The Alien Pollution That Could Prove We're Not Alone
Dr. Sara Seager on Fluorinated Gases as Technosignatures for IASU404
Two industrial gases, like the ones accumulating in Earth’s atmosphere and driving climate change right now, may also be among the most compelling signals we could ever receive from a civilization beyond our solar system.
The question is whether we can build the telescopes and methods capable of finding them, and that’s part of what Dr. Sara Seager presented during IAUS404: Advancing the Search for Technosignatures.
About the Presenter
Sara Seager was previously a professor at the Massachusetts Institute of Technology, and has recently accepted a new position at the University of Toronto (U of T), to join the Canadian Institute for Theoretical Astrophysics (CITA) as North Star Distinguished Professor.
She earned her B.Sc. in mathematics and physics from the University of Toronto and her Ph.D. in astronomy from Harvard University in 1999. Since originally joining MIT in 2007, she has become one of the most decorated scientists in exoplanet research, earning a MacArthur genius grant in 2013, election to the U.S. National Academy of Sciences, appointment as an Officer of the Order of Canada, and most recently the 2024 Kavli Prize in Astrophysics.
Seager’s work spans theoretical models of exoplanet atmospheres, the search for biosignature gases, and novel space mission design. She has spent decades building tools to ask: what chemical fingerprints might betray the presence of life — or technology — on a distant world?
Her talk at IAUS 404 approached that question from an intriguing direction: the chemistry of what most life doesn’t make.
More About Dr. Seager’s Work in Advancing the Search for Technosignatures
Seager opened by recounting a story that frames her entire research agenda. Years ago, at a Vatican symposium on the search for signs of life, she expressed doubt that any biosignature gas could ever be truly unambiguous—there are too many environmental unknowns, too many potential false positives. Jill Tarter, the pioneering SETI researcher, cut her off with two words: “We’ll know.”
Tarter meant a radio signal from a technological civilization would be unmistakable. But Seager’s talk explored the middle ground: technosignature gases that, while not as definitive as a radio broadcast or seeing an alien craft approaching Earth, carry a far stronger case than most biosignatures.
The foundation for this work is a two-decade effort to systematically catalog the chemistry of life.
Seager and collaborators, notably Janusz Petkowski, built an exhaustive database of roughly 200,000 compounds produced by organisms on Earth, mapping what life uses and, crucially, what it avoids.
One striking finding: despite nitrogen and sulfur each appearing in thousands of natural molecules, life almost entirely refuses to bond them to each other. Only about 100 known compounds in all of Earth’s biochemistry contain nitrogen-sulfur bonds, and those are mainly toxins. A 2024 companion paper by Petkowski, Seager, and Bains offers a similar quantified analysis for fluorine, finding that life nearly excludes it from its molecular toolkit.
This biochemical “no-fly zone” around fluorine is the engine of the technosignature argument. Earth life avoids producing or using any N–F or S–F bond-containing molecules and makes no fully fluorinated molecules with any element. Against that backdrop, nitrogen trifluoride (NF₃) and sulfur hexafluoride (SF₆), both of which are produced in large industrial quantities on Earth, stand out as nearly ideal technosignature candidates. The 2023 paper in Scientific Reports by Seager, Petkowski, and colleagues lays out five reasons:
First, fluorine is itself deeply excluded from biological chemistry.
Second, life produces no molecules in which fluorine bonds to anything other than carbon.
Third, both gases have virtually no natural abiotic sources.
Fourth, their extremely low water solubility means they don’t dissolve into rainfall and wash out of an atmosphere.
Fifth, both gases absorb strongly in the 9–12 micron spectral window, a region where major atmospheric gases like CO₂ and nitrogen are relatively quiet, giving them a uniquely prominent spectral fingerprint.
On detection:
Seager was candid that current telescopes aren’t up to the task. Even the James Webb Space Telescope would need tens of transits and concentrations of these gases many orders of magnitude above today’s Earth levels in order to pick up a signal.
A civilization deliberately pumping NF₃ or SF₆ to terraform a cooling planet might reach detectable levels; accidental industrial leakage, as on Earth, almost certainly would not.
Seager highlighted a genuinely futuristic prospect: a solar gravitational lens telescope that would travel to 500 astronomical units and use the Sun itself as an optical element, potentially achieving enough spectral resolution to disentangle a crowded forest of trace atmospheric gases.
She also acknowledged the important caveat: life on a hydrogen-depleted world, lacking easy access to hydrogen, might plausibly use fluorine as a chemical cap in its molecular structures, meaning SF₆ or NF₃ could, in exotic circumstances, emerge from biochemistry rather than technology.
Context will always matter.
Key Takeaways
Life’s chemical avoidances may be universal. Fluorine’s extreme electronegativity makes it energetically costly for biochemistry to exploit — a constraint that may apply to life anywhere, not just on Earth.
NF₃ and SF₆ are “exemplary” but not perfect technosignatures. They have minimal false positives and strong spectral features, but edge cases (waterless planets, hydrogen-depleted chemistries) leave some ambiguity.
Detection requires next-generation instrumentation. JWST cannot identify these gases at realistic concentrations; a solar gravitational lens telescope or future large-aperture missions may be necessary.
Technosignature gases could indicate deliberate planetary engineering. Detectable concentrations might point not just to industrial activity but to civilizations actively terraforming a neighbor world.
Chemical space mapping is still incomplete. Thousands of molecules with potential spectral relevance lack adequate data — a significant gap the community needs to close.
The search for technosignatures has long been anchored to radio astronomy, but the past decade has seen growing interest in atmospheric chemistry as a complementary strategy — one that could work passively, without a civilization ever intending to broadcast its presence.
Seager’s fluorinated gas framework adds rigorous biochemical reasoning to what has often been an intuitive field, grounding candidate molecules in a systematic understanding of life’s chemistry.
As Blue Marble Space and its partners continue expanding the scientific case for technosignature searches, the challenge is now as much instrumental as theoretical: building the telescopes that could actually see these faint molecular whispers across interstellar distances — and knowing how to interpret them when we do.
This video was included in the proceedings of the International Astronomical Union (ISU) symposium #404, Advancing the Search for Technosignatures, hosted by Blue Marble Space. Stay tuned as we explore the boundaries of knowledge in technosignature science!
Thanks for joining us here with Planetary Perspectives, an editorial endeavor and science communication project from Blue Marble Space.
Incredible insight. When we hear the word technosignature we assume radio signals, high frequency signals, etc. But Dr Seager points out that there could be more to technosignatures.