Phosphine as a Biosignature Gas in Exoplanet Atmospheres

Abstract: A long-term goal of exoplanet studies is the identification and detection of biosignature gases. Beyond the most discussed biosignature gas O$_2$, only a handful of gases have been considered in detail. Here we evaluate phosphine (PH$_3$). On Earth, PH$_3$ is associated with anaerobic ecosystems, and as such is a potential biosignature gas on anoxic exoplanets. We simulate CO$_2-$ and H$_2-$dominated habitable terrestrial planet atmospheres. We find that PH$_3$ can accumulate to detectable concentrations on planets with surface production fluxes of 10$^{10}$-10$^{14}$ cm$^{-2}$ s$^{-1}$ (corresponding to surface concentrations of 10s of ppb to 100s of ppm), depending on atmospheric composition and UV flux. While high, such surface fluxes are comparable to the global terrestrial production rate of CH$_4$ (10$^{11}$ cm$^{-2}$ s$^{-1}$) and below the maximum local terrestrial PH$_3$ production rate (10$^{14}$ cm$^{-2}$ s$^{-1}$). As with other gases, PH$_3$ can more readily accumulate on low-UV planets, e.g. planets orbiting quiet M-dwarfs or with a photochemical UV shield. If detected, PH$_3$ is a promising biosignature gas, as it has no known abiotic false positives on terrestrial planets that could generate the high fluxes required for detection. PH$_3$ also has 3 strong spectral features such that in any atmosphere scenario 1 of the 3 will be unique compared to other dominant spectroscopic molecules. PH$_3$'s weakness as a biosignature gas is its high reactivity, requiring high outgassing for detectability. We calculate that 10s of hours of JWST time are required for a potential detection of PH$_3$. Yet because PH$_3$ is spectrally active in the same wavelength regions as other atmospherically important molecules (e.g., H$_2$O and CH$_4$), searches for PH$_3$ can be carried out at no additional observational cost to searches for other molecules relevant to exoplanet habitability.

• The paper demonstrates that phosphine can accumulate to detectable levels when surface fluxes range from 10^9 to >10^12 cm⁻² s⁻¹.
• It employs numerical simulations showing that low UV environments and intrinsic atmospheric shields enable phosphine detection.
• The study highlights JWST and future telescopes as crucial tools for confirming phosphine’s unique spectral signatures indicative of anaerobic biospheres.

Phosphine as a Biosignature Gas in Exoplanet Atmospheres

The potential detection of biosignature gases on exoplanets has been an intriguing aspect of astrobiology and planetary science. Among the gases that serve as potential indicators of life, phosphine (PH$_3$) emerges as a viable candidate, chiefly due to its association with anaerobic biomes on Earth, as discussed in the paper titled "Phosphine as a Biosignature Gas in Exoplanet Atmospheres." This analysis provides an academic perspective on the findings and implications of the research, evaluating phosphine's prospects to accumulate to detectable levels in exoplanetary atmospheres and its utility in astrobiological investigations.

The study postulates that phosphine could accrue in detectable quantities in exoplanet atmospheres, provided these environments have low ultraviolet (UV) radiation and high phosphine production surface fluxes. The numerical simulations presented indicate that phosphine can reach observable concentrations, particularly on planets orbiting quiet M-dwarf stars or planets possessing an intrinsic UV shield. These models ascertain that phosphine, with surface fluxes ranging from 10$^9$ cm$^{-2}$s$^{-1}$ to above 10$^12$ cm$^{-2}$s$^{-1}$, can lead to atmospheric accumulation to levels significant enough to be detected using current or near-future astronomical instrumentation. The reactivity of phosphine with atmospheric radical species highlights the necessity of adequate production rates to overcome these destructive processes.

Phosphine's merits as a biosignature are underscored by its unique spectral features at 2.7–3.6 μm, 4.0–4.8 μm, and 7.8–11.5 μm. These regions offer distinct signatures that differ from other prevalent atmospheric compounds such as water vapor, methane, and ammonia. The absence of known significant abiotic sources for phosphine in terrestrial analog environments further bolsters its qualification as an indicator of life.

In terms of detectability, the paper rigorously discusses the prospects of observing phosphine using the James Webb Space Telescope (JWST) and other future observational platforms such as OST, LUVOIR, and 30-meter ground-based telescopes. While the JWST is crucial for evaluating the presence of phosphine in certain wavelength bands, the challenges posed by phosphine's chemical reactivity necessitate significant observation time to ascertain its presence with statistical confidence.

A notable aspect included in the study is the potential "runaway" effect, where phosphine, beginning to accumulate at specific surface flux thresholds, enters a self-sustaining phase significantly enhancing its atmospheric presence. Interestingly, the models indicate that reaching these tipping points might catalyze paramount transformations in atmospheric composition, potentially yielding detectable changes with reduced observational effort.

The theoretical implications are momentous, as detecting phosphine on an exoplanet could provide compelling evidence of anaerobic biospheres. The research contends with potential false positive scenarios comprehensively, evaluating abiotic processes capable of generating phosphine under simulated planetary conditions but finding them unable to replicate the requisite high flux levels observed biologically.

In summary, this paper offers a substantive evaluation of phosphine's candidacy as a biosignature gas under varied exoplanetary conditions, providing hypothetical pathways and practical considerations for its observation. These insights propose that phosphine, due to its unique reactivity, atmospheric chemistry, and spectral properties, could serve a pivotal role in future searches for extraterrestrial life, provided reconciliation with inherent observational challenges, atmospheric models, and surface flux considerations. As observational technologies evolve, phosphine's detection might offer novel insights into the biochemical diversity populating the universe.