Discovery of an Exterior Third Planet Orbiting $\beta$ Pictoris

Abstract

We report the discovery of $\beta$ Pictoris d ($\beta$ Pic d), a third giant planet in the $\beta$ Pictoris system, which now becomes only the second directly imaged system with more than two confirmed planets. $\beta$ Pic d was serendipitously detected in JWST/NIRSpec IFU observations. A second epoch of NIRSpec and MIRI/MRS observations confirm the initial discovery. The extracted spectrum shows clear CH$_4$, CO, and H$_2$O absorption features, and $\beta$ Pic d's measured radial velocity is consistent with its orbital position. Radial velocity and astrometry measurements combined with orbital stability simulations suggest a semi-major axis $>$30 au, consistent with $\beta$ Pic d being responsible for carving the inner edge of the $\beta$ Pictoris debris disk. Using effective temperature estimates from atmosphere model grid fits combined with evolutionary models, we estimate a mass of 2--4 $M_\mathrm{Jup}$. $\beta$ 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.