What if the key to finding extraterrestrial civilizations lies not in listening for signals but in scrutinizing water? A recent study suggests that advanced alien societies might leave behind a unique, long-lasting trace in an anomalously low ratio of deuterium to hydrogen (D/H) in their planetary oceans and atmosphere—a potential technosignature.

“Deuterium-deuterium (DD) fusion is viewed as an ideal energy source for humanity in the far future, given a vast seawater supply of D. Here, we consider long-lived, extraterrestrial, technological societies that develop DD fusion,” researchers wrote. “If such a society persists over geologic timescales, oceanic deuterium would diminish.”

Humanity has long looked to the stars for signs of intelligent life, often focusing on fleeting technosignatures like radio waves or pollutants.

However, a new study led by Dr. David C. Catling, a professor of Earth and space sciences at the University of Washington, proposes a paradigm-shifting approach: hunting for enduring markers of alien technology by measuring deuterium—a heavy isotope of hydrogen—in exoplanetary water.