A technosignature that could detect an extraterrestrial civilization's reliance on nuclear fusion

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Extraterrestrial civilizations need a great deal of energy as they advance up the Kardashev scale. Fossil fuels are finite, wind and solar energy are carbon free but not as efficient as fossil fuels, and traditional nuclear fission power depends on a supply of fissionable material and has a waste problem. Thus, any advanced alien species may well turn to nuclear fusion for their ever-increasing energy needs (unless they've discovered even better energy processes we don't yet know about).

Deuterium (D) fusion is one of the simplest forms of nuclear fusion, where D fuses with tritium or another D. As life needs water as far as we know, oceans on an advanced world could supply plenty of it in ocean water.

On Earth, water contains a natural miniscule amount of heavy water, with deuterium replacing one or both hydrogen atoms to exist as HOD or DOH and rarely as D₂O. Extracting deuterium from an ocean would decrease its ratio of deuterium-to-hydrogen, D/H, including in atmospheric water vapor, while the helium produced in the nuclear reactions would escape to space. Could low values of D/H in an exoplanet's atmosphere be a technosignature of long-lived, uber-advanced extraterrestrial life?

That's what David C. Catling of the University of Washington started wondering a while back. "I didn't do much with this germ of idea until I was co-organizing an astrobiology meeting last year at Green Bank Observatory in West Virginia," he said.

When a brainstorming session on SETI was in the works, he did some preliminary analysis that eventually led to a three-party collaboration and a paper on the subject that will appear in the Astrophysical Journal and is published on the arXiv preprint server.

"Measuring the D/H ratio in water vapor on exoplanets is certainly not a piece of cake," he said. "But it's not a pipe dream either" according to his analysis. One big advantage of looking for low D/H values in an exoplanet's atmosphere is that it would persist even if advanced life died out on their planet or migrated away, increasing the chances of detecting this technosignature.

On Earth, where humanity is currently at 0.73 on the Kardashev scale, natural deuterium in the ocean accounts for about one atom in every 6,240 atoms of hydrogen, or 35 grams of deuterium for every ton of seawater. (That's a collective 4.85 × 10¹³ tons of deuterium.) The D/H ratio is nearly the same in our atmosphere. Deuterium can fuse with itself and, in a chain of nuclear reactions, ultimately produces 335 gigajoules of energy per gram of deuterium.

Using Earth as a model for an exoplanet with advanced life, Catling and his colleagues calculated fusion power of roughly 10 times that projected for humans next century, about 100 TW in 2100 for a population of 10.4 billion (five times more than today). That 1,000 terawatts (TW)—which could be a low amount for an advanced species (or their robotic descendants!)—would deplete an Earth-like ocean's D/H value to a value found in the local interstellar medium, about 16 parts per million, in about 170 million years.

"If the D/H ratio in the water of an exoplanet was found to be substantially below [interstellar medium] values...it would be strange and anomalous," the group writes in their paper.

If, by chance, their exoplanet had an ocean only a few percent of Earth's—a so-called "land planet"—D/H would reach anomalously low values in roughly 1 to 10 million years. That's on the order of the average lifetime of a mammalian species since the Chicxulub impact ended the dinosaurs, about 3 million years.

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Other planets have higher D/H values, like Venus and Mars, but processes like Venus's runaway greenhouse effect and physical escape processes on Mars have left both uninhabitable. Thus a higher D/H than Earth's "probably indicates a planet that is problematic for habitability on geologic timescales."

Calculations like these led the group to propose looking for unusually low D/H in planetary water vapor as a potential technosignature, which they call "potentially remotely detectable."

Using the Spectral Mapping Atmospheric Radiative Transfer (SMART) model, they proposed specific wavelengths to look for among the emission lines for of HDO and H₂O. HDO has strong lines in the infrared and near-infrared part of the electromagnetic spectrum, and in 2019 scientists first detected water vapor in the atmosphere of a potentially habitable planet.

Two missions in development, NASA's Habitable Worlds Observatory (HWO) that would follow the James Webb Space Telescope, and the European-led Large Interferometer For Exoplanets (LIFE), could possibly measure D/H.

"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," said Catling, noting that one scientist on the LIFE team is looking at the issue after reading their paper.

"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."

More information: David C. Catling et al, Potential technosignature from anomalously low deuterium/hydrogen (D/H) in planetary water depleted by nuclear fusion technology, arXiv (2024). DOI: 10.48550/arxiv.2411.18595

Journal information: Astrophysical Journal , arXiv

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