Humans have searched for signs of intelligent life beyond Earth for over a century. The most well-known effort, the Search for Extraterrestrial Intelligence (SETI), was popularized by Carl Sagan’s 1985 novel, Contact, and its later film adaptation. Like Sagan’s protagonist, many SETI researchers use telescopes to listen for radio signals from distant civilizations. But radio waves are only one way scientists search for alien life.

Astronomers sweep the skies for measurable signs of advanced technology, known as technosignatures. In 1906, astronomer Percival Lowell mapped what he believed to be a vast series of artificial “canals” on Mars. In 1960, physicist Freeman J. Dyson proposed that an advanced civilization might build a structure around its star to harvest energy, sometimes called a Dyson sphere. Lowell’s canals were natural erosion features, and Dyson spheres remain hypothetical, but the hunt for technosignatures persists to this day.

These gases could signify intelligent life beyond Earth. Modern astronomers use powerful instruments like JWST to analyze the chemical fingerprints of distant planetary atmospheres in search of signs of life or advanced technology. Researchers have proposed looking for industrial gases, such as chlorofluorocarbons or hydrofluorocarbons, to detect alien civilizations on exoplanets. However, these gases are in Earth’s atmosphere at extremely low levels, which suggests that detecting them on an exoplanet would be challenging. Astronomers recently proposed that the industrial gases nitrogen trifluoride and sulfur hexafluoride could provide measurable signs of advanced technology on distant exoplanets. Read more here.

Are aliens receiving radio signals from Earth? Radio signals are a staple of the first-contact subgenre of science fiction. Carl Sagan’s Contact famously revolves around the discovery of encoded radio signals from the star Vega, Liu Cixin’s The Three-Body Problem follows what happens after a scientist secretly makes radio contact with aliens, and Vince Gilligan’s Pluribus centers on what happens after scientists follow instructions communicated to Earth via radio signals. But how likely is it that we could actually receive an alien radio signal, or that aliens could receive an outgoing signal from Earth? To answer this question, scientists recently calculated the maximum distance at which extraterrestrials with modern human-level technology could detect radio signals from Earth. Read more here.

“Radical mundanity” could explain why we haven’t met aliens. “Where are they?” is the question reportedly asked by the famous Italian-American physicist Enrico Fermi during a conversation with fellow scientists in the early 1950s. The other scientists understood that Fermi was referring to aliens. Over the rest of the conversation, Fermi went through a series of calculations showing not only that extraterrestrials should exist, but that they should have come to Earth eons ago, multiple times. But if other alien civilizations exist throughout the Galaxy and we can theoretically detect signs of their existence, then why haven’t we seen them? One researcher argued that if alien civilizations are technologically limited like humans, then it shouldn’t be surprising that we haven’t encountered any. Read more here.

Has Earth received any radio signals from aliens? In 1984, researchers founded the independent research institute known as the Search for Extraterrestrial Intelligence (SETI), dedicated to searching specifically for technosignatures in the form of radio signals. From 2006 to 2020, the SETI@home project worked alongside researchers who were surveying the sky for excess radio emissions from space for their own projects using the Arecibo Telescope, until it collapsed in December 2020. The SETI@home team gathered 400 days of combined observation time over these 14 years, producing billions of excess radio wave detections from different points in space. They narrowed these down to 92 incoming radio signals that could potentially be from alien technology. Read more here.

Telescopes could search for alien farms. Farming is one of the oldest forms of technology in human history. Although farming today may include the use of heavy machinery and other modern tools, the true innovation in farming began about 10,000-20,000 years ago when people established permanent settlements and learned to increase their food production by fertilizing the soil. One breakthrough in farming was the ability to produce ammonia-based fertilizers by combining the nitrogen in air with hydrogen gas using a high-temperature industrial process. Researchers demonstrated that ammonia and other gas byproducts of modern farming could be detectable with space telescopes, providing evidence of technology if seen on another planet. Read more here.