Universe Today
One of the challenges of searching for “technosignatures” (i.e. signs that intelligent life somewhere in the universe has created technology) is understanding what to look for. Technology is a very broad area, and different types would show up as different features. One of the most commonly cited is a Dyson sphere, which attempts to encapsulate a star and capture it’s outgoing light to produce energy. But while we’ve looked for the mid-infrared waste heat these structures would produce for decades, we haven’t found a definitive instance of one. According to a new paper, available in pre-print on arXiv by Turkish high school student Sahin Torlakcik, that might be because we are looking for the wrong type of energy all together.
In his paper, Torlakcik introduces the concept of Stellar J-Harvesting - building a system that could deliberately extract a star’s rotational angular momentum. To be clear, this solution wouldn’t capture the same energy output of a full Dyson sphere (or, more accurately, swarm), but it does have two massive advantages - it requires much less physical material to build, and it would generate waste heat that was millions of times lower than the stars luminosity, essentially making it “invisible” to most infrared surveys.
So how would this work in practice? You can’t simply set up a frictional braking system on a star. Instead, you would have to sap its energy by using one of a few different electromagnetic coupling techniques.
Fraser talks about how we can search for technosignatures.According to the paper, one technique would be to build a massive conducting structure that is embedded in the solar wind, and extracts angular momentum using Alfvén-wave coupling. These low-frequency oscillations of magnetic fields can interact with the tether, transferring some of the stars angular momentum into the tether itself.
An alternative approach is to create what is essentially a giant orbital flywheel. Building a massive ring at approximately 1 AU (the distance from the Sun to the Earth) could capture angular momentum from the star via Lorentz-force coupling. Using the Lorentz force, the fundamental force exerted on a charged particule by a magnetic field, this megastructure could scavenge angular momentum from the star by using its magnetic field to push on a massive electrically conductive ring.
Another interesting alternative is a synchrotron spindown array. Synchrotron radiation is a form of radiation that occurs when a particle, such as an electron, travels a curved path near the speed of light. When these particles travel along a magnetic field line, they lose energy by emitting highly directional beams of radiation, known as synchrotron radiation.
Fraser talks about what happens if we do find one of these megastructures out in the galaxy.By placing an array of extremely strong electrically conductive structures in the flow of the solar wind, this technosignature would accelerate those charged particles to near relativistic speeds, and thereby emit synchrotron radiation. Since Newton’s Third Law holds that every action has an equal and opposite reaction, this radiation has a reactive torque that pushes the array, which, since it is coupled to the star’s stellar wind, thereby opposes and slows down the star’s rotation. Crucially, this technique would also emit a specific technosignature of its own, in the form of radio or X-rays, depending on the speed and field strength of the system.
Some of these techniques are similar to those seen in another technosignature idea - starlifting - whereby an advanced civilization intentionally harvests material from a star, either to use that material to build megastructures or to artificially extend the star’s life by lowering its mass. However, this particular use case has a very distinct pattern that we can easily search for - stars that are rotating more slowly than their peers.
So Torlakcik began that search himself. He selected the Kepler field of stars for a first look, and sorted the stars by color and surface gravity, while also applying strict filters to eliminate natural false positively (like subgiants and pre-main-sequence stars). He was left with a dataset of 6,725 FGK main-sequence stars. Out of this group, two interesting candidates emerged - KIC 67606183 and KIC 9834255. Both are G-type main sequence stars, but spin at a relatively slow 61 and 65 days respectively, in comparison to the 5 to 10 days stars their age normally take.
To be clear, Torlakcik does not claim to have found a rotational sapping megastructure at these two locations. In fact, their slow rotation rates are likely due to some other mundane astrophysical phenomena such as an unresolved binary or low metallicity. But at the very least, maybe it's worth a look at these two stars in particular to see if we can figure out why, exactly, they are rotating so much slower than their peers.
Time for more observations, then, and Torlakcik has several ideas of follow up studies that could help narrow the search for hints of these types of megastructures in the future. Luckily, he will have a long time to continue that search. After all, he’s only in high school, and already starting to make a name for himself in the search for technosignatures.
Learn More:
S. Torlakcik - Stellar J-Harvesting: a novel angular momentum technosignature and first search in the Kepler field
UT - Could Alien Solar Panels Be Technosignatures?
UT - How Hidden Stars Shape Our Search for Technosignatures
UT - A New Search for Evidence of Technological Civilizations in the Milky Way
