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.
The research, published in a preprint on arXiv, delves into how advanced extraterrestrial civilizations’ use of nuclear fusion technology could affect a planet’s water composition.
Fusion reactors, particularly those relying on deuterium-deuterium (DD) reactions, consume deuterium as fuel. Over millennia, this would drastically lower the D/H ratio in oceans and water vapor.
Unlike radio signals, which require an active civilization, deuterium depletion persists for eons. So even if a civilization disappears or moves on, this signature could remain—a ghostly trace of its technological existence.
The D/H ratio in water is a well-known marker in planetary science. Earth’s oceans have a relatively high D/H ratio due to chemical processes during the planet’s formation.
By contrast, the interstellar medium (ISM) has a much lower D/H ratio, shaped by Big Bang nucleosynthesis and stellar evolution processes. If an exoplanet’s water shows an abnormally low D/H ratio well below the ISM baseline, it could signal prolonged DD fusion activity.
Detecting such anomalies hinges on advanced telescopic technologies. Instruments like the James Webb Space Telescope (JWST) and future missions like NASA’s Habitable Worlds Observatory (HWO) or the European Large Interferometer for Exoplanets (LIFE) could analyze water vapor in the atmospheres of distant worlds. Key spectral features, such as the 3.7-micron wavelength of deuterium-enriched water, provide a promising window for these investigations.
For an Earth-like planet with a comparable ocean mass and initial D/H ratio, the study estimates that a civilization consuming 1,000 terawatts of power through DD fusion—10 times the energy projected for humanity in the next century—could deplete deuterium to anomalous levels within 167 million years. This timeline could shrink to as little as 10 million years on planets with smaller oceans.
Researchers note that if advanced civilizations exist and rely on DD fusion, as hypothesized, their technological activities could create detectable technosignatures that persist long after their presence on a planet.
The proposal of using DD fusion as a detectable signature of advanced alien life adds to the growing list of innovative technosignature ideas. Traditional methods, such as searching for radio waves or atmospheric pollutants like chlorofluorocarbons (CFCs), are constrained by their short lifespans. However, the deuterium depletion signature offers a near-permanent record, potentially revolutionizing how we think about finding alien life. In essence, it doesn’t rely on detecting life as it happens. Instead, it opens the doorway to searching for technological fingerprints etched into the fabric of a planet.
The search for deuterium depletion represents a novel approach, but it is part of a broader effort to investigate unusual planetary features as potential indicators of life or advanced technology.
Among these intriguing possibilities is the concept of Dyson Spheres, theoretical megastructures constructed around stars to capture energy, which could be identified through anomalous infrared signatures.
Another idea involves detecting artificial illumination; some researchers propose that city lights on exoplanets might emit unique, detectable wavelengths.
Additionally, planets with irregular or erratic orbital patterns could signal the gravitational influence of massive alien engineering projects. Even the spectral signatures of uniquely evolved alien ecosystems have been considered, as they could offer biosignatures that hint at the presence of life far different from what we know on Earth.
Together, these ideas expand the scope of technosignature studies, pushing the boundaries of how we search for extraterrestrial civilizations.
Beyond theoretical technosignatures, the oceans on Earth continue to astound scientists with bizarre discoveries, like the giant tube worms recently discovered in deep-sea hydrothermal vents.
In the context of alien worlds, such phenomena remind us how varied life can be and how it might appear elsewhere. Similarly, deuterium depletion as a technosignature could represent the tip of the iceberg in detecting alien life.
Ultimately, the quest for life beyond Earth has entered an exciting era, with telescopes like JWST already unveiling detailed atmospheric compositions of exoplanets. The idea of searching for technosignatures like deuterium depletion promises to broaden this effort, challenging scientists to develop even more sensitive tools and techniques.
Whether or not humanity discovers such a technosignature, the journey promises to deepen our understanding of the cosmos—and perhaps our place within it.
“It’s up to the engineers and scientists designing [HWO] and [LIFE] to see if measuring D/H on exoplanets might be an achievable goal,” Dr. Catling told Phys.org.
“What we can say, so far, is that looking for D/H from LIFE appears to be feasible for exoplanets with plenty of atmospheric water vapor in a region of the spectrum around 8 microns wavelength.”
Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com