Neptune Trojans as a Test Bed for Planet Formation

The problem of accretion in the Trojan 1 : 1 resonance is akin to the standard problem of planet formation, transplanted from a star-centered disk to a disk centered on the Lagrange point. The newly discovered class of Neptune Trojans promises to test theories of planet formation by coagulation. Neptune Trojans resembling the prototype 2001 QR₃₂₂ ("QR")—whose radius of ~100 km is comparable to that of the largest Jupiter Trojan—may outnumber their Jovian counterparts by a factor of ~10. We develop and test three theories for the origin of large Neptune Trojans: pull-down capture, direct collisional emplacement, and in situ accretion. These theories are staged after Neptune's orbit anneals, that is, after dynamical friction eliminates any large orbital eccentricity and after the planet ceases to migrate. We discover that seeding the 1 : 1 resonance with debris from planetesimal collisions and having the seed particles accrete in situ naturally reproduces the inferred number of QR-sized Trojans. We analyze accretion in the Trojan subdisk by applying the two-groups method, accounting for kinematics specific to the resonance. We find that a Trojan subdisk comprising decimeter-sized seed particles and having a surface density ~10^-3 times that of the local minimum-mass disk produces ~10 QR-sized objects in ~1 Gyr, in accord with observation. Further growth is halted by collisional diffusion of seed particles out of resonance. In our picture, the number and sizes of the largest Neptune Trojans represent the unadulterated outcome of dispersion-dominated, oligarchic accretion. Large Neptune Trojans, perhaps the most newly accreted objects in our solar system, may today have a dispersion in orbital inclination of less than ~10°, despite the existence of niches of stability at higher inclinations. Such a vertically thin disk, born of a dynamically cold environment necessary for accretion and raised in minimal contact with external perturbations, contrasts with the thick disks of other minor body belts.