Discovery of an Exterior Third Planet Orbiting ${\hat{I}}^2$ Pictoris

Abstract

We report the discovery of ${\hat{I}}^2$ Pictoris d (${\hat{I}}^2$ Pic d), a third giant planet in the ${\hat{I}}^2$ Pictoris system, which now becomes only the second directly imaged system with more than two confirmed planets. ${\hat{I}}^2$ 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 ${\hat{I}}^2$ 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 ${\hat{I}}^2$ Pic d being responsible for carving the inner edge of the ${\hat{I}}^2$ Pictoris debris disk. Using effective temperature estimates from atmosphere model grid fits combined with evolutionary models, we estimate a mass of 2--4 $M_{\mathrm{Jup}}$. ${\hat{I}}^2$ 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.

14 pages, 4 figures, accepted for publication in ApJ Letters, this article is embargoed for discussion in the press until formal publication in ApJ Letters; press releases are pending