Decoding Biosignatures: A Key to Unlocking the Mystery of Life Beyond Earth

In this article we discuss the ways through which we can find signs of life on distant planets. We explore the significance of biosignatures and how studying our own planet aids in identifying similar signs on distant celestial bodies like exoplanets. We find ways to search for extraterrestrial life, spotlighting biosignatures—key evidence that could signify life on other planets. Following Meadows' recommendations, we classify biosignatures into gaseous, surface, and temporal types, examining each with observations and examples. Briefly, we look into the prevalent method of detecting these signs—IR spectroscopy. Furthermore, we have listed out details of two potential exoplanets from NASA's catalog and emphasized the remarkable capability of NASA's JWST to detect excess infrared light from remote planets. By the end of this article, you will be a few steps closer to understanding the methodology behind finding life beyond earth as well as analytical techniques.

Keywords: Astrobiology, Exoplanets, Biosignatures, spectroscopy, JWST observations.

Introduction:?

For millions of years, planet Earth has sustained life and is predicted to do so for many more years to come. However, the presence of human life is limited to this pale blue dot. This fact alone has made many, scientists and researchers alike, question the surety of our continued existence as a species. And has led to the need to find other planets in this universe that can sustain life similar to ours, along with the curiosity if some other form of life exists in the universe apart from the one we see around us. Life leaves behind many common clues of its existence that are identified as biosignatures. There is a potential chance of finding life in mysterious parts of the universe that are yet to be explored. Biosignatures help us recognize the ability of a planet to inhabit life along with possible indications of its existence there. As we dive deeper into these clues of life we come across different categories of biosignatures, and different methods of identifying and finding potential exoplanets. In this article, we will first explore the understanding of biosignatures, later leading to its different categories and, making the study more efficient and resourceful. This article also discusses different ways of detecting these biosignatures and the observations made regarding the same.

Methodology:

Defining biosignatures:

The universe is too vast to be summed up in an easy catalog of the existing planets and similar celestial bodies with their composition and other features. Thus arises the need to narrow down the features that are most essential in this search and the requirements that are to be fulfilled by a planet for it to be considered suitable for life. For this, we ask a simple question, what is the evidence of life?

The answer to the above question is what we call biosignatures. In simple terms, these findings provide evidence of life's presence. Life alters the chemistry of a planet. Spacecraft have observed the biological alteration of modern Earth's global spectrum in ways that should also be detectable across interstellar space but very coarsely resolved by telescopes.[3]

Biosignatures are the observable effects of life on a planet. It can be a phenomenon, an observation ( by visible or radio means), a pattern, or a substance whose presence is a consequence of a living thing.

We use life on Earth as an analogy for what may be discovered on exoplanets simply because Earth hosts the only known reservoir for life, and more importantly, it is the only body in the solar system for which evidence of life is remotely detectable. This does not mean we exclude the possibility of different types of life. However, the evidence for life must be remotely detectable and we must have the capacity to recognize it. The search for analogs to Earth life, or conceptual extensions thereof, is the central and essential conceit of exoplanet biosignature science in its current state [14]

To study biosignatures effectively we must first classify them, currently, there is no universally accepted scheme for classifying the vast array of potential exoplanet biosignatures. For convenience, we group biosignatures into three broad categories following a suggestion by Meadows [12]: gaseous, surface, and temporal biosignatures.[6]

Categorizing biosignature:

Gaseous biosignatures:

Gaseous biosignatures, as the name suggests, are the gases that are evidence of life. These gases result from either a biological or environmental process on some biological matter. It is important to note that not all biogenic gases are uniquely biological and their biosignature depends heavily on their surrounding environment. A good example of this is what we call false positives(determining something is true when in reality it is false). These biosignatures are inferred from the spectroscopic signatures of the gases.

Oxygen, for example, is considered a significant biosignature and has been extensively studied. It fulfills all three requirements met by the most useful biosignatures: reliability, survivability, and detectability, which together enhance the probability that the biosignature can be detected and interpreted as being due to life [8]. In the article by Meadows [12], they thoroughly examine oxygen as a biosignature. They cover everything about oxygen: how it comes about, its detection, and the uncertainties related to the evolution of oxygen-producing processes on Earth. This includes the history of atmospheric oxygen, processes unrelated to life that can also generate oxygen, various situations that might give incorrect results (both false positives and false negatives), and clear-cut answers. The article also dives into a comprehensive review of methods to measure oxygen and the specific wavelengths that offer crucial information, aiming to provide both direct evidence and a broader understanding of the context.[6]

However, the current understanding of biosignatures and much research conducted on oxygen formation and retention in the atmosphere suggests that oxygen is not an unambiguous sign of life on a planet. Many studies have shown the possibility of false positives in this context. This was also discussed by Meadows [12]. Certain non-biological processes, like breaking down carbon dioxide with light or losing hydrogen, can also create detectable oxygen (O2) or ozone (O3). On the flip side, a life that produces oxygen, like oxygenic photosynthesis, might not be easy to spot. Earth's history shows that even though oxygen-producing life existed long before high oxygen levels, it didn't necessarily translate to detectable atmospheric oxygen. This means thriving life forms might not show up in the usual tests for signs of life. The possibility of both false positives and negatives emphasizes the need to explore alternative signs of life, possibly ones that change over time.[7]

All of this is to say that our evolving understanding of oxygen (O2) as a biosignature doesn't show it's unsuitable but enhances its reliability. Studying O2 has taught us to consider false positives and negatives, improving how we interpret observations of exoplanets and boosting our confidence in detecting life. Lessons from Earth's history, where O2 didn't rise immediately, help us recognize situations where O2 might not be detected (false negatives). Interdisciplinary studies of exoplanets help identify stellar and planetary traits leading to potential false negatives. This knowledge helps us choose which planets to study for signs of life and design observations to enhance our confidence in the results. These insights also set a foundation for developing biosignatures on exoplanets, contributing to a broader framework for detecting signs of life beyond our solar system.[8]

Surface biosignatures:

The spectral signatures of a planet provide us with a lot of information regarding its surface and ongoing activities. Life through various mechanisms has a huge impact on the spectrum of the surface of a planet. These mechanisms include absorption and reflection of light by pigments in living organisms, scattering by the physical structures of organisms (including individual organisms and community architectures), degradation products of biological molecules, fluorescence of pigments, and bioluminescence. However, not all of these biological signals may be common enough or easily seen globally on Earth, and some may resemble non-biological processes. [6]

A surface biosignature results from living material imprinting an inferable spectral or polarization marker on reflected, transmitted, or scattered light. [14]

We look at the unique features of known biomolecules on Earth, highlighting that only one, the vegetation red edge (VRE), is confirmed as a distinct sign of life on our planet. Other suggested biomolecules are still being researched. Even on Earth, the spectra of these molecules can vary due to different environmental conditions, the health of the organisms, and species differences, making it challenging to have specific features that uniquely identify these molecules, unlike the case for gaseous absorption spectra. However, certain "edge" spectra, occurring in the visible and near-infrared range, are potentially strong indicators of life on a planet's surface. These spectra depend on factors like the inherent chemistry of major biomolecules, how pigments adjust to the surrounding light and evolutionary influences. [6]

Vegetation Red Edge (VRE): This remains the most well-studied surface biosignature, with its various extensions such as spectral ‘edges’. This distinctive feature involves a sharp increase in reflectance, particularly noticeable in green vascular plants, occurring at the boundary between visible and near-infrared wavelengths, around 700 nanometers. This effect results from the contrast between chlorophyll absorption in the red wavelengths and the scattering properties of cell and leaf structures in the near infrared]. Oxygenic photosynthesizers, including various plants and algae, share a VRE break clustered between 0.69-0.73 micrometers. The VRE is a robust indicator, more prominent than the weaker chlorophyll "green bump," as illustrated in Figure 2, depicting reflectance spectra of vegetation and other oxygenic photosynthesizers compared to non-biological surfaces like soil, snow, and seawater.[14]

Temporal biosignatures:?

Temporal biosignatures refer to time-dependent patterns in measurable aspects, like gas levels or planetary reflectivity, that signal the presence of biological activity. These patterns may unfold over seasons, daily cycles, or even in unpredictable ways, but they must directly link to a response from the biosphere. Unlike gaseous or static surface biosignatures, temporal biosignatures are less explored due to the complexity of additional factors like planetary asymmetry, axial tilt, and orbital eccentricity. Despite the challenges, we can draw inspiration from Earth's living processes, where variations over time serve as proofs of concept for detecting signs of life on other planets.[14]

Earth's biosphere influences the atmospheric composition, creating observable fluctuations in key gases like CO2, O2, O3, and CH4 (Fig. 3). One well-known example is the seasonal oscillation in CO2 levels, driven by the growth and decay of land vegetation. During spring, CO2 decreases as plants absorb it for growth, rising in fall and winter as vegetation decomposes. This change varies with hemisphere and latitude, being more pronounced in the northern hemisphere due to its larger continental area. O2 concentrations show a corresponding seasonal pattern, linked to the photosynthesis and decomposition reactions involving CO2. Unlike CO2, O2 exhibits larger absolute amplitude variations because of its lower solubility in seawater. This solubility difference also explains why O2's seasonal variability is comparable between hemispheres, with an amplitude of around ～50 ppm at midlatitudes[15]. These temporal variations serve as compelling proofs of concept for detecting signs of life on other planets.[6]

Seasonal variation in atmospheric composition arises on Earth from the interplay between the biosphere and axial tilt. For example, seasonal patterns in insolation shift the balance between various processes like photosynthesis and the reverse reaction, aerobic respiration, resulting in antithetic oscillations of atmospheric CO2 and O2. That is as the concentrations of CO2 increase the concentrations of O2 are simultaneously decreasing and vice versa. Seasonality as a biosignature refers to the potential for seasonal variation in atmospheric composition to serve as an indicator of the presence of life on an exoplanet. This is because seasonal variation in atmospheric composition is a biologically modulated phenomenon on Earth that may occur elsewhere due to the interplay between the biosphere and time-variable insolation. Searching seasonality as a biosignature would avoid many assumptions about specific metabolisms and provide an opportunity to directly quantify biological fluxes, allowing us to characterize, rather than simply recognize, biospheres on exoplanets. However, seasonality also poses observational challenges for remote life detection.

Now that we have defined and classified biosignatures, let's learn how these biosignatures are detected.

Methods of Detecting Biosignatures:

Infrared spectroscopy: Spectroscopy is the study of the interaction of electromagnetic waves (EM) with matter. Infrared spectroscopy, as the name suggests, is the study of the interaction of infrared (IR) radiation with matter. ?IR spectroscopy is useful for identifying and characterizing substances and confirming their identity since the IR spectrum is the “fingerprint” of a substance. [2]

Below is a simple visual representation by NASA that explains the basic idea of spectroscopy and how it is used to detect biosignatures on distant exoplanets.

Observations and Applications:

Remarkable Achievements:

James Webb Space Telescope: The JWST can detect planets with Earth-like temperatures or even colder or warmer habitable temperature ranges by detecting the excess infrared light emitted by the respective planets. The MIRI Mid-Infrared instrument used in the JWST operates within the mid-infrared wavelength ranges of 5-28.5 micrometers and is designed to provide imaging, coronagraphy, and spectroscopy capabilities. The JWST can detect habitable planets within the range of 287 Kelvin, and even hotter planets like Mercury with temperatures ranging from 400 to 1000 Kelvin. This is possible after observing the combined light coming from the 15 nearest white dwarfs. Additionally, exceptionally cold Jupiter-sized exoplanets with even temperatures colder than 200 Kelvin are also detected by the MIRI instrument. [11]

Potential Exoplanets:

1. GJ1132 b: Emerged as a mini-Neptune, it is now a super earth that is 1.66 times the mass of the Earth. GJ 1132 b is a super Earth exoplanet that orbits an M-type star. Its mass is 1.66 times that of Earth. It takes 1.6 days to complete one orbit of its star and is 0.0153 AU from its star.GJ 1132 b, began as a gaseous planet with a thick hydrogen layer of atmosphere. This planet was believed to have quickly lost its primordial hydrogen and helium atmosphere due to the intense radiation of the hot, young star it orbits. In a short time, it was believed that the planet would lose everything and end up as a small, Earth-sized core.[9]

But then according to the Hubble Telescope observations a “secondary atmosphere” was observed that is still present. The observational reports suggest that the atmosphere consists of molecular hydrogen, hydrogen cyanide, and methane and also contains an aerosol haze. Modeling suggests the aerosol haze is based on photochemically produced hydrocarbons, similar to smog on Earth. Scientists interpret the current atmospheric hydrogen in GJ 1132 b as hydrogen from the original atmosphere which was absorbed into the planet’s molten magma mantle and is now being slowly released through volcanic processes to form a new atmosphere. The atmosphere we see today is believed to be continually replenished to balance the hydrogen escaping into space.[9]

2. Kepler 22b: Kepler 22 b is a super-Earth that could be covered in a super-ocean. The jury is still out on Kepler-22b’s true nature; at 2.4 times Earth’s radius, it might even be gaseous. But theoretically, an ocean world tipped on its side – a bit like our solar system’s ice giant, Uranus – turns out to be comfortably habitable based on recent computer modeling. Researchers found that an exoplanet in Earth’s size range, at a comparable distance from its sun and covered in water, could have an average surface temperature of about 60 degrees Fahrenheit (15.5 Celsius). Because of its radical tilt, its north and south poles would be alternately bathed in sunlight and darkness, for half a year each, as the planet circled its star.[10]

Conclusion:

Till now, using different methods and techniques, over 5000 exoplanets have been found. And many of these are predicted to be habitable. But here comes the question of whether life can survive in these habitable parts or not. The answer to this question cannot be accurate but precise. The probability of survival on such planets can only be determined by minutely studying the biosignatures available there. Hence, when it comes to astrobiology, studying biosignatures becomes crucial. As we dive deeper, there are various categories of biosignatures. Gaseous biosignatures, particularly oxygen, act as a major pillar for a specific environment, suspecting its ability to sustain life. Surface biosignature explores life based on spectral signals, one of its successful examples being the Vegetation Red Edge (VRE). The VRE's robustness, compared to other biomolecules, underscores its potential as a key indicator of life on a planet's surface. Temporal biosignatures, linked to time-dependent patterns in measurable aspects, are presented as an essential avenue for life detection with the oscillations in the level of lively glasses. Lessons from Earth's history and current exoplanetary studies helps the research, enhancing confidence in interpreting potential extraterrestrial life. Efforts to detect extraterrestrial life involve predicting visual signatures and systematically searching for them. Instruments capable of detecting authentic organism morphologies, organic molecules, and biogenic gases like methane, hydrogen, oxygen, carbon dioxide, and nitrous oxide are crucial. Biofabrics, isotopic signature detectors, biomineralization, and spatial chemical distribution analyzers can provide tentative evidence of life. Hence, various of the above methods are employed to detect biosignatures. Detecting biosignatures is an intricate yet crucial task. Hence, it is carried out with the utmost accuracy and with minimum errors. For this purpose, there have been various types of techniques implemented. A glimpse of one such technique Infrared Spectroscopy is given above. Cutting-edge technologies such as infrared spectroscopy, coupled with groundbreaking instruments like the James Webb Space Telescope, have expanded our ability to observe exoplanets and investigate their suitability for life. Notable achievements, including the detection of exoplanets like GJ1132 b and Kepler-22b, offer tantalizing glimpses into the diversity and potential habitability of worlds beyond our solar system. Through the study of biosignatures, the clues for the presence of life have fueled the discovery of several seemingly habitable planets. With the advancements in the field of technology and in the field of astrobiology, the probability of finding life other than ours might turn into a reality.?

References:

[1] Michael Perryman, ‘The Exoplanet Handbook, 2nd Edition’, Cambridge University Press, 2018?

[2] Theophanides, Theophile. 2012. ‘Introduction to Infrared Spectroscopy’. Infrared Spectroscopy - Materials Science, Engineering and Technology. InTech. https://doi.org/10.5772/49106.

[3] Nancy Y. Kiang, Shawn Domagal-Goldman, Mary N. Parenteau, David C. Catling, Yuka Fujii, Victoria S. Meadows, Edward W. Schwieterman, and Sara I. Walker. 2018.

Exoplanet Biosignatures: At the Dawn of a New Era of Planetary Observations.

Astrobiology.https://doi.org/10.1089/ast.2018.1862

[4] Grenfell John. 2017. A review of exoplanetary biosignatures, Physics Reports, Volume 713.

https://doi.org/10.1016/j.physrep.2017.08.003

[5] Popescu, Marcel & Birlan, M. & Gherase, R.M. & Sonka, Adrian & Naiman, M. & Cristescu, C.P.. (2012). Applications of visible and infrared spectroscopy to astronomy. UPB Scientific Bulletin, Series A: Applied Mathematics and Physics. 74. 107-120.?

[6] Edward W. Schwieterman, et al. Volume: 18, Issue 6(June 1, 2018) pg no. 663-708.

Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life.

Astrobiology.?

https://doi.org/10.1089/ast.2017.1729

[7] Stephanie L. Olson et al 2018 ApJL 858 L14

Atmospheric Seasonality as an Exoplanet Biosignature?

https://doi.org/10.3847/2041-8213/aac171

[8] Victoria S. Meadows et al. Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment. Astrobiology.?

Volume: 18 Issue 6: June 1, 2018 . Jun 2018.630-662. https://doi.org/10.1089/ast.2017.1727

[9] GJ 1132 b?

Composition https://exoplanets.nasa.gov/exoplanet-catalog/7106/gj-1132-b/

[10] Kepler-22b

https://exoplanets.nasa.gov/exoplanet-catalog/1599/kepler-22b/?

[11] JWST observations

Mary Anne Limbach,et al, A new method for finding nearby white dwarfs exoplanets and detecting biosignatures, Monthly Notices of the Royal Astronomical Society, Volume 517, Issue 2, December 2022, Pages 2622–2638, https://doi.org/10.1093/mnras/stac2823

[12] Modelling the Diversity of Extrasolar Terrestrial Planets?

Meadows VS. Modelling the Diversity of Extrasolar Terrestrial Planets. Proceedings of the International Astronomical Union. 2005;1(C200):25-34. doi:10.1017/S1743921306009033

Limbach, M.A., Vanderburg, A., Stevenson, K.B., Blouin, S., Morley, C., Lustig-Yaeger, J., Soares-Furtado, M., & Janson, M. (2022). A new method for finding nearby white dwarfs exoplanets and detecting biosignatures. Published on 03 October 2022.?

[13] How to find an exoplanet https://www.esa.int/Science_Exploration/Space_Science/Exoplanets/How_to_find_an_exoplanet

[14] Surface and Temporal Biosignatures?

Noack, L., Hoinkis, J., Breuer, D., & Spohn, T. (2019). The interior structure and composition of Mars: Implications for the extent and history of habitability. In R. Deiters, A. Gucsik, & D. Topa (Eds.), Geochemistry of Mars (pp. 1763-1797). Springer International Publishing.

[15] Seasonal variations in the atmospheric O2/N2 ratio about the kinetics of air-sea gas exchange Keeling, R.F., Stephens, B.B., Najjar, R.G., Doney, S.C., Archer, D., & Heimann, M. (1998). A three-dimensional model of atmospheric CO2 transport based on observed winds: 1. Analysis of observational data. Journal of Geophysical Research: Atmospheres, 103(D3), 3381-3390.