Detectability of biosignatures on LHS 1140 b (2012.11426v1)

Abstract: Terrestrial extrasolar planets around low-mass stars are prime targets when searching for atmospheric biosignatures with current and near-future telescopes. The habitable-zone Super-Earth LHS 1140 b could hold a hydrogen-dominated atmosphere and is an excellent candidate for detecting atmospheric features. In this study, we investigate how the instellation and planetary parameters influence the atmospheric climate, chemistry, and spectral appearance of LHS 1140 b. We study the detectability of selected molecules, in particular potential biosignatures, with the upcoming James Webb Space Telescope (JWST) and Extremely Large Telescope (ELT). In a first step we use the coupled climate-chemistry model, 1D-TERRA, to simulate a range of assumed atmospheric chemical compositions dominated by H$_2$ and CO$_2$. Further, we vary the concentrations of CH$_4$ by several orders of magnitude. In a second step we calculate transmission spectra of the simulated atmospheres and compare them to recent transit observations. Finally, we determine the observation time required to detect spectral bands with low resolution spectroscopy using JWST and the cross-correlation technique using ELT. In H$_2$-dominated and CH$_4$-rich atmospheres O$_2$ has strong chemical sinks, leading to low concentrations of O$_2$ and O$_3$. The potential biosignatures NH$_3$, PH$_3$, CH$_3$Cl and N$_2$O are less sensitive to the concentration of H$_2$, CO$_2$ and CH$_4$ in the atmosphere. In the simulated H$_2$-dominated atmosphere the detection of these gases might be feasible within 20 to 100 observation hours with ELT or JWST, when assuming weak extinction by hazes. If further observations of LHS 1140 b suggest a thin, clear, hydrogen-dominated atmosphere, the planet would be one of the best known targets to detect biosignature gases in the atmosphere of a habitable-zone rocky exoplanet with upcoming telescopes.

• The paper demonstrates that H2-dominated, CH4-rich atmospheres significantly reduce O2 and O3 levels due to strong chemical sinks.
• It employs the 1D-TERRA model and transmission spectroscopy to identify key biosignature gases such as NH3, PH3, CH3Cl, and N2O.
• The study finds that clear, low-haze conditions enable biosignature detection within 20–100 hours, though spectral overlaps complicate analysis.

Detectability of Biosignatures on LHS 1140 b

The paper presents an in-depth examination of the detectability of atmospheric biosignatures on the exoplanet LHS 1140 b using the upcoming James Webb Space Telescope (JWST) and the Extremely Large Telescope (ELT). The paper explores various atmospheric conditions under which potential biosignatures such as NH3, PH3, CH3Cl, and N2O might be detected, focusing extensively on the role that the primary atmospheric gases (H2 and CO2) play in the overall atmospheric chemistry and climate.

Atmospheric Modeling

The authors employ the 1D climate-photochemistry model, 1D-TERRA, to simulate potential atmospheric compositions of LHS 1140 b. Their simulations span a range of scenarios, dominated by molecular hydrogen (H2) and carbon dioxide (CO2), each interlaced with significant variations in methane (CH4). Critical findings show that in H2-dominated and CH4-rich atmospheres, oxygen (O2) encounters strong chemical sinks, reducing its concentration significantly, which diminishes the levels of ozone (O3).

Transmission Spectroscopy

Transmission spectra were calculated for the modeled atmospheres to determine observable spectral features using JWST and ELT. The paper identifies that potential biosignatures such as ammonia (NH3), phosphine (PH3), chloromethane (CH3Cl), and nitrous oxide (N2O) could be detectable within a reasonable observation window given optimal atmospheric conditions. Key spectral bands for these gases were identified, albeit noting that many of these features overlap with those of other prominent gases like CH4 and CO2, complicating unambiguous detection.

Detectability and Observational Considerations

The analysis suggests that, under clear sky conditions with weak haze extinction, potential biosignature gases in H2-dominated atmospheres could be discernible within 20 to 100 hours of observation time using the capabilities of JWST or ELT. However, the presence of significant atmospheric hazes could substantially hinder detection. The paper underscores that while the identification of certain biosignatures may be feasible, one must also contend with the challenges posed by overlapping spectral features and potential false positives.

Implications and Future Prospects

This investigation provides critical insights for the characterization of habitable-zone rocky exoplanets. If LHS 1140 b possesses a clear, thin, hydrogen-dominated atmosphere, it might rank among the top candidates for detecting biosignature gases using forthcoming observational facilities. From a broader perspective, the detailed consideration of chemical interactions and photolytic reactions highlights the intricate balance required in atmospheric compositions that could foster potential biosignatures, further informing theoretical models of atmospheric evolution and stability.

In recognizing both the capabilities and the challenges associated with current and next-generation telescopic instrumentation, this paper lays a solid groundwork for subsequent observational strategies aiming to distinguish between truly biosignature-indicating conditions and those that might merely mimic biological activity through abiotic processes. This research will undoubtedly influence future mission designs and observation prioritizations in the pursuit of extraterrestrial life detection.