FORMATION OF SUPER-EARTH MASS PLANETS AT 125–250 AU FROM A SOLAR-TYPE STAR

We investigate pathways for the formation of icy super-Earth mass planets orbiting at 125–250 AU around a 1 $M_{\odot }$ star. An extensive suite of coagulation calculations demonstrates that swarms of 1 cm–10 m planetesimals can form super-Earth mass planets on timescales of 1–3 Gyr. Collisional damping of 10⁻²−10² cm particles during oligarchic growth is a highlight of these simulations. In some situations, damping initiates a second runaway growth phase where 1000–3000 km protoplanets grow to super-Earth sizes. Our results establish the initial conditions and physical processes required for in situ formation of super-Earth planets at large distances from the host star. For nearby dusty disks in HD 107146, HD 202628, and HD 207129, ongoing super-Earth formation at 80–150 AU could produce gaps and other structures in the debris. In the solar system, forming a putative planet X at $a\lesssim 300$ AU ($a\gtrsim 1000$ AU) requires a modest (very massive) protosolar nebula.