Detectability of Chlorofluorocarbons in the Atmospheres of Habitable M-dwarf Planets

Abstract: The presence of chlorofluorocarbons (CFCs) in Earth's atmosphere is a direct result of technology. Ozone-depleting CFCs have been banned by most countries, but some CFCs have persistent in elevated concentrations due to their long stratospheric lifetimes. CFCs are effective greenhouse gases and could serve as a remotely detectable spectral signature of technology. Here we use a three-dimensional climate model and a synthetic spectrum generator to assess the detectability of CFC-11 and CFC-12 as a technosignature on exoplanets. We consider the case of TRAPPIST-1e as well as a habitable Earth-like planet around a 3300 K M-dwarf star, with CFC abundances ranging from one to five times present-day levels. Assuming an optimistic James Webb Space Telescope (JWST) Mid Infrared Instrument (MIRI) low resolution spectrometer (LRS) noise floor level of 10 ppm to multiple co-added observations, we find that spectral features potentially attributable to present or historic Earth-level CFC features could be detected with a SNR $\ge 3-5$ on TRAPPIST-1e, if present, in $\sim 100$ hours of in-transit time. However, applying a very conservative 50 ppm noise floor to co-added observations, even a 5x Earth-level CFC would not be detectable no matter the observation time. Such observations could be carried out simultaneously and at no additional cost with searches for biosignature gases. Non-detection would place upper limits on the CFC concentration. We find that with the launch of JWST, humanity may be approaching the cusp of being able to detect passive atmospheric technosignatures equal in strength to its own around the nearest stars.

• The paper demonstrates that current CFC levels on TRAPPIST-1e are marginally detectable with about 100 hours of JWST transit observations under optimistic noise conditions.
• It employs 3D climate modeling and synthetic spectrum generation to evaluate how doubled or quintupled CFC levels enhance signal-to-noise ratios.
• Findings emphasize that detection feasibility critically depends on noise floor assumptions and future advances in observational instrumentation.

Detectability of Chlorofluorocarbons in the Atmospheres of Habitable M-Dwarf Planets

The detection of extraterrestrial technosignatures, particularly industrial pollutants such as chlorofluorocarbons (CFCs), represents an intriguing avenue for the search for technologically advanced civilizations. In their study, Haqq-Misra et al. explore the prospects of identifying CFCs in the atmospheres of exoplanets, focusing specifically on planets orbiting M-dwarf stars, like TRAPPIST-1e. These efforts align with the broader scientific goal of expanding the search for extraterrestrial intelligence beyond traditional radio signals.

Methodology

The study utilizes a combination of 3D climate modeling and synthetic spectrum generation to estimate the detectability of CFC-11 and CFC-12 as potential technosignatures. M-dwarfs, which are smaller and cooler than solar-type stars, provide advantageous targets due to their prevalence and relatively favorable profile for transit spectroscopy. Two primary scenarios are examined:

1. TRAPPIST-1e: Utilizing updated planetary and stellar parameters, simulations were conducted to determine the spectral features of CFCs assuming various concentrations relative to Earth's. The simulations aimed to assess the feasibility of detecting these features using the James Webb Space Telescope (JWST).
2. An Earth-like Planet around a 3300 K M-dwarf: This scenario estimated the detection capabilities using the Origins Space Telescope concept, focusing on the habitable zone of a typical M-dwarf at a relatively close distance.

Results

The analysis reveals several critical insights:

• At current Earth's CFC levels, with an optimistic noise floor assumption of 10 ppm in JWST's Mid Infrared Instrument (MIRI), spectral features are marginally detectable on TRAPPIST-1e with approximately 100 hours of in-transit observation achieving a signal-to-noise ratio (SNR) of around 3. Doubling or quintupling the Earth's CFC levels notably improves the SNR.
• However, adopting a more conservative noise floor of 50 ppm negates the detectability of these features, even with increased levels of CFCs and extended observation times.

For planets orbiting 3300 K M-dwarfs, results similarly show sensitivity to noise characteristics but suggest that, over feasible observing campaigns, elevated CFC levels could achieve detectable SNRs with future mission concepts like Origins.

Implications and Future Directions

The findings underscore the conditional potential for CFC detecting as technosignatures with current and conceptual astronomical instruments. They point to atmospheric constituents as a promising, albeit challenging, complement to traditional SETI approaches. Yet, the ability to distinguish these features from overlapping spectral lines of natural molecules remains a substantial challenge.

Future work will need to refine strategies for untangling these ambiguities, possibly utilizing high-dispersion spectroscopy or new instrumentation capabilities. Moreover, the question of abiotic processes that might mimic or obscure technosignatures is critical in contextual analysis. In parallel, increasing the catalog of potential industrial pollutants with unique spectral signatures could enhance search strategies.

This research illustrates that as we advance in our observational capabilities, the detectability of earthly technosignatures across interstellar distances is approaching feasibility. It encourages an interdisciplinary approach integrating atmospheric science, climatology, and instrumentation development to better understand and identify promising targets in the search for extraterrestrial technology.