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 $\sim17.5$ at $\sim0.41$ arcsec ($\sim14$ au) from the host star, with no detection of the source in F200W. A follow-up observation with LBTI/LMIRCam in $L'$ band re-detects the source 16 months after the JWST epoch, providing evidence for common proper motion over a chance alignment with a background interloper at the $6-7σ$ level. Atmospheric grid model fits to the available photometry yield bolometric luminosity log$_{10}(L/L_\odot) = -5.48^{+0.10}_{-0.19}$ dex. At an age of $18-26$ Myr, hot-start evolutionary models predict $M=2.8^{+0.5}_{-0.5}$ M$_{\text{Jup}}$ and $T_{\text{eff}}=674^{+57}_{-49}$ K. From the $L'-F444W$ color and magnitudes we find evidence for disequilibrium chemistry or enhanced metallicity in the planet atmosphere. Additionally, an extended structure is detected in the JWST F200W observation, consistent with a resolved debris disk with peak density radius of $79^{+3}_{-3}$ au and an inclination of $56.5^{+1.5}_{-1.5}$ deg. 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 50 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.