Astronomers report a candidate fifth planet in the well-known HR 8799 system, imaged with the James Webb Space Telescope (JWST) using aperture masking interferometry. The source appears in the F380M filter at a projected separation of about 150 milliarcseconds, corresponding to roughly 7 astronomical units from the star. Its brightness is reported at a contrast of about 2×10^-4 relative to the star, which the authors say would correspond to a few-to-several times the mass of Jupiter if the object is a planet.

The data come from JWST’s NIRISS instrument using a technique called Aperture Masking Interferometry (AMI). AMI inserts a mask with several small holes into the telescope’s pupil to turn the telescope into a small interferometer. That lets observers measure tiny differences in light between holes and build up precise information about very close-in, faint companions using so-called closure phases, which help remove some kinds of error. The observations used a single filter (F380M) and were taken on 2023 August 3; the target star was also observed with a calibrator star to help the analysis.

This detection was made possible by a new data pipeline called amigo, which forward-models the instrument optics and the detector’s electronic response. According to the authors, amigo accounts for optical and electronic systematics that limited previous analyses and brings the AMI data down to the photon-noise limit. The candidate source lies just above the three-sigma (3σ) contrast curve in that single-band data, meaning it is only marginally above the formal detection threshold in this analysis.

If confirmed, the object would be the innermost member of the HR 8799 system and make it the first directly imaged five-planet system. The candidate’s position is compatible with proper-motion limits from Gaia and Hipparcos if the object is bound to the star, and it sits near a dynamically stable orbital solution in a 3:1 mean motion resonance with the known innermost planet (planet e). A resonant arrangement like this can help keep multiple massive planets in long-term stable orbits.