Planetary Climate Dynamics

Abstract:

We report the detection of photometric variability in the directly imaged super-Jupiter β Pictoris b (β Pic b). Using JWST NIRCam dual-band coronagraphic imaging, we conducted a 16 hr continuous photometric monitoring campaign in the F210M and F410M filters. We developed and validated a time-series photometry framework that combines point-spread function subtraction, principal component analysis for systematic noise removal, and injection-and-recovery tests to confirm signal fidelity. Both light curves show consistent sinusoidal variability at ∼5σ and ≫5σ significance in the F210M and F410M bands, respectively. A joint sinusoidal fit yields a rotation period of Prot = 9.00 ± 0.13 hr and variability amplitudes of 0.85% ± 0.07% and 0.89% ± 0.04% in F210M and F410M, respectively. The near-identical amplitudes and periods in both bands confirm a common astrophysical origin in a heterogeneous atmosphere. Combining Prot with the previously measured projected rotational velocity, we constrain the line-of-sight spin axis inclination of β Pic b. The result favors an equator-on viewing geometry, consistent with line-of-sight spin–orbit alignment: the planetary spin axis, orbital plane, debris disk, and stellar equator are all mutually aligned. This stands in sharp contrast to the large obliquities of wide-separation companions that are likely formed via gravitational fragmentation. Together with the system’s young age, this observation provides independent dynamical evidence that β Pic b formed via core accretion. This result constitutes the first detection of rotational modulation in a close-in, high-contrast exoplanet that likely formed via core accretion, demonstrating that time-series coronagraphic imaging with JWST opens a powerful new window onto the rotation, atmospheric dynamics, and spin–orbit architecture of this population.