Kepler-1649 c (foreground) is an exoplanet similar in size and temperature to Earth, orbiting a red dwarf star. Credit: NASA/Ames Research Center/Daniel Rutter
To find advanced civilizations it is not necessary to hunt for megastructures or hypothetical space probes. You might find civilizations just a few centuries ahead of us by searching for a key element: hydrogen.
Even though hydrogen is everywhere, it’s not all the same type. There are several isotopes of hydrogen and deuterium is one of the most stable. It contains a neutron and a proton, making it a heavier neutron than hydrogen. But deuterium is also very promising for nuclear fusion. It would create much more energy than methods studied on Earth and would also burn at higher temperatures.
To harvest deuterium, an alien civilization could tap into its oceans to continue meeting its 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 an article recently accepted for publication on Astrophysics Journal.
Telltale sign
In that paper, David Catling of the University of Washington and his colleagues lay out this idea. Furthermore, since deuterium and hydrogen are quite stable, this imbalance would exist forever, and therefore be easy to find, even if the alien civilization had long since disappeared. But megastructures like a Dyson sphere – an artificial object that captures the energy of a star – may only be available to the most advanced civilizations, and so may never reach it.
“This is a pretty drastic concept where you’re basically designing an entire solar system,” Catling says. “No civilization will be able to make this leap in one go.”
On Earth in recent years there has been a lot of talk about fusion. Laboratories like Lawrence Livermore National Laboratory have, with great effort, achieved the Holy Grail of fusion: the plasma at the center of the reactor produced more energy than was used. It’s essentially the same concept as how the Sun works: you use hydrogen plasma to fuse heavier elements like helium. But fusion reactors on Earth use deuterium and tritium, an unstable isotope of hydrogen that has two neutrons.
Tritium has a half-life of about 12 years and so, although it is stable enough to be used in the laboratory, it breaks down over time. It must therefore be produced, often from an isotope of lithium called lithium-6 which is itself rare. Catling and colleagues think that a civilization might eventually be able to capture stable deuterium-deuterium reactions, enough to create a small “artificial sun” that produces large amounts of energy.
Finding worlds where deuterium has been harvested, however, could be a challenge. You need a planet very similar to Earth, with abundant oceans to act as a deuterium reservoir. You also need to understand the chemical ratio of the star to understand how much deuterium or hydrogen that planet has Should Have.
Find the sweet spot
Telescopes like the James Webb Space Telescope (JWST) aren’t really equipped to observe truly Earth-like worlds, those with months-long orbits around their stars. Instead, it is more limited to Earth-sized planets around smaller stars, for which it is not yet clear whether or not they are hospitable to life due to capricious stars. JWST has spectrometers that analyze the chemical elements of a stellar object. To detect planetary features, that means looking for atmospheric signatures as a planet passes in front of its star, something that happens around these small stars (called red dwarfs) every week or couple of weeks or so. While the telescope could, under the right conditions, find a report bad enough to warrant further investigation, it would still be a challenge.
But Catling and his colleagues instead hope the paper will influence the astronomical community to build spectrometers with a sweet spot that would make the deuterium-to-hydrogen ratio much more apparent. And even if he doesn’t find the aliens, having these abilities will still serve other useful purposes.
“Even if you don’t find advanced extraterrestrials, it will still tell you something about the history of water for these planets, where it came from, and how similar these solar systems are to our solar system,” Catling says.
