The Aligned Compact

Compact multi-planet systems orbit in the plane their star spins. No other architecture does so consistently.

Stellar obliquity — the angle between a star's spin axis and its planet's orbital plane — encodes the dynamical history of the system. A planet that formed in the protoplanetary disk and migrated gently should be aligned. A planet that was scattered by gravitational interactions or underwent Kozai-Lidov oscillations can be tilted to any angle.

Giant planets show a wide range of obliquities: some aligned, some polar, some retrograde. This confirmed that violent dynamical histories are common for hot Jupiters. But giant planets are poor tracers of system-level dynamics because their mass produces tidal back-reactions on the star, gradually realigning the spin axis and erasing the dynamical memory.

Small planets don't have this problem. Their masses are too low to torque the star. The obliquity measurement is a clean record of the system's dynamical past.

The SLOPE survey (arXiv:2603.23713) measures obliquities of sub-Saturn planets with the Keck Planet Finder. Four new measurements: all aligned. Combined with the full sample, the statistical result at 6-sigma confidence: planets in compact multi-planet systems (tightly spaced, multiple small planets) are preferentially aligned with the stellar equator.

This distinguishes compact multis from other architectures. Isolated planets, planets in wide binaries, planets with distant giant companions — these show no consistent alignment preference. Only the compact systems stay in the plane.

The interpretation: compact multi-planet systems are dynamically cold. They formed in the disk, migrated gently or not at all, and avoided the gravitational scattering that scrambles obliquities. The compactness is not just a spatial property — it's a dynamical fingerprint of an undisturbed history.

Compact means calm. Calm means aligned.