<i>JWST</i> NIRSpec finds no clear signs of an atmosphere on TOI-1685 b

ABSTRACT Determining the prevalence of atmospheres on terrestrial planets is a core objective in exoplanetary science. While M dwarf systems offer a promising opportunity, conclusive observations of terrestrial atmospheres have remained elusive, with many yielding flat transmission spectra. We observe four transits of the hot terrestrial planet TOI-1685 b using James Webb Space Telescope (JWST)’s Near Infrared Spectrograph (NIRSpec) G395H instrument. Combining this with the transit from the previously observed phase curve of the planet with the same instrument, we perform a detailed analysis to determine the possibility of an atmosphere on TOI-1685 b. From our retrievals, the Bayesian evidence favours a simple flat line model, indicating no evidence for an atmosphere on TOI-1685 b, in line with results from the phase curve analysis. Our results show that hydrogen-dominated atmospheres can be confidently ruled out. For heavier, secondary atmospheres we find a lower limit on the mean molecular weight of $\gtrsim 10$, at a significance of ~5σ. Pure ${\rm CO}_{2}$, ${\rm SO}_{2}$, ${\rm H}_{2}{\rm O}$, and ${\rm CH}_{4}$ atmospheres, or a mixed secondary atmosphere (${\rm CO}+{\rm CO}_{2}+{\rm SO}_{2}$) could explain the data ($\Delta \ln Z< 3$). However, pure ${\rm CH}_{4}$ atmospheres may be physically unlikely, and the pure ${\rm H}_{2}{\rm O}$ and ${\rm CO}_{2}$ cases require a high-altitude cloud, which could also be interpreted as a thin cloud-free atmosphere. We discuss the theoretical possibility for different types of atmosphere on this planet, and consider the effects of atmospheric escape and stellar activity on the system. Though we find that TOI-1685 b is likely a bare rock, this study also highlights the challenges of detecting secondary atmospheres on rocky planets with JWST.