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.