Discovery of an Exterior Third Planet Orbiting β Pictoris

Abstract We report the discovery of β Pictoris d ( β Pic d), a third giant planet in the β Pictoris system, which now becomes only the second directly imaged system with more than two confirmed planets. β Pic d was serendipitously detected in JWST/NIRSpec integral field unit observations. A second epoch of NIRSpec and MIRI medium-resolution spectrometer observations confirms the initial discovery. The extracted spectrum shows clear CH 4 , CO, and H 2 O absorption features, and the planet’s measured radial velocity is consistent with its orbital position. Radial velocity and astrometry measurements combined with orbital stability simulations suggest a semimajor axis >30 au, consistent with β Pic d being responsible for carving the inner edge of the β Pictoris debris disk. Using effective temperature estimates from atmospheric model grid fits combined with evolutionary models, we estimate a mass of ∼2–4 M Jup . β Pic d is the first planet discovered using spectral template matching with moderate-resolution spectroscopy, highlighting its sensitivity to planetary molecular features hidden within bright extrasolar debris disks that are difficult to access with broadband imaging.