Evaluating Habitability and Biosignature Detection on TOI-700 d: The Role of UV Environment and Atmospheric Pressure

TL;DR

This study investigates the habitability and biosignature detectability of the Earth-sized TOI-700 d orbiting an M dwarf by coupling a 1D photochemical model with a climate model across Archean and modern Earth-like atmospheres. It explores two UV regimes, $f_\text{UV}=1$ and $f_\text{UV}=10$, and four surface pressures (0.5–4 bar), revealing that surface temperatures remain compatible with liquid water in all scenarios. The dominant UV-shielding mechanism transitions from methane-derived hazes under low UV to ozone formation under high UV, with haze presence also amplifying the atmospheric signal in transmission spectra. However, hazes and UV-driven chemistry complicate biosignature detection, suggesting JWST-era observations would require extensive campaigns; the results highlight the intricate interplay between photochemistry, atmospheric pressure, hazes, and UV flux in shaping habitability and spectroscopic observability for exoplanets around M dwarfs.

Abstract

M dwarfs have long been prime targets in the search for habitable exoplanets, owing to their abundance in the galaxy and the relative ease of detecting Earth-sized worlds within their narrower habitable zones. Yet, these low-mass stars can emit high-energy radiation that may gradually erode planetary atmospheres, raising concerns about long-term habitability. TOI-700, a relatively quiescent M dwarf that hosts four known planets, stands out due to its Earth-sized TOI-700 d in the star's habitable zone. Here, we assess whether a habitable environment can be sustained on TOI-700 d by analyzing different UV flux levels and atmospheric pressures. We focus on two atmospheric scenarios - one analogous to the Archean Earth and another representing a modern Earth-like environment - using a 1D photochemistry-climate model. Our results indicate that all simulated cases can maintain temperatures compatible with liquid water on the surface. However, the dominant photochemical pathways differ substantially with UV levels: under low-UV conditions, haze formation in the Archean-like atmosphere provides the main UV shielding, whereas under intensified UV, ozone production in the modern-like atmospheres can protect the surface from harmful doses. Interestingly, although haze can impede the detection of certain biosignatures, such as CH4, CO2 and O2, it also enhances the overall atmospheric signal by increasing scattering and transit depth, potentially aiding in revealing the presence of an atmosphere. These findings underscore the dual role of hazes as both a challenge for biosignature detection and a potential protection of surface habitability.

Figures (8)

• Figure 1: Each panel, from the upper left to the lower right, represents the atmospheric pressure of O, O$_2$, O$_3$, CO, CO$_2$, H$_2$O, CH$_4$, SO$_2$ and SO$_3$ as a function of mixing ratios. The initial conditions and the produced surface mixing ratios can be seen either in \ref{['tab:inital_produced_mix_ratios']} or in the files available at DOI: https://doi.org/10.5281/zenodo.13947863. The Archean atmospheres with $f_\mathrm{UV}$=1 and $f_\mathrm{UV}$=10 are depicted in purple and red, while modern Earth-like atmospheres with $f_\mathrm{UV}$=1 and $f_\mathrm{UV}$=10 are shown in green and blue, respectively. The different surface pressures, i.e., 4.0, 2.0, 1.0, and 0.5 bar, are represented by tones gradually shifting towards lighter shades, indicating a transition from higher to lower pressure.
• Figure 2: Atmospheric pressure profiles plotted against temperature for different pressures. The Archean atmospheres with $f_\mathrm{UV}$=1 and $f_\mathrm{UV}$=10 are depicted in purple and red, while modern Earth-like atmospheres with $f_\mathrm{UV}$=1 and $f_\mathrm{UV}$=10 are shown in green and blue, respectively. The Modern Earth atmosphere models of $f_\mathrm{UV}$=1 and $f_\mathrm{UV}$=10 can only be distinguished from one another in the case where the surface pressure ($P_0$) equals 1 (top right panel), otherwise for all other pressures, the curve for $f_\mathrm{UV}$=10 (blue) superimposes on the $f_\mathrm{UV}$=1 (green) curve.
• Figure 3: Upper panel: UV flux reaching the surface of the planet TOI-700 d for an Archean atmosphere, depicted in purple ($f_\mathrm{UV}$=1) and red ($f_\mathrm{UV}$=10), and a modern Earth-like atmosphere, in green ($f_\mathrm{UV}$=1) and blue ($f_\mathrm{UV}$=10). The different surface pressures, namely 4.0, 2.0, 1.0, and 0.5, are represented by tones gradually shifting towards lighter shades, indicating a transition from higher to lower pressure. The solar flux reaching both Earth's exosphere and surface is depicted by solid black line and dashed black line, respectively, primarily for the purpose of comparison. Lower panel: a blowup of the flux scale depicted on the upper panel.
• Figure 4: Atmospheric optical depth as a function of wavelength for $f_\mathrm{UV}$=1 (purple curves) and $f_\mathrm{UV}$=10 (red curves) Archean atmospheres. Different surface pressures, namely 4.0, 2.0, 1.0, and 0.5, are represented by tones gradually shifting towards lighter shades, indicating a transition from higher to lower pressure. Under 10 times stellar UV, the aerosol column thins (lower $\tau$), consistent with non-monotonic haze production in CO$_2$-rich anoxic atmospheres where short-wave UV also enhances oxidative destruction of intermediates.
• Figure 5: Action spectra, or biological response, for D. radiodurans (left) and E. coli (right).