Dyson Spheres in the Milky Way?

Title: Project Hephaistos – II. Dyson sphere candidates from Gaia DR3, 2MASS, and WISE

Authors: Matías Suazo, Erik Zackrisson, Priyatam K. Mahto, Fabian Lundell, Carl Nettelblad, Andreas J. Korn, Jason T. Wright, Suman Majumdar

First Author’s Institution: Observational Astrophysics, Department of Physics and Astronomy, Uppsala University, Sweden

Status: Published in Monthly Notices of the Royal Astronomical Society [open access]

Civilizations of the Future

As civilizations progress, so does their need for greater sources of energy. Such is the simple but sound concept that drove astronomer Nikolai Kardashev to formulate his hypothetical classification of advanced civilizations based on the amount of energy they are able to control. Presented in 1964, the Kardashev scale proposes three stages: Type I: civilizations that utilize all available energy from their planet; Type II: civilizations that draw energy directly from their star; and Type III: civilizations that tap the energy of their entire galaxy. (Further stages are commonly accepted today and place our own civilization at Type 0.7, as we have not harnessed the full potential of our planet’s energy.)

In 1960, a similar minded physicist Freeman Dyson imagined that a futuristic civilization, with technology requiring more energy than their planet could provide, would create some sort of spherical structure around their host star to mine more of its energy, now called a Dyson sphere. Furthermore, he realized that if such a “Type II” civilization exists, we would be able to detect the waste-heat emission of their Dyson sphere as an excess of infrared (IR) radiation from the star it surrounds. Thus began the search for Dysonian technosignatures of advanced extraterrestrial life.

Project Hephaistos (appropriately named for the Greek god of blacksmiths and fire) heeds Dyson’s suggestion, with the help of modern observations and analysis. In 2018, their search for nearly complete Dyson spheres which dim the observed light of the star unfortunately left them empty-handed. But remarkably, in today’s paper their search for partial Dyson spheres, which only partially obscure the star’s light, has revealed seven candidates within the Milky Way!

Two spherical structures made of metallic hexagons. One structure has metallic mesh on the face of each hexagon, covering the entire surface of the sphere.
Figure 1: Example of a potential Dyson sphere structure. Left: Partial Dyson sphere. Right: Fully completed Dyson sphere. Image credit: https://hammer.bas.lv/, licensed under CC BY-SA 3.0 DEED
Two graphs with wavelength on the x-axis and magnitude on the y-axis. Each has colored vertical bars representing wavelength bands. On the graphs are black lines and gray lines, and the gray lines fall above the blacks lines in the infrared region of wavelengths.
Figure 2: Models of how a Dyson sphere modifies the photometry of a Sun-like star. The standard Sun-like star is the black line, while the gray lines are the varying Dyson sphere models. Each colored column represents an observed wavelength band, with W1-W4 covering the MIR, while the y-axis is magnitude. Top: Models with varying covering factors (the amount that the Dyson sphere blocks the light of the star). Bottom: Models with varying Dyson sphere temperatures. Figure 2 in the paper.

The Search for Dyson Spheres

The search began with a sample of stars exhibiting excess IR emission from Gaia Data Release 3 which maps stars in the Milky Way via parallax along with their fluxes in optical wavelengths, 2MASS which provides fluxes in the near-infrared (NIR), and AllWISE with fluxes in the mid-infrared (MIR).

This initial sample was then narrowed down to identify potential Dyson sphere candidates by removing stars for which the IR emission can be explained by natural phenomena.

First, stars were selected with distances less than 300 parsecs from Earth and with detections in the 12 and 22 micron MIR wavelengths, bands in which the IR excess is expected to be prominent. They then created theoretical models of how a star’s photometry, or magnitude in different wavelengths, would change if it were surrounded by a Dyson sphere. Specifically, some of the star’s light would be obscured, and the Dyson sphere would re-emit some of the light it had absorbed at longer wavelengths. Each star in the sample was matched to a best-fitting model, and those that could not be matched within reasonable errors were removed.

The authors then realized that young stars had made it through their selection criteria, as they were emitting excess IR for a variety of reasons. They removed stars that were in prominent nebulae which were causing contamination, and stars that were still forming with accretion disks, identified by optical brightness variability and Hydrogen-alpha emission from ionization as a star heats up. (And, of course, an advanced civilization isn’t likely to be building an enormous structure around a star that is still forming.)

Final cuts to the sample of potential candidates included ensuring the parallax-based distances were well constrained, each source was a point source, and the MIR detection signal-to-noise ratio was sufficiently high. By now, the initial sample of 5 million sources had been reduced to 368, a reasonable number for humans to evaluate by eye. After removing any sources that looked to have irregular structures or nebular features, seven Dyson sphere candidates remained.

A grid with two graphs in the leftmost column, and images in all other columns. The graphs plot wavelength on the x-axis and magnitude on the y-axis, and show black (data) and gray (model) lines with consistent shapes. The images are of the candidate objects in different wavelength bands, and show a black circle at the center of each.
Figure 3: Two Dyson sphere candidates photometric images and fits to Dyson sphere models. Figure 7 in the paper.

Could it be?

Before you call home – other possibilities remain. The excess IR emission could be explained by a chance alignment with a distant galaxy, or be a result of processes we don’t yet understand, like curious young stars with no optical brightness variation, or warm debris disks around M-dwarf stars. More analysis is definitely needed to understand the true nature of these seven sources, but like Jocelyn Bell Burnell and her Little Green Man, sometimes the thrill of finding life out there leads to the discovery of astronomical objects we didn’t even know to look for. Or aliens.

Astrobite edited by Samantha Wong

Featured image credit: Kevin Gill, https://www.flickr.com/photos/kevinmgill/29401385502/, licensed under CC BY 2.0 DEED

About Annelia Anderson

I'm a 4th year Astrophysics Ph.D. student at the University of Alabama. My current research uses simulations to study the circumgalactic medium and inform interpretations of observations. Beyond research, I enjoy playing piano and guitar, cooking, spending time with my cat, and I hope to someday write astronomy children's books.

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