Seismic Oscillations Excited by Giant Impacts in Directly-Imaged Giant Planets

The radii and masses of many giant exoplanets imply their interiors each contain more than $\sim$100 $M_\oplus$ of solids. A large metal content may arise when a giant planet grows by colliding and merging with multiple $\sim$10 $M_\oplus$ solid cores. Here we show that a giant impact with a young gas giant excites long-lived seismic oscillations that can be detected photometrically. Mode lifetimes are close to the planet's Kelvin-Helmholtz time, a significant fraction of a young planet's age. Oscillation periods lie between tens of minutes to an hour, and variability amplitudes can exceed a percent for several million years. Beta Pictoris b is a young super-Jupiter known to be highly metal-enriched. If a Neptune-mass (17 $M_\oplus$) body impacted $\beta$ Pictoris b in the past $\sim$9--18 Myr, the planet could still be ringing with a percent-level photometric variability measurable with JWST.