Summary in seconds: the historical prospective of “Are we alone in this Universe?” – from imaginary creatures to biosignatures, the ten signatures of life, and the topic of my next article in this series.
Are we alone in this universe? A historical prospective
This question is as old as ancient human myths that linked twinkling lights of the night sky to spirits, gods, and other imaginary creatures. Like they did to their gods, our ancestors endowed these imaginary creatures with human characteristics and sometimes even features.
This mindset of projecting human traits on alien creatures continues to pervade our cultures till the dawn of the space age in the last century. All along, we were not looking for “life;” rather we were searching for intelligent creatures that shared some of our human features.
In 1686, Bernard le Bovier de Fontenelle1 argued that alien beings might attempt to communicate with us or even visit us using some advanced form of flight. Ever since, each age has featured its own version yearning for contact with “intelligent life” from beyond, always anchored to the technological themes of the day.
In 1818, the German mathematician Carl Friedrich Gauss2 proposed communicating with aliens using a system of mirrors (heliotrope) to send coded signals using reflected sunlight. A few years later, a French inventor, Charles Cros3, suggested that lamps could be amplified to beam messages to Venus or Mars. In 1900, Nikola Tesla4 suggested communicating with other planets using newfangled radio waves.
A full century after Tesla, researchers engaged in the search for extraterrestrial intelligence (SETI)5 were still scanning the heavens with antennas and listening for artificial radio transmissions incoming from other worlds. These efforts led a handful of briefly exciting false alarms, but mostly, a whole lot of nothing.
Getting the SETI field moving again required more than a specific new technology; it required a new way of thinking about technology as a whole. In 1995, two Swiss astronomers6 managed to do that. They revitalized the enthusiasm for SETI when they found the first known planet orbiting another star similar to the Sun. Since then, the number of discovered exoplanets has been booming and currently stands at more than 50007. The discovery of exoplanets changed the possibility of life on other planets from a matter of speculation to a topic within the realm of empirical investigation. In response to this exoplanet boom, researchers coined the term ‘biosignatures,’8 which represented a new way to search for “life” beyond our blue planet.
Biosignatures
A biosignature is any substance, such as an element, isotope, molecule or phenomenon that provides scientific evidence of past or present life. Measurable features of life include its chemical or physical makeup, or energy use, and its production of biomass and waste. Biosignatures can provide clues to the existence of extraterrestrial organisms either directly or indirectly by searching for their unique byproducts. In general, biosignatures can be grouped into ten broad categories:
1. Isotope patterns: includes any evidence of an isotope or patterns of isotopes that require biological processes to exist.
2. Chemistry: any chemical features that need biological activities to happen.
3. Organic matter: organic compounds that are formed by biological processes.
4. Minerals: minerals or biomineral-phases whose composition and/or morphology indicate biological activity (e.g., bio-magnetite).
5. Microscopic structures and textures: for example, biologically formed cements, micro-textures, microfossils, and films.
6. Macroscopic physical structures and textures: these are structures that indicate the presence of microbial ecosystems, biofilms (e.g., stromatolites), or fossils of larger organisms.
7. Temporal variability: any changes over time in the atmospheric gases, or reflectivity, or macroscopic appearance that indicate the presence of life.
8. Surface reflectance features: any large-scale reflectance features due to biological pigments, which could be detected remotely.
9. Atmospheric gases: detecting gases that are formed as a result of metabolic and/or aqueous processes, and their presence on a planet-wide scale.
10. Techno-signatures: signatures that indicate a technologically advanced civilization.
In my next article of this series “life in Our Backyard,” the topic of “Signatures of Life” will be revisited when I cover searching for life in our Solar System.
References
1. Bernard le Bovier de Fontenelle (1657-1757). A French author and a long-term secretary of the French Academy of Science during the unfolding of the Age of Enlightenment.
https://mathshistory.st-andrews.ac.uk/Biographies/Fontenelle/
2. Carl Friedrich Gauss (1777-1855). A German mathematician and physicist.
https://mathshistory.st-andrews.ac.uk/Biographies/Gauss/
3. Charles Cros (1842-1888). A French poet and inventor, and author of “Studies on the Means of Communication with the Planets”.
https://www.britannica.com/biography/Charles-Cros
4. Nicola Tesla (1856-1943). A Serbian-American inventor, electrical engineer, mechanical engineer, and futurist best known for his contributions to the design of the modern alternating current electricity supply system.
https://www.britannica.com/biography/Nikola-Tesla
5. Search for extraterrestrial intelligence (SETI). A collective term for scientific searches for intelligent extraterrestrial life, for example, monitoring electromagnetic radiation for signs of transmissions from civilizations on other planets.
https://en.wikipedia.org/wiki/Search_for_extraterrestrial_intelligence
6. Michel Mayor (1942- ) and Didier Queloz (1966- ). Two Swiss astronomers, famous for discovering the first exoplanet orbiting a sun-like star in 1995. In 2019 they won the Nobel Prize in Physics.
https://www.nature.com/articles/d41586-019-02964-z
7. Exoplanets: are planets located outside our solar system and orbiting sun-like stars. As of the 1 of July 2022, 5,108 exoplanets have been discovered outside our solar system.
8. Biosignatures: are any substance that provides scientific evidence of past or present life.