We live in extraordinary times. Not a month goes by without a new terrestrial exoplanet getting its shiny entry in the NASA Exoplanet Database. And with each new addition, our enthusiasm for peering into their atmospheres, if they have one, in search of biosignatures grows—because who doesn’t want to find life on a distant rock?
Yet, contrary to the hype often portrayed in the media, numerous problems abound when looking for signs of life in exoplanets. For a start, characterizing the atmospheres of terrestrial exoplanets is science at its cutting edge. In 2020, scientists announced the detection of phosphine (PH₃) in Venus's atmosphere, sparking excitement as this gas is considered a potential biosignature. However, follow-up observations, including those by the Stratospheric Observatory for Infrared Astronomy (SOFIA), failed to confirm the presence of phosphine. And that's just analyzing the atmosphere of our closest planet. When the James Webb Space Telescope (JWST) attempts to detect biosignatures in the atmospheres of the Trappist and other exoplanets light years away, it is pushed to its limits, which can introduce even further uncertainties into the data and its interpretations. Can we truly conclude extraterrestrial life would be present on a distant world from just a single dataset? Additional observations using other instruments than the JWST would be necessary to verify any detection.
Still, even when we obtain a set of trustworthy data, interpreting it is far more challenging than one might think. This is primarily due to our knowledge being limited to a single class of life-form, specifically carbon-based organisms, which have developed on one planet, Earth. The lack of analogous instances results in interpretative biases from both scientific and philosophical viewpoints.
To exemplify this, we need to meet a prominent scientist at the Zarclan Academy of Sciences. In a past lecture, which I wholeheartedly invite you to view, the scientist provides four compelling reasons why Naknar3—also known as Earth—is not habitable and could not have intelligent life, given the current scientific knowledge held by the Zarclans. The four points raised to back this claim are the following:
Presence of high concentrations of oxygen
Presence of a moon
Land movements due to tectonic plates.
Low UV radiation
This outstanding and amusing lecture is actually delivered by Prof. Charles Cockell, Director of the UK Astrobiological Institute who, full disclaimer, generously assisted me during the writing of my first book. With the lecture, Charles encourages us to reconsider our beliefs about habitability and life and whether these beliefs are correct, serving as an excellent reminder of how narrow-minded our search for life might be. Maybe we are missing obvious biosignatures out there, things we haven't really considered. These would be false negatives.
Another challenge we encounter when interpreting a set of 'biosignatures' is our limited understanding of geochemical processes and geological pathways present in an alien world. These processes involve the transformation and movement of minerals, gases, and fluids within a specific environment. The vast range of extreme conditions on exoplanets (such as WASP-76b, where iron rains, HD 189733b, where glass rains, or Kepler-70b, the hottest known exoplanet with surface temperatures over 7,000°C) presents scientists with conditions about which we can only speculate. Grasping the complexities of hydrothermal mineral alterations on a planet completely enveloped in liquid water, or the geochemical cycling on a planetary body subjected to high temperatures, is a major challenge. There are many unknown unknowns in this field, which can lead to false positives. An example is the discovery of so-called 'Dark oxygen' last year. Found in the Clarion-Clipperton Zone of the Pacific Ocean, this phenomenon involves polymetallic nodules on the seafloor generating oxygen through electrochemical processes, independent of photosynthesis. Dark oxygen complicates the search for extraterrestrial life, as it suggests that oxygen in a planet's atmosphere might not always indicate biological activity, potentially leading to false positives in the hunt for biosignatures. In other words, we would now need to refine our detection methods to distinguish between biological and non-biological oxygen production. What about other biosignatures frequently considered as sure signs of life such as phosphine (PH₃), nitrous oxide (N₂O), ozone (O₃), or methane (CH₃). Could we soon uncover non-biological pathways that could generate these? Your guess is as good as mine.
Considering all these points, the idea that we can decode signs of life present in an atmosphere tens of light-years away might seem preposterous. This was the main reason I wrote this post in July two years ago, suggesting that searching for technosignatures might be more fruitful. Nevertheless, there is significant optimism. The emergence of artificial intelligence is enhancing our pattern detection abilities, revolutionizing the field, and leading to improvements in several key areas, from data analysis to signal processing, from predictive modeling to detection systems. No stone is left unturned.
The integration of AI into astrobiology is the way to go, bringing us forward to potentially finding an answer as to whether we are alone in the universe. Did I mention that we live in extraordinary times? Image: Tarantual Nebula by JWST. Credit NASA
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