JWST-TST High Contrast: Living on the Wedge, or, NIRCam Bar Coronagraphy Reveals CO₂ in the HR 8799 and 51 Eri Exoplanets’ Atmospheres

High-contrast observations with JWST can reveal key composition and vertical mixing dependent absorption features in the spectra of directly imaged planets across the 3-5 μm wavelength range. We present novel coronagraphic images of the HR 8799 and 51 Eri planetary systems using the NIRCam Long Wavelength Bar in an offset “narrow” position. These observations have revealed the four known gas giant planets encircling HR 8799, even at spatial separations challenging for a 6.5 m telescope in the mid-infrared, including the first ever detection of HR 8799 e at 4.6 μm. The chosen filters constrain the strength of CO, CH₄, and CO₂ absorption in each planet’s photosphere. The planets display a diversity of 3-5 μm colors that could be due to differences in composition and ultimately be used to trace their formation history. They also show stronger CO₂ absorption than expected from solar metallicity models, indicating that they are metal enriched. We detected 51 Eri b at 4.1 μm and not at longer wavelengths, which, given the planet’s temperature, is indicative of out-of-equilibrium carbon chemistry and an enhanced metallicity. Updated orbits fit to the new measurement of 51 Eri b validate previous studies that find a preference for high eccentricities ( e = 0.5 7 − 0.09 + 0.03 ), which likely indicates some dynamical processing in the system’s past. These results present an exciting opportunity to model the atmospheres and formation histories of these planets in more detail in the near future, and are complementary to future higher-resolution, continuum-subtracted JWST spectroscopy.