As the Worlds Turn: Constraining Spin Evolution in the Planetary-Mass Regime

TL;DR

This study measures and analyzes the spin velocities of planetary-mass objects across various ages, finding that their spin is conserved during cooling and contraction, and suggesting disk interactions influence their initial spin rates.

Contribution

It provides the first comprehensive dataset of PMO spin velocities across a wide age range, supporting spin conservation and disk-driven spin-down hypotheses.

Findings

Spin velocities scale inversely with planetary radius.

PMOs spin below break-up speeds, indicating spin-down during formation.

Significant variation in spin velocities remains unexplained.

Abstract

To understand how planetary spin evolves and traces planet formation processes, we measure rotational line broadening in eight planetary-mass objects (PMOs) of various ages (1--800 Myr) using near-infrared high-resolution spectra from NIRSPEC/Keck. Combining these with published rotation rates, we compile 27 PMO spin velocities, 16 of which derive from our NIRSPEC/Keck program. Our data are consistent with spin velocities $v$ scaling with planetary radius $R$ as $v \propto 1/R$. We conclude that spin angular momentum is conserved as objects cool and contract over the sampled age range. The PMOs in our sample spin at rates that are approximately an order of magnitude below their break-up values, consistent with the hypothesis that they were spun down by magnetized circum-PMO disks (CPDs) during the formation era at ages $\lesssim$ a few Myr. There is a factor of 4--5 variation in spin velocity that has yet to be understood theoretically. It also remains to be seen whether spin evolves on timescales $\gtrsim$ 1 Gyr for PMOs, as it does for stars and high-mass brown dwarfs emitting magnetized winds.