Heavy hydrogen could be important in long-range detections of civilizations, says a pioneering astronomical study.
Kepler-1649 c (foreground) is an exoplanet similar in size and temperature to Earth, which orbits a red dwarf star. Credit: NASA/Ames Research Center/Daniel Rutter
To find advanced civilizations, you don’t need to go hunting for megastructures or hypothetical space probes. You could find civilizations just a few centuries ahead of us by looking for a key element: hydrogen.
While hydrogen is everywhere, not all of it is of the same type. Several hydrogen isotopes exist, and deuterium is one of the most stable. It contains one neutron and one proton, making it one neutron heavier than hydrogen. But deuterium also holds a whole lot of promise for nuclear fusion. It would create a lot more energy than the methods investigated on Earth, and burn hotter too.
In order to harvest deuterium, an alien civilization might draw from its oceans to continue the need for fuel. This, in turn, would cause a visible imbalance in the deuterium-to-hydrogen ratio, something potentially visible with next-generation telescopes. At least that’s according to a paper recently accepted for publication in the Astrophysical Journal.
Telltale sign
In that paper, David Catling of the University of Washington and his colleagues lay out this idea. What’s more, because deuterium and hydrogen are fairly stable, this imbalance would exist perpetually, and thus be easy to find, even if the alien civilization was long gone. But megastructures like a Dyson sphere — an artificial object that captures the energy of a star — may be available to only the most advanced civilizations, and thus may never reach it.
“That’s a rather drastic concept where you’re basically engineering an entire solar system,” Catling says. “No civilizations will make that leap in one go.”
On Earth, there’s been a lot of news about fusion in the last few years. Labs like the Lawrence Livermore National Laboratory have achieved, with great effort, a holy grail of fusion: The plasma at the center of the reactor has produced more energy than it used. It’s essentially the same concept as how the Sun works: You use hydrogen plasmas to fuse heavier elements like helium. But the fusion reactors on Earth use deuterium and tritium, an unstable isotope of hydrogen that has two neutrons.
Tritium has a half-life of around 12 years, and thus while it’s stable enough to use in the lab, it falls apart over time. It thus has to be produced, often from an isotope of lithium called lithium-6 that is itself rare. Catling and colleagues think that a civilization may eventually be able to capture stable deuterium-deuterium reactions, enough to make a small “artificial sun” that produces a whole lot of energy.
Finding worlds where the deuterium has been harvested, however, may be a challenge. You need a planet a lot like Earth, with abundant oceans acting as a deuterium reservoir. You also need to understand the chemical ratio of the star to figure out how much deuterium or hydrogen that planet should have.
Finding the sweet spot
Telescopes like the James Webb Space Telescope (JWST) aren’t really equipped to look at truly Earth-like worlds, ones with months-long orbits around their stars. Instead, it is more limited to Earth-size planets around the smallest stars, for which the jury is out on whether or not they’re even hospitable to life due to temperamental stars. JWST has spectrometers that analyze chemical elements of a stellar object. To detect characteristics of planets, this means looking for atmospheric fingerprints while a planet passes in front of its star, something that happens around these small stars (called red dwarfs) every week or couple weeks or so. While the telescope could, under the right conditions, find a ratio off enough to warrant further investigation, it would be challenging.
But Catling and his colleagues instead hope the paper can influence the astronomical community to build spectrometers with a sweet spot that would make a deuterium-to-hydrogen ratio much more apparent. And even if it doesn’t find aliens, it will still serve other useful purposes to have these capabilities.
“Even if you don’t find advanced extraterrestrials, it will still tell you something about the history of water for these planets and where it came from and how similar these solar systems are to our solar system,” Catling says.
