Biosignatures: Presence of Life in the Universe
"Pale Blue Dot" is a photograph of Earth taken on February 14, 1990, by Voyager 1 from the NASA (new window) at a distance of 3.7 billion miles (6 billion kilometers) from the Sun. The almost invisible Earth is the pale blue dot in the first third of the pale vertical streak. The image inspired the title of scientist Carl Sagan's book, "Pale Blue Dot: A Vision of the Human Future in Space," in which he writes: "Look again at that dot. That's here. That's home. That's us."
Scientific summary
Astronoo's article explores the notion of a biosignature, a chemical or physical clue that may indicate past or present biological activity. It distinguishes between direct biosignatures, such as complex organic molecules, and indirect biosignatures, such as atmospheric gases (oxygen, methane, phosphine) detected by spectroscopy. The James Webb Space Telescope plays a key role in observing exoplanet atmospheres, but its capabilities are limited. The article highlights the difficulty of interpreting these signals, which are often ambiguous, and emphasizes the need for solid evidence before claiming the discovery of extraterrestrial life.
How can scientists detect life in the Universe using biosignatures?
The search for extraterrestrial life relies on identifying biosignatures, chemical or physical signals that could indicate the presence of living organisms. Scientists use space telescopes like the James Webb Space Telescope (JWST) to analyze light from exoplanets and infer the composition of their atmospheres. Among indirect biosignatures, oxygen, ozone, methane, and phosphine are particularly scrutinized because they can be produced by biological processes. However, these gases can also have non-biological origins (volcanism, atmospheric chemistry), making their interpretation delicate. Direct biosignatures, such as nucleic acids or proteins, would be more convincing, but their detection is extremely difficult due to the faintness of exoplanets. The JWST, while powerful, can only observe atmospheres, not surfaces. Astrobiology is advancing through combined approaches: spectroscopy, photometry, and modeling. Future missions, such as the PLATO space telescope or the Extremely Large Telescope, could offer new opportunities. In the meantime, scientists remain cautious, because as Carl Sagan reminded us, extraordinary claims require extraordinary evidence. This Astronoo article reviews these methods and the challenges to be met before one day announcing the discovery of life beyond Earth.
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Signal of life
What is a biosignature?
A biosignature is a signal that suggests the presence of life or a particular chemistry formed by living organisms. We have not yet identified any extraterrestrial biosignatures.
The James Webb Space Telescope and the detection of biosignatures
The search for biosignatures to detect life in the Universe is a fascinating field of astrobiology.
The James Webb Space Telescope (JWST), launched on December 25, 2021, has the potential to detect biosignatures. It is equipped with four scientific instruments that allow it to observe exoplanets and their atmospheres in the infrared. This wavelength range is particularly interesting for the search for biosignatures because it allows the detection of complex organic molecules that could be produced by living organisms. The observation techniques used are spectroscopy, photometry, polarimetry, all based on the analysis of light emitted or reflected by planets.
The James Webb Space Telescope is not capable of detecting traces of life on the surface of exoplanets. To detect traces of life on the surface of exoplanets, it would be necessary to observe the planet directly, without being hindered by the atmosphere. This would require a much more powerful telescope than the JWST. It is only able to observe their atmosphere.
However, the James Webb Space Telescope is capable of observing both direct and indirect biosignatures in the atmosphere of exoplanets.
Direct biosignatures
Definition and detection challenges
Direct biosignatures concern the presence of complex organic molecules, such as nucleic acids, proteins, and lipids, biomolecules, or living cells.
Direct biosignatures are difficult to detect because of the faintness of exoplanets, which are much smaller than stars. The light from an exoplanet is mixed with the light from its host star. This makes it difficult to identify the specific spectral signatures of biomolecules.
Moreover, exoplanet atmospheres can be complex, and some biosignatures can be confused with non-biological processes. For example, the presence of oxygen in an atmosphere can be related to geochemical processes rather than biological processes. Finally, it is very difficult to predict which biosignatures we should be looking for.
Although difficult to detect, direct biosignatures would constitute irrefutable proof of life. Because of these challenges, scientists also rely on indirect techniques, such as spectral analysis of exoplanet atmospheres.
Indirect biosignatures
Atmospheric gases as indicators of life
Indirect biosignatures concern the presence of atmospheric gases, such as oxygen, ozone, methane, phosphine, sulfur dioxide, carbon dioxide, or water vapor. These gases can be produced by living organisms. They are easier to detect but also more ambiguous. However, these traces can provide valuable clues about the presence of life, even if they do not constitute conclusive proof.
- Oxygen (O2) in Earth's atmosphere is mainly produced by photosynthesis in plants. The detection of a large amount of oxygen in a planet's atmosphere could suggest the presence of photosynthetic life.
- Ozone (O3) is produced by the interaction of oxygen with ultraviolet light and may suggest active biological processes.
- Methane (CH4) is produced by certain types of bacteria. The detection of significant levels of methane may indicate ongoing biological processes.
- Phosphine (PH3) is an inorganic chemical compound, meaning it does not contain carbon, but it is naturally produced by the decomposition of organic matter, such as decaying animal or plant matter.
- Sulfur dioxide (SO2) can be emitted by some forms of life but also by volcanoes like on Earth.
- Carbon dioxide (CO2) is a common byproduct of respiration and decomposition of living organisms. A high concentration of CO2 in an exoplanet's atmosphere could indicate the presence of life, however volcanic activity also produces CO2.
- Water vapor (H2O) in the atmosphere is also an important factor, as water is essential for life as we know it.
What to remember
Caution in interpretation
While these gases can be indirect biosignatures, their presence alone is not sufficient as definitive proof. Ultimately, the search for biosignatures requires extraordinary evidence to support extraordinary claims, as Carl Sagan (1934-1996), American scientist and astronomer, said.
References
FAQ: Everything you need to know about biosignatures and the search for extraterrestrial life
What is a biosignature?
A biosignature is a signal, whether chemical, physical, or spectral, that may indicate the presence of life, past or present. It can be complex organic molecules, atmospheric gases (oxygen, methane), or variations in light. The identification of an extraterrestrial biosignature would be a major step in astrobiology, but it requires ruling out any non-biological origin.
Why is oxygen considered a biosignature?
Oxygen is produced in large quantities by photosynthesis in plants and cyanobacteria on Earth. Its presence in a planetary atmosphere is therefore a potential indicator of photosynthetic life. However, oxygen can also be produced by abiotic processes, such as photolysis of water or volcanic activity, hence the need to combine it with other clues.
Is methane always a sign of life?
No, methane can have geological origins (volcanism, serpentinization) or biological origins (methanogenic bacteria). To distinguish between these sources, scientists analyze isotopic ratios and the presence of other associated gases. An atmosphere rich in methane and oxygen would be a particularly intriguing signal, as these two gases react with each other and require continuous production.
What are the limitations of JWST for detecting biosignatures?
The James Webb Space Telescope (JWST) is designed to observe exoplanet atmospheres using transit spectroscopy. It cannot directly resolve the surface of planets or detect complex organic molecules in small quantities. Furthermore, it is limited to planets that pass in front of their star from our line of sight. Finally, the interpretation of spectra remains delicate due to possible false positives.
Are there biosignatures other than gases?
Yes, we distinguish between direct biosignatures (nucleic acids, proteins, lipids) and other indicators such as the polarization of reflected light, surface color (for example, the "redness" of vegetation), or seasonal cycles of gases like CO2. Future telescopes, such as the Extremely Large Telescope, could attempt to directly image exoplanets to look for these signals.
Why is it so difficult to confirm extraterrestrial life?
The major difficulty lies in the ambiguity of signals: a biosignature can have abiotic explanations. Moreover, exoplanets are very distant and their light is drowned out by that of their star. The scientific community requires an extremely high level of proof, as Carl Sagan reminded us, to avoid false discoveries. A definitive confirmation would likely require several independent observations and the exclusion of all alternative hypotheses.
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