The JWST Sub-Jupiters Survey: Direct-imaging Discovery of a Giant Planet and a Debris Disk around the Young M Dwarf RX J0534.0-0221

Abstract

We present the discovery of RX J0534.0-0221 b, a giant planet orbiting an M dwarf star in the β Pictoris moving group. RX J0534 was originally observed with JWST/NIRCam in the F444W and F200W filters. Observations in F444W reveal a point source at signal-to-noise ratio ∼17.5 at ∼0.″41 (∼14 au) from the host star, with no detection of the source in F200W. A follow-up observation with LBTI/LMIRCam in the L′ band redetects the source 16 months after the JWST epoch, providing evidence for common proper motion over a chance alignment with a background interloper at the 6σ–7σ level. Atmospheric grid model fits to the available photometry yield the bolometric luminosity log 10(L/L⊙)=−5.48−0.19+0.10 dex. At an age of 18–26 Myr, hot-start evolutionary models predict M=2.8−0.5+0.5 MJup and Teff=674−49+57 K. From the L′−F444W color and magnitudes, we find evidence for disequilibrium chemistry or enhanced metallicity in the planet's atmosphere. Additionally, an extended structure is detected in the JWST F200W observation, consistent with a resolved debris disk with a peak density radius of 79−3+3 au and an inclination of 56.°5−1.5+1.5 RX J0534 b is one of the lowest-mass planets imaged to date. After TWA 7 b, it is the second imaged planet around an M dwarf orbiting at solar system scales (the first within 40 au) and the first to be confirmed via common proper motion. Future orbital monitoring and atmospheric characterization will shed light on its formation history, a particularly interesting question given the challenging nature of giant planet formation around M dwarfs.