*K2-18b made the news this week [BBC – Scientists find ‘strongest evidence yet’ of life on distant planet], it’s only 700 trillion miles away, for us to visit with current propulsion tech it would take over 2 million years – thats about the amount of time its taken us to evolve from being Homo habilis, the “handy man”, chipping away at things with stone tools to now. I wonder what we’d evolve into if we spent 2 million years on a spaceship in space? Maybe that could be a future post?

Look up at the night sky on a clear evening, far away from city lights. The sheer number of stars scattered across the blackness is staggering. Our own galaxy, the Milky Way, contains hundreds of billions of stars, and astronomers estimate there are hundreds of billions, perhaps even trillions, of galaxies in the observable universe. Each star is a potential sun, many likely orbited by planets. Confronted with this almost incomprehensible vastness, humanity has long pondered a fundamental question: Are we alone? The search for extraterrestrial intelligence and life, often shortened to SETI for the intelligence aspect, is arguably one of the most profound and exciting scientific quests ever undertaken. It touches upon our origins, our place in the cosmos, and potentially, our future. This exploration delves into why we search, how we search, and what the silence – or a potential discovery – might mean.

The idea of other worlds isn’t new. Ancient Greek philosophers debated the possibility, and figures like Giordano Bruno were persecuted in the 16th century for suggesting that stars were distant suns with their own planets, possibly inhabited. For centuries, however, these ideas remained largely in the realm of philosophy and, later, imaginative fiction like H.G. Wells’s The War of the Worlds. The scientific search only became feasible in the 20th century, catalysed by advancements in our understanding of physics, astronomy, biology, and technology. We began to grasp the true scale of the universe, understand the chemical basis of life on Earth, and develop tools capable of peering far beyond our own solar system. A pivotal moment came with the dawn of radio astronomy. After World War II, scientists realised that the radio waves used for communication and radar could also be used to study celestial objects. If another civilisation had developed similar technology, perhaps we could detect their signals.

This possibility spurred astronomer Frank Drake to conduct the first modern SETI experiment in 1960. Called Project Ozma, he used a radio telescope in West Virginia to listen for structured, artificial-sounding radio signals coming from two nearby sun-like stars, Tau Ceti and Epsilon Eridani. Although Ozma detected no alien broadcasts, it marked the beginning of a new scientific field. The following year, Drake organised a meeting with other scientists interested in the problem, including a young Carl Sagan. To focus the discussion, he formulated what is now known as the Drake Equation. It doesn’t provide a definitive answer, but rather breaks down the enormous question “How many detectable civilisations exist in our galaxy?” into smaller, more manageable factors. The equation looks like this: N = R* × fp × ne × fl × fi × fc × L. Here, N is the number of civilisations we might detect; R* is the average rate of star formation; fp is the fraction of those stars with planets; ne is the average number of planets that could potentially support life per star with planets; fl is the fraction of those planets that actually develop life; fi is the fraction of life-bearing planets where intelligent life evolves; fc is the fraction of civilisations that develop technology releasing detectable signs of their existence; and L is the length of time such civilisations release detectable signals. As Drake himself clarified, its main value wasn’t calculation, but “to summarise clearly the factors which determined how many civilizations might exist” [1]. When Drake first wrote it, almost all the terms were wild guesses. Today, thanks largely to discoveries in astronomy, we have much better estimates for the early terms, especially the prevalence of planets (fp), confirming that planets are incredibly common, perhaps even outnumbering stars [2]. The later terms, concerning the emergence and longevity of life and intelligence, remain deeply uncertain.

The Drake Equation highlights a profound cosmic mystery often called the Fermi Paradox, named after physicist Enrico Fermi. During a casual lunchtime conversation in 1950, considering the age and size of the universe and the likelihood of planets, Fermi supposedly asked, “But where is everybody?” [3]. If life and intelligence are not extraordinarily rare, the galaxy should logically be teeming with civilisations, some far older and more advanced than ours. Yet, despite decades of searching, we’ve found no confirmed evidence of their existence. This “Great Silence” has spawned numerous potential explanations. Perhaps life itself is incredibly rare (the Rare Earth hypothesis) [4]. Maybe the development of intelligence or technological societies is the bottleneck. Perhaps civilisations inevitably destroy themselves before they can expand across the galaxy (a concept known as the Great Filter) [5]. Maybe they exist but are using technologies we can’t detect, or they have no interest in contacting us. Or perhaps, as Carl Sagan mused, “The universe is a pretty big place. If it’s just us, seems like an awful waste of space” [6], implying the search simply needs more time and better tools.

The search itself proceeds along two main pathways. The first is traditional SETI, focusing on detecting signals deliberately or accidentally broadcast by intelligent civilisations. This largely involves using large radio telescopes to scan the skies for narrow-bandwidth signals – patterns unlikely to be produced by natural astrophysical phenomena. Projects like the SETI Institute’s Allen Telescope Array and the ambitious Breakthrough Listen initiative, funded by billionaire Yuri Milner, are conducting the most extensive searches to date, scanning millions of stars across billions of radio channels [7, 8]. Another approach is Optical SETI, which looks for powerful, brief pulses of laser light that could be used for interstellar communication. The challenge for all SETI efforts is immense. The search space is vast (billions of stars, billions of frequencies), signals would be incredibly faint after travelling interstellar distances, and we don’t even know what kind of signal to truly expect. As SETI pioneer Jill Tarter has often emphasised, the search requires persistence and technological advancement: “We are looking for signals that are manufactured, that have some structure imposed upon them that nature doesn’t seem to produce… We haven’t explored the haystack very well yet” [9].

The second major pathway is the broader field of astrobiology, which searches for any evidence of life, past or present, intelligent or not. This involves looking for ‘biosignatures’ – chemical or physical indicators that can only be explained by biological processes. One exciting frontier is the study of exoplanet atmospheres. Since the first confirmed discovery of planets orbiting other sun-like stars in the 1990s, thousands have been found [10]. Telescopes like Hubble and Spitzer, and now especially the James Webb Space Telescope (JWST), can analyse the starlight filtering through an exoplanet’s atmosphere [11]. By looking for the spectral fingerprints of gases like oxygen, methane, or water vapour – potentially in combinations that suggest biological activity – scientists hope to find hints of life. JWST’s power allows it to probe the atmospheres of smaller, rocky planets in the habitable zones of their stars – the region where temperatures could allow liquid water to exist on the surface. The definition of ‘habitable’ is also expanding. Life on Earth exists in extreme environments – deep-sea vents, acidic pools, Antarctic ice. This suggests life might arise in places we once thought impossible, such as the subsurface oceans suspected to exist on Jupiter’s moon Europa or Saturn’s moon Enceladus [12]. Future missions aim to explore these icy moons more closely. Mars remains a key target, with rovers like NASA’s Perseverance searching for signs of ancient microbial life in rocks and sediments formed when Mars was warmer and wetter [13].

Technological advancements are crucial to both SETI and astrobiology. Bigger, more sensitive telescopes (like the upcoming Square Kilometre Array in radio astronomy [14] and Extremely Large Telescopes in optical astronomy) will improve our ability to detect faint signals or analyse distant atmospheres. Increased computing power allows SETI projects to process unprecedented amounts of data, searching for complex signal patterns. Advances in spectroscopy and modelling help astrobiologists interpret the atmospheric data from exoplanets and understand what constitutes a reliable biosignature, distinguishing it from non-biological processes that might mimic life.

The implications of discovering extraterrestrial life, in any form, would be staggering. Finding even microbial fossils on Mars or simple organisms swimming in Europa’s ocean would revolutionise biology, proving that life is not unique to Earth. It would suggest that life might be common throughout the universe. Discovering an intelligent signal would be even more profound, forcing a complete re-evaluation of humanity’s place in the cosmos. It would answer the question “Are we alone?” with a definitive “No.” The societal, philosophical, and religious impacts would be immense and unpredictable. Would it foster global unity or fear? How would we respond? There’s even debate about whether we should actively broadcast messages into space (Active SETI or METI – Messaging Extraterrestrial Intelligence) or just listen, given the unknown nature and intentions of any potential listeners [15]. Conversely, if the search continues for centuries with no success, the implications are also profound. Does the continued silence strengthen the idea of a Great Filter, suggesting intelligent life is either incredibly rare or tragically short-lived? Does it mean humanity holds a uniquely precious position in the galaxy, perhaps even the universe?

The search for extraterrestrial intelligence and life is more active and technologically advanced today than ever before. We’ve moved from philosophical speculation to targeted scientific investigation. We know planets are common, and the ingredients for life as we know it appear widespread. We have powerful tools like JWST opening new windows onto potentially habitable worlds, and dedicated projects scanning the skies for signals. Yet, the fundamental question remains unanswered. The silence persists, but the search continues, driven by human curiosity and the sheer statistical likelihood suggested by the vastness of the cosmos. It’s a search that requires patience, ingenuity, and perhaps a little bit of hope. Whether we eventually detect a faint whisper from a distant star, find microbes in Martian soil, or continue to find only silence, the quest itself teaches us about the universe and forces us to confront our own existence. The cosmos is vast and ancient, and our journey to understand our place within it has only just begun. Perhaps the silence we perceive isn’t empty, but rather filled with possibilities we haven’t yet learned how to recognise. What secrets might the universe reveal when we finally develop the right ways to look and listen?

References and Further Reading:

1. SETI Institute. (n.d.). Drake Equation. Retrieved from https://www.seti.org/drake-equation-index (Note: While Drake discussed its purpose widely, a single definitive quote source can be elusive; the SETI Institute accurately reflects its commonly understood function).
2. Cassan, A., Kubas, D., Beaulieu, J.-P., Dominik, M., et al. (2012). One or more bound planets per Milky Way star from microlensing observations. Nature, 481(7380), 167–169. DOI: 10.1038/nature10684
3. Jones, E. M. (1985). “Where is everybody?”: An account of Fermi’s question. Los Alamos National Laboratory report LA-10311-MS. Retrieved from https://www.osti.gov/biblio/7111851
4. Ward, P. D., & Brownlee, D. (2000). Rare Earth: Why Complex Life Is Uncommon in the Universe. Copernicus Books. ISBN: 978-0387987019
5. Bostrom, N. (2008). Where Are They? Why I Hope the Search for Extraterrestrial Life Finds Nothing. MIT Technology Review. Retrieved from https://www.technologyreview.com/2008/05/29/219978/where-are-they/
6. Sagan, C. (1985). Contact. Simon & Schuster. ISBN: 978-0671434007 (The quote is famously associated with the film adaptation, but reflects Sagan’s views expressed throughout his work). A similar sentiment is found in Cosmos.
7. SETI Institute. (n.d.). Allen Telescope Array. Retrieved from https://www.seti.org/ata
8. Breakthrough Initiatives. (n.d.). Breakthrough Listen. Retrieved from https://breakthroughinitiatives.org/initiative/1
9. Tarter, J. (2009). Jill Tarter: Why the search for alien intelligence matters. TED Talk. Retrieved from https://www.ted.com/talks/jilltarterwhythesearchforalienintelligencematters (Quote is representative of many similar statements she has made).
10. NASA Exoplanet Exploration. (n.d.). Exoplanet Exploration: Planets Beyond our Solar System. Retrieved from https://exoplanets.nasa.gov/
11. NASA. (n.d.). James Webb Space Telescope. Retrieved from https://webb.nasa.gov/ (Check specific science goals sections).
12. NASA Solar System Exploration. (n.d.). Europa and Enceladus fact pages. Retrieved from https://solarsystem.nasa.gov/moons/jupiter-moons/europa/in-depth/ and https://solarsystem.nasa.gov/moons/saturn-moons/enceladus/in-depth/
13. NASA Mars Exploration Program. (n.d.). Mars Perseverance Rover. Retrieved from https://mars.nasa.gov/mars2020/
14. SKA Telescope. (n.d.). The SKA Project. Retrieved from https://www.skao.int/en/explore/ska-project
15. Billingham, J., & Benford, J. (Eds.). (2014). Special Issue: Searching for Cost Optimized Interstellar Beacons and the METI Debate. Journal of the British Interplanetary Society, 67.

Further Reading Suggestions:

• Cosmos by Carl Sagan (Book and TV series) – A classic exploration of the universe and the search for life.
• Pale Blue Dot: A Vision of the Human Future in Space by Carl Sagan – Reflections on Earth’s place in the cosmos.
• Websites: NASA Astrobiology (https://astrobiology.nasa.gov/), SETI Institute (https://www.seti.org/), European Space Agency (ESA) Science & Exploration (https://www.esa.int/Science_Exploration).
• Documentaries like BBC’s Wonders of the Universe or The Planets.
• Check out science news sites like New Scientist or Scientific American for the latest discoveries in exoplanet research and astrobiology.

---