Deep Limits on Planets Exterior to HR 8799’s Known Planetary System with the James Webb Space Telescope

Abstract We observed the HR 8799 planetary system with JWST’s NIRCam instrument using its F356W and F444W filters to search for additional, lower-mass planets that might reside between or exterior to the four previously detected Jovian-mass planets. After blocking out the star with a circular occulting mask with gradual radial opacity, the four known planets (b, c, d, and e) are detected in both filters, even though the innermost planet lies so close to its parent star that the coronagraphic mask nominally blocks ∼86% of the planet’s light. The observed fluxes and positions of the planets are consistent with previous models of the planets’ atmospheres and with predictions of orbital fits. Two background sources are detected interior to and within the outer debris ring, respectively, both of which are visible in ALMA images at submillimeter wavelengths. No new planet candidates ≥1 M Jup are seen within the region defined by the four known planets. Planets larger than a Saturn mass (∼0.3 M Jup ) are ruled out within the outer gap between HR 8799 b (at 70 au) and the cold belt of debris (≥120 au), limiting the ability of a potential planet in that region to sculpt the inner edge of the outer disk.