The HR 8799 system is iconic among wide-orbit multi-planet systems, but how did its planets form? Core accretion struggles to form $\sim$10 Earth-mass cores beyond 10 au within disk lifetimes. Yet, a debris belt at $\sim$120 au hints at a rich planetesimal reservoir. We propose the planets originated in a dense "pebble ring" at this location—analogous to axisymmetric substructures seen in ALMA images. Multi-wavelength analyses suggest these rings contain mm–cm grains, ideal for planetesimal formation and rapid growth via pebble accretion. Using an N-body model for ring-driven formation (Jiang & Ormel 2023), we show that protoplanets form sequentially and migrate inward, reproducing HR 8799's architecture. Our MCMC analysis confirms this pathway matches constraints on planet masses, locations (likely in 8:4:2:1 resonance), and the outer belt. This model links ALMA rings to planetary architectures and will inspire future planet searches and further our understanding of the formation of giant planets in wide orbits.
| Subject : | : | Oral presentation |
| Topics | : | Formation |
| PDF version | : |  PDF version |
