JWST NIRSpec finds no clear signs of an atmosphere on TOI-1685 b

TL;DR

This study interrogates the atmosphere of the hot rocky planet TOI-1685 b with five JWST/NIRSpec transits (four new transits plus a transit from a phase-curve program) analyzed independently by three pipelines. Across reductions, the transmission spectrum is consistent with a flat line, with only weak, nonrobust hints of slope or a negative Gaussian likely related to systematics or stellar activity. Atmospheric modelling shows that hydrogen-dominated atmospheres are confidently ruled out, while heavier, secondary atmospheres could be present only under high-altitude clouds or thin envelopes; methane-dominated atmospheres are disfavored on physical grounds. A refined planetary radius of about 1.429 R⊕ places TOI-1685 b in a rocky regime, and, together with the lack of clear atmospheric signatures, supports a bare-rock interpretation, though detecting secondary atmospheres remains challenging for JWST with current observational strategies. The work underscores both the potential and limits of JWST in probing atmospheres on rocky planets and highlights the need for broader wavelength coverage and multiple epochs to robustly identify subtle atmospheric signatures around M-dwarf hosts.

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 JWST's 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 ~10, at a significance of ~5 sigma. Pure CO2, SO2, H2O, and CH4 atmospheres, or a mixed secondary atmosphere (CO+CO2+SO2) could explain the data (Delta lnZ < 3). However, pure CH4 atmospheres may be physically unlikely, and the pure H2O and CO2 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.

Figures (14)

• Figure 1: Density versus radius values for super-Earths observed with JWST, coloured by their equilibrium temperature. Filled circles represent planets with hints of an atmosphere, whilst empty circles represent flat transmission spectra, or secondary eclipse measurements indicating bare rocks. The squares show the measurements for TOI-1685 b from various papers.
• Figure 2: The light curves from the NIRSpec G395H observations of TOI-1685, with the five visits shown chronologically from left to right. Top panel: raw, undetrended light curves at wavelength resolution of 6.5 nm, which was used in our adopted fit. NRS1 is shown in blue colours and NRS2 is shown in red colours, with the darkening in the centre of each panel corresponding to the flux drop during the transit of TOI-1685 b. Middle panel: Raw, undetrended NRS1 and NRS2 white light curves for each visit. Unbinned data are shown in colour, with data in bins of 9 minutes shown in empty black markers, with error bars generally too small to be visible. Bottom panel: Light curves in 6 wavelength bins across each detector in the same structure as the middle panel, with the detectors separated by a dashed line.
• Figure 3: Comparison between the derived MPS-ATLAS-1 values of $u_\mathrm{1}$ and $u_\mathrm{2}$ and those sampled in the combined fit. The values were sampled under a Gaussian prior with a mean of the corresponding MPS-ATLAS-1 value and a standard deviation of 0.1
• Figure 4: Comparison of the Eureka! and transitspectroscopy reductions for our four transits. The top panels show the transit depths and their error bars. The bottom panels show the difference between the spectra, measured in $\sigma$ values relative to the Eureka! error bars.
• Figure 5: Comparison of the Eureka! and transitspectroscopy reductions of the transit from the phase curve program (GO 3263) and the Eureka! reduction of this transit from luque25. The top panel shows the transit depths and their error bars. The bottom panel shows the difference between the spectra, with the circles and diamonds indicating the difference between the Eureka! spectrum and the transitspectroscopy and luque25 spectra, respectively. The differences are measured in $\sigma$ values relative to the Eureka! error bars.