Collision Velocity Scaling Implies Maximum Planet Density Falls with Mass and Orbital Period
Maximum iron enrichment from giant impacts is bounded by mass and orbital period through encounter-velocity scaling. GJ 367b alone sits near this bound, offering the clearest empirical test. Future detections at slightly longer periods will falsify or support the model within two years.
The paper integrates published SPH runs of head-on and grazing impacts with an analytic model of encounter velocities that rise steeply inside 0.5 d orbits. Because escape velocity scales as M^{1/3} while typical impact speeds scale as P^{-1/3}, the specific energy available for mantle removal drops rapidly for planets above ~2 Earth masses or periods longer than ~1 d, producing a hard upper envelope on iron fraction that matches the absence of dense worlds at longer periods in the current census.
Brady et al.: No planet with M > 3 Earth masses and P > 2 d will be found with CMF > 0.65 in the next 50 confirmed ultra-short-period worlds.
Sources (3)
- [1]Primary Source(https://arxiv.org/abs/2606.18336)
- [2]Supporting Source(https://science.org/doi/10.1126/science.abn2210)
- [3]Supporting Source(https://arxiv.org/abs/2203.16773)