In the search for "technosignatures"—traces of extraterrestrial intelligent life—a seed planted by a single physicist more than half a century ago continues to powerfully stimulate the imagination of astronomers today. The idea was proposed in 1960 by Freeman Dyson in a short paper published in the journal Science. It is the hypothesis that a highly advanced civilization, in order to meet its ever-growing energy demands, would eventually surround an entire star with countless energy collectors, harnessing all of that star's radiated energy without waste.
This colossal artificial structure later came to be known as a "Dyson sphere." In science fiction, Dyson spheres are often depicted as a single rigid shell completely enveloping a star. However, from a physical standpoint, no material in the universe could maintain a thin, giant solid shell while balancing the powerful gravity and rotational centrifugal force of a star. Even a slight shift in the center of mass would cause it to collide with the star. For this reason, modern scientists envision not a single continuous shell but rather a "Dyson swarm"—countless independent orbiting satellites and heat collectors flying around the star in a swarm. This can be seen as a symbol of what Russian astronomer Nikolai Kardashev, in a 1964 scale describing civilizational development, called a "Type II civilization": one that fully controls the energy emitted by its own home star.
In searching for this tremendous structure across the cosmos, astronomers have long focused on main-sequence stars similar in mass and temperature to our own Sun. Given that life arose on the planet Earth and a technology-wielding civilization developed here, it was an extremely natural approach to assume that advanced technological civilizations might exist around stars with similar conditions.
However, targeting Sun-like stars presents a major engineering obstacle. Consider the scale required to build a Dyson swarm around our Sun. If it were to envelop the Sun at a radius equal to Earth's orbit, the surface area would reach hundreds of millions of times that of Earth's surface. Estimates suggest that even if Jupiter—the largest planet in the solar system—were completely disassembled and all its matter converted into construction material, the resulting structure would be only a few meters thick. Launching countless space shuttles, each weighing 1,950 tons (equivalent to 1,300 small cars), would fall far short of what's needed; manipulation of matter on a planetary scale would be required.
On top of that, Sun-like stars exhaust their nuclear fusion fuel within a few billion years, expanding into red giants. In this process, any planets in inner orbits, along with any painstakingly built megastructures, would be engulfed and destroyed by the star's superheated gas. For a hyper-advanced civilization aiming to thrive on astronomical timescales, investing enormous costs into a star with an expiration date of only a few billion years hardly seems like a rational choice.
The Rational Choice of a Hyper-Advanced Civilization Avoiding Sun-Like Stars
Physicist Amirnezam Amiri of the University of Arkansas and colleagues, in a new study published on the preprint server arXiv, present a target that fundamentally overturns the premise of the search. After discussions with Harvard University astronomer Avi Loeb and others, the team chose to simulate objects that have often been overlooked as protagonists of technosignature searches: the dim, small stars of the galaxy—namely red dwarfs (M-type dwarfs) and white dwarfs.
Red dwarfs, making up about 70 percent of the stars in the galaxy, are the most common type of star in the universe. Because their mass is only a fraction to about a tenth of the Sun's, their core temperatures never rise very high, and nuclear fusion proceeds extremely slowly. Burning quietly while conserving fuel, their lifespans reach trillions of years—far exceeding the current age of the universe, 13.8 billion years—making them stable energy sources over an extraordinarily long period.
Red dwarfs also have a violent side: in their youth, they emit intense flares that can strip away the atmospheres of surrounding planets. From the standpoint of the origin of life, this is a harsh environment, but the situation differs for a civilization that has already established advanced technology. Such a civilization would either choose an older star whose flare activity has calmed down, or possess the technology to capture even sudden bursts of energy release with a vast network of swarms and convert it into electricity. For a civilization envisioning a long-term survival strategy spanning tens of millions or even billions of years, there is no more attractive base than a red dwarf.
White dwarfs, on the other hand, are the dense remnants left behind after a Sun-like star reaches the end of its life and blows off its outer layers of gas into space. Nuclear fusion in the core has already ceased, but the enormous stored heat allows them to keep glowing for billions of years or more. While retaining about half the mass of the Sun, their physical size shrinks dramatically to about 1 percent of the original star—roughly the same size as Earth.
This small size makes white dwarfs an ideal stage for cosmic engineering. Because the star itself is so small, a swarm of energy collectors could be placed just a few million kilometers from the star's surface—an extremely close distance. The entire system could be compactly contained within a range only a few times the distance between Earth and the Moon. A smaller orbital radius means far less material is needed to completely cover the same area without gaps. Compared to building around a Sun-like star, the amount of resources required is reduced by several orders of magnitude, greatly lowering the barrier to construction. Although any former planetary system would likely have been destroyed when the original star expanded into a red giant, this scenario is consistent with either a surviving civilization rebuilding around the remnant, or a civilization that migrated from another star system using it as a base.
| Comparison Item | Conventional Search Target (Sun-like Stars) | New Search Target (Red Dwarfs / White Dwarfs) |
|---|---|---|
| Rationale for search | Similarity to the environment in which life arose on Earth | Lifespans of trillions of years, or drastically reduced construction material requirements |
| Physical constraints | Requires disassembling a Jupiter-scale mass of material to cover a huge surface area | For white dwarfs, since the size has shrunk to about 1 percent of the original, construction is possible at close range |
| Observational characteristics | Easily blends into regions of known main-sequence stars, making it hard to distinguish from natural phenomena | Shifts unnaturally into a void region of extremely low temperature (e.g., 3000 K to 50 K) while maintaining its luminosity |
An Unnatural Void Appearing in a Star's Vital Records
So, if a Dyson swarm were built around one of these dim stars, how could it actually be detected through astronomical observation?
Amiri's research team calculated where these structures would appear on the coordinates of the most fundamental tool in astronomy: the Hertzsprung-Russell diagram (H-R diagram). Devised in the 1910s by Ejnar Hertzsprung and Henry Norris Russell, this diagram plots a star's surface temperature on the horizontal axis and its total energy output (luminosity) on the vertical axis. Most of the stars shining in the night sky fall within a band-shaped region called the main sequence, occupying specific positions depending on their mass and age. The H-R diagram can be thought of as a kind of vital record classifying the evolutionary stage of stars.
A Dyson swarm absorbs the star's light, and the structure itself, warmed by that heat, re-radiates long-wavelength infrared light into space. In this process, the total amount of energy emitted by the entire star system does not change. What happens is that while the luminosity is preserved, the observed temperature drops to an extreme degree. Even though the heat output of the star inside remains unchanged, only the heat leaking from the outside is observed at an extremely low temperature.
The peak wavelength of thermal radiation follows Wien's displacement law, which states that it is inversely proportional to surface temperature. Expressed as an equation, this is . This is a simple physical law stating that, with the constant $b$ in the numerator and the absolute temperature $T$ in the denominator, the wavelength lengthens as the temperature drops.
Looking at specific numbers highlights just how anomalous this would be. For example, suppose a red dwarf with a surface temperature of 3000 K is completely enveloped by a swarm. This star system would be observed as an extremely cold object at 50 K, without any reduction in luminosity whatsoever. As the temperature drops to one-sixtieth of its original value, the peak wavelength of the emitted light becomes 60 times longer, shifting entirely from visible light into the far-infrared. It would fade into the darkness of space, invisible to the naked eye or optical telescopes, but through the eyes of infrared sensors, the star's inherently enormous energy would eerily emerge as heat. Under the physical laws of nature and the rules of stellar evolution, no star occupies this region of the H-R diagram—high luminosity combined with extremely low temperature. It would be recorded as an anomalous heat source located in a void where no natural star could possibly exist.
The Absence of Silicates and Irregular Light Leakage Determine Authenticity
However, discovering an object with extremely low temperature and strong infrared emission is not enough to immediately conclude that it is artificial. The universe contains many natural phenomena that emit strong infrared radiation, such as dust disks surrounding young stars or distant galaxies shrouded in thick dust.
A Smooth Spectrum Close to Blackbody Radiation and Non-Periodic Variability
Amiri's team also presents a clear dividing line for distinguishing these natural phenomena from genuine technosignatures. The ordinary dust disks surrounding young stars exhibit a distinctive infrared spectrum caused by silicate components. Structures made of advanced metals or artificial materials, on the other hand, lack this feature and instead trace an extremely smooth spectrum close to blackbody radiation.
Furthermore, if what covers the star is not a completely continuous, gap-free shell but rather a swarm of countless independent heat collectors in orbit, then the star's original light would intermittently leak out through gaps in the structure. As countless massive artificial objects orbit the star at high speed, this would produce irregular, non-periodic flickering clearly distinct from the natural activity of the star itself, such as starspots or flares. It is extremely difficult to explain a natural phenomenon that simultaneously possesses all three characteristics: an anomalous temperature shift, the absence of silicates, and irregular light variation.
The Latest Project "Hephaistos" Identifies 7 Candidates
This theoretical framework has already begun to be cross-referenced with actual observational data. In 2024, a search project called "Project Hephaistos," led by Matías Suazo of Uppsala University and colleagues, combined multiple wide-field observational datasets, including those from the European Space Agency's Gaia satellite and NASA's WISE satellite.
The research team used convolutional neural networks—a type of deep learning also used in image recognition—to narrow down candidates from data on approximately 5 million stars. The mid-infrared data captured by the WISE satellite contains a large number of false signals caused by overlapping interstellar material and instrumental glitches. The neural network learned these spatial image patterns and automatically separated genuine point-source radiation from noise.
Afterward, rigorous filtering was applied based on multiple parameters, including the precision of distance measurements to stars derived from Gaia's parallax data and the energy distribution from near-infrared to mid-infrared wavelengths, narrowing the candidates down to 368. Finally, after human visual confirmation excluded objects with irregular structures or nebular characteristics, seven red dwarfs were ultimately extracted as Dyson sphere candidates.
The Wall of False Positives and Hopes for the James Webb Space Telescope
But whether these are genuine technosignatures remains uncertain. In astronomical searches, false positives—natural phenomena that appear artificial—are an ever-present risk.
Even if a candidate object appears to emit strong infrared radiation, it's possible that a distant background galaxy simply happens to overlap along the same line of sight. Given that we observe the vast three-dimensional universe projected onto a two-dimensional celestial sphere, it is not easy to distinguish between radiation from an active galactic nucleus billions of light-years away and thermal radiation from a Dyson sphere just a few thousand light-years in the foreground. In particular, the thick rings of dust surrounding active galactic nuclei have characteristics close to extremely low-temperature blackbody radiation, closely resembling the infrared spectrum of an artificial object. Indeed, for one candidate object, subsequent analysis revealed that a background black hole was emitting strong infrared radiation.
Alternatively, there remains the possibility that these are peculiar young stars we don't yet fully understand—ones that don't show accompanying optical brightness variation—or unknown warm dust disks surrounding red dwarfs.
The highly sensitive infrared instruments of the James Webb Space Telescope, along with the large-scale survey to be conducted by the Nancy Grace Roman Space Telescope scheduled for launch later this year, will help reveal the true nature of these peculiar heat sources. Observations of the darkness of the universe will provide the answer to where advanced civilizations might be hiding.