It is customary to search for signs of life on rocky planets in the habitable zone of their host star. There, the planet’s surface is not too hot or too cold, allowing for the chemistry of life in liquid water in the presence of a sufficiently dense atmosphere.
One could imagine that other forms of life-as-we-do-not-know-it thrive in other fluids. NASA’s Dragonfly mission, scheduled for launch in July 2028, will search for signs of life in the liquid oceans, lakes and rivers of methane and ethane on the surface of Titan, a moon of Saturn. The surface temperature on Titan is a third of that on Earth, 90–94 degrees Kelvin.
As I showed in a paper with my former postdoc, Manasvi Lingam, life could potentially exist in the liquid water under the icy surface of even cooler objects. Any forms of life that are born under a globally opaque layer of ice would never see the stars and might never contemplate the science and technology for interstellar travel.
However, a technological civilization like ours which has noticed the stars since its inception, is capable of building rockets that reach other stars. Despite the imaginative scripts of science fiction films, interstellar journeys are long, boring and dangerous. With chemical propulsion, they take millions to billions of years, and holes drilled by impacts of energetic cosmic-rays or micrometeorites could have devastating consequences for biological entities. It makes more sense to launch purely technological objects with artificial intelligence for interstellar trips, rather than biological astronauts with natural intelligence.
Technological objects could potentially land on any surface, including that of non-habitable planets. Across the Milky-Way disk, the travel time of communication signals from distant destinations to the senders would be tens of millennia at the speed of light. Since this signal propagation time is longer than recorded human history, it would make most sense for technological interstellar travelers to be autonomous. After landing on a solid planetary surface, the technological ambassadors could potentially be programmed to establish technological infrastructure out of the raw materials they find near the landing site. The constructions they make would mark ownership of local resources. It could also flag the technological prominence of the senders. In such a case, the motivation of the senders would echo the sentiment expressed by President Donald Trump during a speech before the joint session of the U.S. Congress on March 5, 2025: “We are going to conquer the vast frontiers of science, and we are going to lead humanity into space and plant the American flag on the planet Mars and even far beyond.”
Given these possibilities, astronomers should also search for technological infrastructure on planets that are not conducive for life. The locations could involve free-floating planets throughout interstellar space with no star near them. Such rogue planets were discovered over the past two decades as magnifiers of the light emitted by background stars through gravitational microlensing, and their abundance might be larger than that of planets bound to stars. Free-floating planets could have been ejected gravitationally from their birth sites through dynamical instabilities. Some of them might be frozen Earth-like planets, worlds offering mineral resources similarly to their analogs around stars.
The landing sites could also involve warm planets without an atmosphere. A densely-packed planetary system, such as the seven rocky planets around the star TRAPPIST-1, offers ample real-estate opportunities for interstellar travelers who can use rocky raw-materials to construct their technological infrastructure.
It is therefore reasonable to search for technological signatures on non-habitable planets. Finding them would not only indicate that we are not at the technological center of the Universe, but also that other technological civilizations predated us by millions to billions of years since they reached Galactic destinations well outside their birth planetary system.
Exploring technological interstellar ambassadors will resemble the experience of the prisoners in Plato’s allegory of the cave: we will only witness the shadows of the senders and try to figure out their nature from their technological products.
It is possible that we will discover the products of technological civilizations that died by now. In that case, we will have no other option but to reconstruct their abilities and motivations from their relics. Finding technological signatures on non-habitable or free-floating planets could inspire us to imitate them. As Oscar Wilde noted: “Imitation is the sincerest form of flattery.”
This all makes sense in Nature. The Dandelion flower does not expect its seeds to report back, nor does it expect to witness their growth on fertile grounds far away.
We are all transient actors that come and go in an ever-evolving Universe. It is fun to watch the cosmic play and find out who was casted before our act started.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
