A Good Place to Look for E.T.

What's a Dyson Sphere? Hint: It's not a round-shaped vacuum cleaner.

Fig. 1: Midjourney AI-generated image of a Dyson sphere.

The Dyson Sphere is a hypothetical megastructure physicist Freeman Dyson proposed in 1960.

According to his paper in Science magazine, a technologically advanced alien civilization would use increasing energy as it grew. As the most significant source of energy in any solar system is the parent star, sooner or later, the civilization will build orbiting solar panels to try to capture it. Such structures would take up more and more space until they eventually covered the entire star like a sphere.

In a 2008 interview with Slate, Dyson credited the concept to writer Olaf Stapledon, who introduced it in his novel Star Maker in 1937.

Dyson’s hypothesis was complicated to verify because a complete Dyson sphere, absorbing all of the light from the star, would be invisible to an exo-planet hunting telescope (such as NASA’s Kepler). Only half-completed spheres would have a chance to be discovered.

Unfortunately, a Dyson sphere is unlikely to remain under construction for long. The time it takes to make a Dyson sphere is relatively short. A 2013 paper by Stuart Armstrong and Anders Sandberg estimates that disassembling Mercury to make a partial Dyson shell could be done in 31 years.

An alternative would be to look for waste heat in the infrared. After being absorbed and used, the energy from a star needs to be reradiated, or else it would build up and eventually melt the Dyson sphere. This energy would be shifted to longer wavelengths, so a Dyson sphere might give off a peculiar energy signature in the infrared. In other words, Freeman Dyson saw a search for his namesake spheres as a complement in the infrared to what Frank Drake’s Search for extraterrestrial intelligence had begun to do with radiotelescopes (SETI.)

Carl Sagan and Russell Walker first voiced an issue with Dyson’s SETI notion in their 1966 paper “The Infrared Detectability of Dyson’s Civilizations” for the Astrophysical Journal. The authors noted that:

discrimination of Dyson civilizations from naturally occurring low temperature objects is very difficult, unless Dyson civilizations have some further distinguishing feature, such as monocromatic radio-frequency emission.

In the following decades, the search for Dyson spheres expanded dramatically. Starting in 1980, researchers used sources identified by the Infrared Astronomical Satellite (IRAS). These early searches produced little or no results, as most Dyson sphere candidates had non-technological explanations or needed further study. Subsequent investigations using NASA’s space-based WISE (Wide Field Infrared Survey), with higher resolution than IRAS, have all concluded that the identification of a promising source would not in itself be proof of an extraterrestrial civilization unless the object could be followed up with more conventional methods, such as laser or radio search.

Fig. 2: Midjourney AI-generated image of a Dyson sphere still under construction.

Among the latest developments concerning Dyson spheres are the following:

• Dyson spheres could be built around black holes instead of stars.

Black holes can radiate incredible amounts of energy (10⁵ more than the Sun) produced by the so-called “accretion disk” of gas and dust falling into the black hole’s maw. Due to their spiraling and spinning motions, these materials heat up through friction to millions of degrees, emitting extremely energetic X-ray photons.

But why would an alien civilization decide to build a Dyson sphere around a distant black hole (if it weren’t “distant,” the civilization would have been “eaten” long before it managed to construct the sphere) rather than using their much closer parent star? Black holes concentrate an enormous mass into a space area that is orders of magnitude smaller than a star’s and are, therefore, easier to encircle. On the downside, black holes often have bursts of activity followed by quiet periods as they consume varying lumps of matter in their disks. An alien species must protect their orbiting structures from the explosions that might destroy them.

• Dyson spheres could circle the husks of sunlike stars known as white dwarfs.

Every star has a finite lifetime. If a civilization arose around a typical sun-like star, that star would someday turn into a red giant and leave behind a white dwarf. That process would roast its solar system’s inner planets and freeze the outer ones as the white dwarf cooled off. Consequently, the civilization would have to choose between moving to another system or building a series of habitats that harvest the radiation from the remaining white dwarf. It seems unlikely that a civilization, no matter how advanced, would go through the enormous effort of traveling to another star only to build a Dyson sphere.

This allows a direct connection between stellar lifetimes and the prevalence of Dyson spheres.

If enough aliens decided to build Dyson spheres around their white dwarf homes, astronomers should find at least one Dyson sphere in white dwarf surveys. The presence of a megastructure like a Dyson sphere around a white dwarf would absorb part of its radiation and convert it into reusable energy. Since no conversion is 100% efficient, this process would leave behind waste heat that would escape as infrared light.

Fig. 3: Midjourney AI-generated image of a fully functional Dyson sphere.

Astronomers have already found many white dwarfs with excess infrared emission, usually explained as dust in those systems, not megastructures. According to a 2022 paper by retired UCLA astrophysicist Ben Zuckerman, no more than 3% of habitable planets around sunlike stars give rise to a white dwarf sphere-building civilization. Still, there are so many planets orbiting sunlike stars that this calculation only provides an upper limit of 9 million potential alien civilizations in the Milky Way.