. They are brightest, and thus easiest to find, when they are extremely young, and therefore still in the clusters in which they are born, and so they will tend to be highly reddened. This method is particularly well suited for finding reddened brown dwarfs and is currently being used in searches for low-mass brown dwarfs in star forming regions.
E. Thommes has been developing a hybrid code to study "planet-planet-disk" dynamics. It combines an existing N-body integrator (SyMBA, Duncan, Levison and Lee 1998) with a co-evolving one-dimensional viscous disk, coupling the two parts via azimuthally averaged prescriptions for planet-disk torques. This code reproduces a number of key features of planet-disk interaction found in full hydrodynamic simulations (e.g. migration and gap formation), but due to its simplicity it runs much faster, making possible simulations spanning the entire lifetime (up to 10 Myrs) of a protoplanetary gas disk. In part using this code, Thommes is investigating interactions among massive gap-opening bodies and smaller sub-gap-opening bodies in a protoplanetary disk. The former migrate more slowly and tend to act as a "safety barrier" for the latter, trapping the smaller, faster-migrating bodies in external mean-motion resonances. This constitutes a possible mechanism for the origin of the mean-motion resonances between planets which are observed in a number of extrasolar systems. (A paper based on this work was accepted to ApJ in February of 2005).
In collaboration with D. Lin and M. Nagasawa (UC Santa Cruz), Thommes is studying the late stages of terrestrial planet formation in the Solar System. In the standard model, this stage plays out over 100 Myrs or more, long after all nebular gas has been depleted. Two puzzles associated with this picture are the excessively large final eccentricities of Venus and Earth analogues produced in numerical simulations, and recent cosmochemical evidence which suggests a formation time of the Earth, from start to finish, of only a few tens of Myrs. Lin, Nagasawa and Thommes are developing a new variation on the standard model, in which the last stage of terrestrial planet formation proceeds rapidly, being triggered by the dynamical "shakeup" accompanying the depletion of the nebular gas. This mechanism is able to produce final planets on very circular orbits. Thommes spent February and early March of 2004 at a workshop on terrestrial and extrasolar planet formation hosted by the Kavli Institute for Theoretical Physics at UC Santa Barbara, where, together with Lin and Nagasawa, much of the initial work on this project was undertaken.
During the early stages of planet formation, dust grains in a protoplanetary gas disk settle to the midplane and grow. Dust is thought to be quickly swept onto the central star as it approaches 1m in size (at ~1AU). Humble and collaborators model how various populations of dust grains settle and migrate using a 3D two-phase hydrodynamic (gas and dust) self-gravitating global disk code. Rapid migration and settling of dust can be seen in some areas of the disks, as can large enhancements in dust surface density. Investigations are continuing into whether (and when, where) the dust layer becomes thin and dense enough to undergo gravitational instability and directly form long lived ~1km planetesimals.
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