Upcoming space missions could possibly look for these wavelength emissions and provide compelling evidence for advanced life.
Scientists employ many methods in the search for alien life. One approach is using both terrestrial and space-based advanced telescopes for some good, old-fashioned stargazing. Radio astronomy has also produced some exciting results, even if they didn't yield any aliens (yet).
Then there are the strategies that involve imagining ways that advanced civilizations would try to meet their increasing energy demands. While some scientists have speculated how to detect Dyson rings -- megastructures encircling a star -- from Earth, University of Washington's David Catling is considering another not-so-far-future energy signature: nuclear fusion.
Fusion is the end-goal of terrestrial energy generation on Earth. By fusing together plasma of two light nuclei (in this example, two atoms of the hydrogen isotope deuterium), the strong nuclear force binds together these isotopes into Helium-3 and releases tremendous amounts of energy.
The one benefit of using only deuterium is that unlike many other fusion candidates (especially tritium), Earth's oceans and atmosphere contain a lot of the isotope, also known as heavy water -- around 35 grams for every ton of seawater. According to Catling's new paper published on the preprint server arXiv -- it's been accepted for publication in The Astrophysical Journal -- astronomers could possibly detect the presence of such a deuterium-deuterium fusion regime on another planet by closely analyzing its atmosphere.
"Deuterium-deuterium (DD) fusion is viewed as an ideal energy source for humanity in the far future, given a vast seawater supply of D," the authors write. "Here, we consider long-lived, extraterrestrial, technological societies that develop DD fusion. If such a society persists over geologic timescales, oceanic deuterium would diminish."
To figure out what this depletion would look like, Catling and his team devised an energy expedition to an advanced civilization at around 10 times what's expected to be humanity's energy needs by 2100, which comes in at about 1,000 terawatts. The authors found that a planet's deuterium-hydrogen (DH) value would decrease about 16 parts per million in 170 million years when compared to the surrounding interstellar medium. If a planet has more land than Earth, it could take only a few millions years for the DH value to be detectable.
Of course, Earth's stellar neighbors -- Venus and Mars -- both have higher D/H values, but they're also uninhabitable, and the authors note that planets with higher DH values than Earth likely point to the inhabitability.
So, how do we exactly find these planets? The authors suggest scanning for specific wavelengths for semi-heavy water (HDO) and H2O. This isn't impossible, as scientists successfully detected water vapor on an exoplanet back in 2019. According to Phys.org, two space missions in development -- the Habitable Worlds Observatory and the Large Interferometer For Exoplanets -- could potentially measure D/H values indicative of advanced civilizations.