Detectability of Chlorofluorocarbons in the Atmospheres of Habitable M-dwarf Planets
Jacob Haqq-Misra · Ravi Kopparapu · Thomas J. Fauchez · Adam Frank · Jason T. Wright · Manasvi Lingam
The Planetary Science Journal · 2022
Using ROCKE-3D climate simulations and the Planetary Spectrum Generator, this study finds that Earth-level CFC-11/CFC-12 on TRAPPIST-1e is marginally detectable (SNR ~3) with JWST MIRI LRS in ~100 in-transit hours under an optimistic 10 ppm noise floor, but remains undetectable under a conservative 50 ppm floor regardless of observation time.
Brief
Haqq-Misra et al. (2022) combine the ROCKE-3D GCM and PSG/GlobES synthetic spectrum pipeline to compute CFC-11 and CFC-12 transit spectra for synchronously rotating aquaplanets around TRAPPIST-1 and a 3300 K M-dwarf, at 1x, 2x, and 5x present-Earth CFC abundances (0.225 ppb and 0.515 ppb at 1x). With an optimistic JWST MIRI LRS noise floor of 10 ppm applied to co-added observations, present-day Earth-level CFCs on TRAPPIST-1e reach SNR ~3 in approximately 100 in-transit hours, while 2x concentrations reach SNR ~5 in the same time. However, raising the noise floor to a conservative 50 ppm eliminates any detectable signal even at 5x Earth levels, and reaching 100 in-transit hours on TRAPPIST-1e would require observing nearly every available transit across JWST's initial six-year mission window.
Metadata
- Category
- Search
- Venue
- The Planetary Science Journal
- Type
- Peer-reviewed
- Year
- 2022
- Authors
- Jacob Haqq-Misra, Ravi Kopparapu, Thomas J. Fauchez, Adam Frank, Jason T. Wright, Manasvi Lingam
- arXiv
- 2202.05858
- Access
- Open access
- Length
- 471.8 K
- Programs
- Breakthrough Listen, TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI)
- Instruments
- JWST MIRI LRS, Origins Space Telescope, ROCKE-3D GCM, Planetary Spectrum Generator (PSG/GlobES)
- Data sources
- HITRAN database, NOAA Global Monitoring Laboratory, BT-Settl stellar spectra
- Tags
- technosignature, atmospheric spectroscopy, exoplanet, SETI, M-dwarf, biosignature, industrial pollution
Key points
- CFC-11 (CCl3F) and CFC-12 (CCl2F2) show strongest absorption in the 8–14 μm mid-infrared window, where JWST MIRI LRS operates, but overlap with CO2, H2O, and O3 features complicates isolation.p.4
- Present-day Earth abundances used as the 1x baseline: 0.225 ppb CFC-11 and 0.515 ppb CFC-12; atmospheric lifetimes are ~55 years for CFC-11 and ~140 years for CFC-12 in the Earth-Sun system, potentially longer around UV-faint M-dwarfs.p.3
- With a 10 ppm JWST MIRI LRS noise floor, 1x Earth CFCs on TRAPPIST-1e reach SNR ~3 in ~100 in-transit hours; 2x Earth CFCs reach SNR ~5 in the same time.p.8
- With a 50 ppm noise floor applied to co-added observations, 5x Earth-level CFCs on TRAPPIST-1e remain undetectable at any observation time, and no meaningful upper limits can be derived.p.8
- TRAPPIST-1's near-Jupiter angular size produces deeper CFC transit features than a 3300 K star, making very late M-dwarfs better CFC detection targets than earlier ones.p.7
- Only ~100 transits of TRAPPIST-1e are observable in JWST's mission window (June 2022–July 2028), meaning nearly every available transit must be captured to accumulate ~100 in-transit hours.p.8
- For a planet around a 3300 K star at 5 pc, the Origins Space Telescope requires ~600 in-transit hours to achieve SNR ~5 for 2x and 5x CFC cases; the larger stellar disk suppresses feature depth.p.8
- CFC technosignature observations are fully simultaneous with biosignature gas searches, adding no observing cost; non-detections still yield upper limits on atmospheric CFC concentration.p.1
Verbatim
“we find that spectral features potentially attributable to present or historic Earth-level CFC features could be detected with a SNR ≥ 3 − 5 on TRAPPIST-1e, if present, in ∼ 100 hours of in-transit time.”
p.1“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.”
p.1“Observing CFCs in the atmosphere of an exoplanet would be compelling evidence of a technosignature.”
p.2“With a 10 ppm noise floor, concentrations at the current Earth level (1x, blue-dashed) can be detectable with SNR ∼ 3 in ∼ 100 hours.”
p.8“if the noise floor is set at 50 ppm, then even the 5x CFC features would not be detectable with a reliable SNR no matter the JWST MIRI-LRS observation time.”
p.8
Most interesting
- CFC-12 has a ~140-year stratospheric lifetime on Earth; around M-dwarfs, which emit far less UV at 185–210 nm (the photolysis window), this lifetime could be substantially longer, amplifying any accumulation signal.
- The 2x Earth CFC scenario modeled here, detectable at SNR ~5 in 100 hours, approximates what Earth's atmosphere might look like today had the 1987 Montreal Protocol failed to reduce industrial CFC output.
- TRAPPIST-1 is described as only slightly larger than Jupiter, meaning its planetary transit depth fraction is far more favorable for atmospheric spectroscopy than a more typical M-dwarf, directly boosting CFC feature amplitude.
- There are approximately 60 stars within 5 pc of the Solar system, most of them M-dwarfs with about half in binaries, and all are already on the Breakthrough Listen target list, meaning CFC searches could piggyback on existing SETI infrastructure.
- Lin et al. (2014) previously estimated that CFCs at 10x Earth levels around a white dwarf host could be detected by JWST in only ~1.7 days; the far less favorable transit geometry of M-dwarf systems requires orders of magnitude more integration time for comparable concentrations.
- The study's ROCKE-3D simulations ran for 9,800 model orbits to reach steady state for the TRAPPIST-1e case, driven by the short 6.1-day orbital period and the model's slow convergence for synchronously rotating planets.