Some of the galaxy's unusually cold 'stars' could actually be alien megastructures

The most likely way of detecting an alien civilization is to observe its network of solar collectors. As aliens are likely to build a massive network of solar collectors that can harness the energy of a star. In the early 1960s, physicist Freeman Dyson proposed that such a structure could exist as a ‘sphere’, a swarm of smaller components around a star. Since then, the structure has been named after Dyson, as a "Dyson sphere" or "Dyson swarm." Physicists say that, in theory, it could exist. But, how would it look if we were to catch a glimpse of it? Physicist Amirnezam Amiri at the University of Arkansas attempts to answer this question in a paper available on the arXiv pre-print server. In the paper soon to be published in the journal Universe, he describes the types of stars around which we can look for an energy-harvesting sphere of an advanced civilization.

The best candidates to search around are red dwarf stars, the most abundant type in the Milky Way. They burn their nuclear fuel very slowly, allowing them to live for trillions of years. This means that they can live far longer than the age of the universe. An alien civilization could build a Dyson swarm using low-cost materials 0.05 to 0.3 AU (astronomical unit) away from a star. One AU is the distance that separates the Sun from Earth. Physicists argue that the second type of stars whose energy can be harnessed are white dwarfs. When sun-like stars exhaust their nuclear fuel, they become white dwarfs, shrinking incredibly to 1% of their original radii. An alien civilization can build a Dyson sphere a few million kilometers away from the surface of such a star. They don’t need to design a supermassive structure that is needed around a larger star. Furthermore, the white dwarfs will give off their energy for billions of years, providing a long-term source of power. 

What would a star look like if surrounded by such a megastructure? Such a structure will not only absorb a star’s energy but also prevent it from escaping into space, blocking part of its light in the process. To deal with this, astronomers employ a tool called the Hertzsprung-Russell, or H-R diagram. It is used to classify stars based on their temperature and luminosity. Inclusion of a Dyson sphere completely changes the outcome of the diagram and a star’s position in it. According to the laws of physics, energy cannot be created or destroyed. So, a Dyson sphere can be thought of as a shell that harnesses and utilizes a star’s energy to sustain a civilization, then radiates it as heat. Any object absorbs heat and radiates it at infrared wavelengths. A Dyson sphere must do the same. 

This moves the star toward the right part of the diagram, where stars with lower temperatures are found close to each other. However, the star’s original luminosity doesn’t change, but a massive structure around it shifts it to infrared. But, how far on the right part of the diagram would the star go? A red dwarf with a surface temperature of around 3000K will occupy the lower right-hand corner of the diagram. Such a star surrounded by a Dyson sphere would have a temperature down to 50K—two orders of magnitude lower. One more factor to detect a Dyson swarm is to look for signals coming from its dust-free radiator panels. A star without it gives its signature silicate emission that signals the presence of dusty disks. Modern calculations show that a full-fledged Dyson sphere is implausible. 

Since a Dyson sphere would emit light at infrared wavelengths, the suite of infrared detectors on the James Webb Space Telescope could be the best bet to monitor and hunt down such structures. Older telescopes such as Gaia and WISE are being used to locate them. In a study published in May 2024, astronomers identified seven strong Dyson sphere candidates, scanning a catalogue of 5 million stars. These candidates are white dwarfs. One of them has been rejected because a supermassive black hole in the background tweaked its readings. The rest warrant closer inspection. With Amiri’s paper providing key insights, we inch closer to our hunt for technologically advanced aliens.

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