2025ApJ...994..241S

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 climate-photochemistry 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 atmospheres provides the main UV shielding, whereas under intensified UV, ozone production in the modern Earth-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.