Earth is a pretty unique place. Thanks to its location in the habitable zone, life as we know it was able to grow and thrive on the planet. However, in our brief time exploring the surrounding cosmos for signs that we are not alone, we have consistently come up empty-handed.
Astrobiologists are concerned with the origin and dispersion of life throughout the universe, and since Earth is the only place we know where life has emerged, the Blue Planet and its features have thus far guided our attempts to spot life lurking somewhere other than our home. By predicting what Earth could look like in the next 1,000 years, researchers may be able to hone in on possible signs of life throughout the cosmos.
Searching for Molecular Evidence of Intelligent Life
One way astrobiologists have sought to identify life on other worlds has been by spotting biosignatures. These are the molecular or chemical byproducts of living systems. For instance, if we detected oxygen in the atmosphere of a distant planet, we might suspect that some living process could be its source since life fills Earth's atmosphere with oxygen.
One issue with the biosignature approach is that some unknown non-living process could be responsible for whatever chemical 'biosignature' we detect. For this and other reasons, spotting something more unambiguous, such as a signal from an advanced civilization's technology, has also been on the radar.
Read More: The Secret Origins of the Search for Extraterrestrial Intelligence
Looking for Signs of Alien Technology
Though rubbing up against what some might call science fiction, the search for 'technosignatures' examines the possibility that signs of technological intelligence could be waiting for researchers to discover. Since Earth is the only place in the cosmos where we know such technology has emerged, the history and trajectory of our planet are the best guides we have for theorizing about what to look for when it comes to alien technology.
Researchers such as Ravi Kopparapu, a planetary scientist at NASA's Goddard Space Flight Center, have been interested in observing Earth as it is now and imagining how it might look in the future to help inform what type of observations we should expect if we ever train our telescopes on a far-off life-bearing world.
"Observing the current Earth from space helps us in understanding how a habitable (and inhabited) planet sustains life," Kopparapu says.
"Several papers were published where data from Earth-observing satellites were used to assess the detectability of biosignatures of a potential exo-Earth. So, observing the present Earth or assessing the future Earth scenario from space can provide insights into the detectability of an advanced civilization through remote observations," he explains.
If some distant civilization had their life-detection observatory — let's say it has similar capabilities to our own — pointed towards current day, or even future Earth, would any such 'technosignature' give away our existence?
Read More: Fact or Fiction: What Is The Truth Behind Alien Conspiracy Theories?
How Scientists Use Radio Signals to Search for Life
One of the most well-known ways researchers scan the stars for signs of technological intelligence is through radio telescopes. Radio signals from our own communication purposes are leaking into space, and it's plausible that anyone listening could eavesdrop on our conversations. In this sense, we are kind of like the obnoxiously loud table at a restaurant, with the other intergalactic guests listening in.
"We have the electromagnetic spectrum, from Ultraviolet to radio observations. Many people are familiar with the radio search for technology, Search for Extraterrestrial Intelligence, which has been ongoing for the past several decades. Intelligence is not detectable, but the manifestation of intelligence is technology, and that we can potentially detect using our instruments and tools," says Kopparapu.
Beyond the radio search for technosignatures, current telescopes like the James Webb Space Telescope and future telescopes like the Habitable Worlds Observatory are capable of observing industrial pollutants. These include chlorofluorocarbons and nitrogen dioxide from burning fossil fuels.
Radio leakage and industrial pollutants are technosignatures that human civilization is creating with our current level of technological knowledge, but it's also possible, in fact, probably likely, that if we ever become aware of another civilization, it will be at a different point in its technological trajectory. Because of this, it's worth considering where humanity is headed and whether this may render new technosignatures that point to our existence or cloak us to the cosmos.
Read More: New SETI Tool Expands the Search for Intelligent Life in the Universe
Understanding the Kardashev Scale and Its Limitations
In typical discussions of technological development and how this pertains to detectable signs of intelligence in the cosmos, researchers often invoke the Kardashev scale. This is a way of describing civilizations and their technology based on how much energy they can harness. A type I Kardashev civilization can harness the entirety of the solar energy received by its home planet, a type II can harness the entirety of the solar energy output of its home star, and a type III — its entire galaxy.
Kopparapu, though, is wary of using this scale to track the development of civilizations and what types of technology (and therefore technosignatures) they might generate.
"We recently published a paper that discusses the necessity of Kardeshev-type civilizations based on energy arguments," says Kopparapu.
"Our conclusion is that, even with significant population growth, the energy needs of human civilization would be several orders of magnitude below the energy threshold for a Kardashev Type I civilization, or a Dyson sphere/swarm which harnesses the energy of a star," he continues.
Read More: How the Kardashev Scale Could Help Us Find Life on Other Planets
How Might Earth Appear to Extraterrestrial Observers?
Human society could take various trajectories, especially when it comes to timescales in the 1000s of years (considering how fast technology has developed). And with each scenario, observers from the outside may see something completely different. Kopparapu recently co-authored a paper that explored different scenarios for the future of humanity and how these might translate to distant observations of Earth.
"We discussed some scenarios in our paper. They range from a golden age of post-economic scarcity with settlements on the moon, Mars, and beyond to space exploration (similar to the 'Star-Trekkian' world) to the bleak scenario of an autocratic power taking over the planet, destabilizing the biosphere and increasing the presence of large urban landscape of the technosphere (probably more like 'Trantor' from Isaac Asimov’s Foundation novels)," he explaines.
"Intermediate scenarios exist where we pass the peak of our technological existence and live sustainably with the biosphere. It is this scenario, in my opinion, that would elude an extra-terrestrial observer the presence of humans on Earth," he adds.
In our search for life in the cosmos, and even life that is sufficiently intelligent to generate its own technology, there is no guarantee that it will resemble anything like the life that we are familiar with on Earth, which is why life-detection efforts can be so difficult. It is also often assumed that any intelligent civilization that emerges in the cosmos will move through stages of development defined by their energy needs (perhaps because of our own society's rapid development).
However, no such future is guaranteed when considering the future of our own species and its relationship to our planet. So, when we consider our ability to detect and even highly intelligent life, Kopparapu's work highlights that we must be open to a range of possible scenarios. If we don't, an answer to the question of our cosmic loneliness could pass us right by.
Read More: What Is the Fermi Paradox?
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
NASA. What is a biosignature?
NASA. Searching for Signs of Intelligent Life: Technosignatures
Planetary scientist at NASA's Goddard Space Flight Center. Ravi Kopparapu
Monthly Notices of the Royal Astronomical Society. Simulation of the Earth’s radio-leakage from mobile towers as seen from selected nearby stellar systems
Habitable Worlds Observatory. Advancing the Habitable Worlds Observatory Concept
arXiv:2405.04560. Detectability of Solar Panels as a Technosignature
arXiv:2409.00067. Projections of Earth's technosphere. I. Scenario modeling, worldbuilding, and overview of remotely detectable technosignatures
Conor Feehly is a New Zealand-based science writer who covers a wide range of topics, including astronomy and neuroscience, with an eye for research at the intersection of science and philosophy. He received a master's in science communication degree from the University of Otago. Conor is a regular contributor to Discover Magazine, with his work also appearing in New Scientist, Nautilus Magazine, Live Science, and New Humanist among others.
