It’s often been suggested that the first indication of technological life on Earth observable from space would be the program “I Love Lucy”, which was among the first popular broadcast television shows beginning in 1951. At this point in time, the sphere of “I Love Lucy” has a radius of 74 light years, and would be detectable to a technological civilization on any of about 4000 star systems. 
It recently occurred to me that over geologic time, many, many, many more star systems than we can detect would have been able to see Earth as a planet with robust life, and the potential to evolve technological intelligence. Let’s first consider what we can observe in nearby star systems using our current level of technology.
The first discovery of a planet orbiting a main-sequence star occurred in 1995. Since then, using a variety of improving methods, we have discovered nearly 6000 exoplanets. The bulk of these tend to be large planets like our gas giants, because of greater ease of detection. Of these, only 60 or 70 are interpreted to be earth-like, in size, rocky composition, and within the habitable zone of their stars. 
Transiting Exoplanet Survey Satellite
We can investigate the chemical composition of exoplanet atmospheres using absorption spectra of light transmitted through those atmospheres. We’ve succeeded in investigating the atmospheres of 288 planets, identifying 67 different chemical constituent gases. The most distant planet where we have analyzed the atmospheric chemistry is at a distance of 124 light years. That planet, K2-18b, has an atmosphere containing methane and traces of dimethyl sulfide. On earth, dimethyl sulfide is produced by marine bacteria, and is a possible indicator of life. 
Spectrometry of Exoplanet Atmospheres
James Webb Space Telescope
It’s worth reflecting for a moment about where we stand in our technological development. It has been 122 years since the Wright brothers’ first flight, 119 years since the first radio broadcast, 68 years since the first satellite in space, 30 years since the discovery of the first exoplanet and only 9 years since the first chemical analysis of the atmosphere of an exoplanet. Nine years. Today we find it difficult to analyze the composition of planets orbiting close to their stars, but consider what we might be able to do in 300 or 500 years, presuming that human technology continues to advance.
No new technology is needed to observe planets which are closer to their stars or more distant from Earth, at least until we reach a limit of interference from interstellar dust. It just requires building bigger telescopes. Within a radius of 100 light years, there are an estimated 10,000 to 60,000 stars. The number of star systems increases exponentially with distance, so there are an estimated ten million stars within a radius of 1000 light years from Earth. It’s easy to imagine that we could achieve an order of magnitude improvement in exoplanetary exploration in 100 years.
Biosignatures
Certain molecules in the atmosphere are unstable combinations, and only occur together because one or both are being replenished through biological activity. These combinations of coexisting molecules are considered likely indicators of extraterrestrial life. Some of those combinations include methane and oxygen, methane and carbon dioxide (without carbon monoxide), nitrogen, methane and CO2 with liquid water, and high concentrations of oxygen and nitrogen. A number of other individual compounds are independently considered biosignatures.
Earth’s atmosphere has contained abundant coexisting oxygen and nitrogen for at least 2.3 billion years, in volumes that would be detectable by our own instrumentation, if our telescopes gave sufficient resolution. Other biosignature molecules would probably also be evident to any extraterrestrial observer. Compared to the 288 planets already analyzed for atmospheric chemistry, Earth’s atmosphere screams that life is robustly abundant on this planet. And our atmosphere has been advertising that fact to the galaxy for the past 2.3 billion years.
The Fermi Paradox, the Dark Forest Hypothesis and Atmospheric Chemistry
The Fermi Paradox is the apparent contradiction between our observation of billions of stars, representing billions of planets capable of hosting intelligent life, and the absence of any sign of extraterrestrial life or communication. Reportedly, Enrico Fermi once abruptly asked his fellow physicists plaintively, “Where is everybody?” I should note that others preceded Fermi in this question, notably Bernard de Bovier in 1686 (!!) and Jules Verne in 1865.
A variety of explanations have been proposed to explain the paradox. These include (some of these are my own names):
> The Great Filter – some evolutionary process greatly reduces the chance that intelligent life develops on other planets.
> The Prime Directive Hypothesis – the idea that advanced aliens have protected Earth from contact by extraterrestrial species. This hypothesis is usually called The Zoo Hypothesis.
> The Sagan Hypothesis – the idea that technological civilizations are very short-lived, tending to destroy themselves in military conflicts or environmental disasters.
> The Bridge Too Far Hypothesis – the idea that interstellar distances and the limit of lightspeed are prohibitively great barriers to interstellar exploration.
> The City and The Stars Hypothesis – the idea that alien civilizations achieved immortality, and have no need or desire for exploration or expansion. (Concept courtesy of my son and A.C. Clarke.)
> The Gaseous Whale Hypothesis – the idea that most intelligent species evolve in oceans or gas giants, lacking the ability to use fire, observe other stars, to develop radio or space-faring technology due to environmental constraints.
> The Smoke Signal vs. Telegraph Hypothesis – the idea that alien civilizations developed communication technologies far beyond the capability of humans to detect.
> The Peacock’s Tail Hypothesis – the idea that human intelligence resulted from sexual selection rather than environmental adaptation, and that intelligence is as unique as a peacock’s tail.
> The Childhood’s End Hypothesis – the idea that technological civilizations eventually progress beyond physical biological forms, and physical space is no longer of interest.
> The Dark Forest Hypothesis – the idea that extraterrestrial civilizations are abundant, but hiding from other potentially aggressive alien species.
> The Conehead, Lizard-People or X-Files Hypothesis – the idea that extraterrestrial beings exist and are living among us.
The Dark Forest is the name of the second book in a science-fiction (or horror) trilogy by Liu Cixin,The Three-Body Problem. The central concept of the trilogy (spoiler alert!) is that alien civilizations are abundant in the galaxy, but hiding out of fear of other aggressive, expansionistic species. I think that the easy detectability of life through atmospheric chemistry should put this hypothesis to rest. Technological alien civilizations have had over 2 billion years to recognize that Earth is life-bearing, and in an ideal environment for water/oxygen/carbon dioxide-based life. If expansionistic space-faring aliens existed on any of the ten million stars within 1000 light years, they would have colonized the planet long ago. Since we have no evidence that aliens ever visited the earth in the deep past, I think we have to conclude that the Fermi Paradox still holds.
We don't know which of the proposed explanations for the Fermi Paradox is correct. In my opinion, the Fermi Paradox results from the cumulative impact of several explanations. I think that The Great Filter, The Sagan Hypothesis, The Bridge Too Far Hypothesis and the Peacock Tail Hypothesis together account for the absence of observable alien civilizations today.
Conclusion
There was exciting news from NASA’s Perseverance rover on Mars. The robot explorer found evidence of two unusual minerals in a carbon-bearing mudstone deposited in a watery environment. The rover’s spectrometry instrument detected “likely” vivianite, a hydrated iron phosphate mineral, and griegite, a magnetic iron sulfide mineral. On earth, these minerals are only formed either in hydrothermal environments or by metabolic processes of microbes, including sulphate-reducing bacteria. The host mudstones for the minerals show no evidence of hydrothermal heating. Together with the previous discovery of long-chain alkanes (https://dougrobbins.blogspot.com/2025/03/curiosity-rover-on-mars-drills-hole.html), this evidence strongly supports the likelihood of ancient microbial life on Mars. But ancient, sparse microbial life is a long way from technological, intelligent, alien species.
The Earth’s atmosphere is a signpost that the Earth is robustly teeming with life. That signpost has been in place for at least 2.3 billion years for an alien civilization on any one of ten million stars within 1000 light years to observe. And yet, there is no serious evidence that the planet has ever been visited by aliens. There are no monoliths, no relict satellites, no abandoned bases on the moon and no tentacled aliens sipping Mai-Tais on the beaches of Maui. All life on Earth appears to share a common evolutionary lineage, with no strains of bacteria possessing alternative DNA.
We know from the history of our own planet that technological intelligence did not easily evolve. About 3 billion years passed from the first appearance of life until the development of multi-cellular, animate life. After animate life evolved, about 538 million years passed before an animal appeared which could use fire and make basic tools. And after a species learned to make basic tools, use fire and exchange ideas through logical symbolic language, it was a million years before that species could write books, broadcast radio signals, build a heavier-than-air flying machine, or launch astronomical telescopes into orbit. In retrospect, the odds for achieving each of those thresholds seems very low, given the time that passed before each step occurred.
The Fermi Paradox is robustly upheld. There are no aliens crouched around an experimental television, watching the first episode of “I Love Lucy” and planning an invasion. Our galaxy is not the galaxy that is shown in Star Trek, teeming with competing intelligent alien species. Millions of planets are within range of seeing that something interesting was happening on Earth for billions of years, but if anyone visited, they left no trace. All of the evidence and reasonable deduction says that we are alone.
References
The Role of N2 as a Geo-Biosignature for the Detection and Characterization of Earth-like Habitats, Lammer et al, 2019.
https://pubmed.ncbi.nlm.nih.gov/31314591/
Redox-driven mineral and organic associations in Jezero Crater, Mars, Hurowitz et al, 2025.
https://www.nature.com/articles/s41586-025-09413-0
