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      Artificial Greenhouse Gases as Exoplanet Technosignatures

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          Abstract

          Atmospheric pollutants such as CFCs and NO\(_{2}\) have been proposed as potential remotely detectable atmospheric technosignature gases. Here we investigate the potential for artificial greenhouse gases including CF\(_{4}\), C\(_{2}\)F\(_{6}\), C\(_{3}\)F\(_{8}\), SF\(_{6}\), and NF\(_{3}\) to generate detectable atmospheric signatures. In contrast to passive incidental byproducts of industrial processes, artificial greenhouse gases would represent an intentional effort to change the climate of a planet with long-lived, low toxicity gases and would possess low false positive potential. An extraterrestrial civilization may be motivated to undertake such an effort to arrest a predicted snowball state on their home world or to terraform an otherwise uninhabitable terrestrial planet within their system. Because artificial greenhouse gases strongly absorb in the thermal mid-infrared window of temperate atmospheres, a terraformed planet will logically possess strong absorption features from these gases at mid-IR wavelengths (\(\sim\)8-12 \(\mu\)m), possibly accompanied by diagnostic features in the near-IR. As a proof of concept, we calculate the needed observation time to detect 1 [10](100) ppm of C\(_{2}\)F\(_{6}\)/C\(_{3}\)F\(_{8}\)/SF\(_{6}\) on TRAPPIST-1f with JWST MIRI/LRS and NIRSpec. We find that a combination of 1[10](100) ppm each of C\(_{2}\)F\(_{6}\), C\(_{3}\)F\(_{8}\), and SF\(_{6}\) can be detected with an S/N \(\geq\) 5 in as few as 25[10](5) transits with MIRI/LRS. We further explore mid-infrared direct-imaging scenarios with the LIFE mission concept and find these gases are more detectable than standard biosignatures at these concentrations. Consequently, artificial greenhouse gases can be readily detected (or excluded) during normal planetary characterization observations with no additional overhead.

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          Author and article information

          Journal
          17 May 2024
          Article
          2405.11149
          5b57fb9a-7b0c-4336-a76d-b5c968d1201a

          http://creativecommons.org/licenses/by/4.0/

          History
          Custom metadata
          25 pages, 8 figures, 6 tables, accepted in the Astrophysical Journal
          astro-ph.EP

          Planetary astrophysics
          Planetary astrophysics

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