There are currently six known trans-Neptunian objects (TNOs) with diameters larger than 1000 km, and all of them host satellites orbiting the primary (e.g., Brown et al. 2006; Parker et al. 2016; Kiss et al. 2017). The discovery of satellites around large TNOs provided us with a key to understanding the early history of the outer solar system. The secondary-to-primary mass ratios and spin/orbital periods among these systems exhibit a large variety. While the majority of moons around 1000 km sized TNOs have circular orbits, the Gonggong–Xiangliu system has a large eccentricity of (Kiss et al. 2019). The largest Plutonian satellite, Charon, is thought to be an intact fragment of an impactor directly formed via a giant impact. However, whether giant impacts can explain the origin of other satellites has been unclear.
Here, we conduct numerical simulations of giant impacts using the smoothed particle hydrodynamics method. We find that the simulated secondary-to-primary mass ratio overlaps with observed mass ratios when these satellites are directly formed as intact fragments of colliding bodies (Arakawa et al. 2019). Additionally, we succeed in reproducing the satellite systems' current spin/orbital periods and eccentricities by considering their tidal evolution following giant impacts (Arakawa et al. 2021). These results suggest that all satellites around 1000 km-sized TNOs might be formed via giant impacts in the early stage of solar system formation. We also discuss the structure and rheology of large TNOs based on their tidal evolution histories.