The Formation Signature

Giant planets and brown dwarfs overlap in mass. A 13-Jupiter-mass object could be either, depending on how it formed — core accretion building up from a seed, or gravitational fragmentation collapsing from a cloud. The conventional boundary at the deuterium-burning limit is a nuclear physics threshold, not a formation marker. It tells you what the object does, not where it came from.

Sepulveda et al. (arXiv:2601.05976) find that spin does what mass cannot: it distinguishes formation pathways. Surveying 43 benchmark companions and 54 free-floating brown dwarfs with high-resolution spectroscopy, they show that giant planets (2–7 Jupiter masses) rotate significantly faster than brown dwarf companions (10–40 Jupiter masses), at 4–4.5 sigma significance.

The mechanism is circumplanetary disk braking. A planet forming by core accretion builds up a circumplanetary disk that extracts angular momentum — but less effectively than the circumstellar disk surrounding a brown dwarf forming by fragmentation. Brown dwarfs as companions lose more spin than planets do. The formation process writes itself into the rotation rate, and the writing is permanent.

The cleanest separator isn't mass but mass ratio: objects below 0.8% of their host star's mass show planetary rotation profiles; above it, brown dwarf profiles. The threshold is relational — it depends on the companion-to-host ratio, not on the companion alone.

There's a second finding buried in the data. Free-floating brown dwarfs spin faster than companion brown dwarfs. Isolation preserves angular momentum that companionship strips away. The disk braking depends on context — same object, different environment, different spin.

The through-claim: formation isn't an event in the past. It's a signature carried forward in the rotation of every object, permanently distinguishing things that mass alone treats as identical. The birth is still happening, written in the spin.