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 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’sDragonfly 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 liquid water under the icy surface of even cooler objects. Any form of life 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, can build 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. Launching purely technological objects with artificial intelligence for interstellar trips makes more sense than biological astronauts with natural intelligence.
Technological objects could land on any surface, including 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 the most sense for technological interstellar travelers to be autonomous.
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