For more than 60 years, astronomers have searched the skies for artificial signals and other signs of extraterrestrial civilizations, yet no evidence of intelligent life beyond the Earth has so far emerged. We already know that planets are common - more than 2000 planets around other stars have already been detected (including several planets with properties reasonably similar to Earth). If one adopts the position that intelligent life should not be unique to our planet, this then leads to an apparent contradiction: If there is intelligent life beyond elsewhere in the Universe, why don’t we see any signs of this?

One viable explanation, among the many proposed for this so-called Fermi paradox, is that - even if planets and even primitive life is common – intelligent, technologically advanced lifeforms are extremely rare. Most SETI efforts have so far focused on our own cosmic backyard, the Milky Way galaxy, but if the probability for technologically advanced civilisations to arise is very small, we could well be alone in the Milky Way. If this is the case, our only chance of making a positive detection of extraterrestrial intelligence is to extend the search radius to extragalactic scales.

In the first project Hephaistos paper, we present a search for supercivilizations in a sample of more than 1000 galaxies similar to the Milky Way. Technically speaking, this search targets so-called Kardashev type III civilizations that have turned a fair fraction of the stars in their host galaxy into stellar engines (using a technology known as a Dyson sphere). While no strong Kardashev type III candidates are detected in our search, we are able to set an upper limit at 0.3% on the fraction of local disk galaxies hosting such civilizations. We also uncover a number of highly unusual galaxies that may open up new avenues for research within mainstream astronomy. We are now targeting these outliers for follow-up observations to learn more about the astrophysics underlying their extreme properties.

Paper link:

Extragalactic SETI: The Tully-Fisher relation as a probe of Dysonian astroengineering in disk galaxies [pdf]
Zackrisson, E., Calissendorff, P., Asadi, S., Nyholm, A. 2015, Astrophysical Journal, 810, 23

The classical way to search for Dyson spheres is to look for their waste heat signatures at infrared wavelengths. While thermodynamical considerations suggests that this signature is more or less unavoidable, there are many naturally occuring infrared-bright objects in the Milky Way, so finding auxiliary signatures of Dysonian technology would be very useful. In the second project Hephaistos paper, we explore one such secondary indicator - discrepant distance estimates.

By studing the optical spectrum of a star, astronomers are able to deduce its intrinsic luminosity. By comparing its apparent brightness to this inferred absolute brightness, a so-called spectrophotometric distance can be derived - this is a standard way of estimating the distance to stars. However, a near-complete Dyson sphere will significantly dim the optical light of the star it surrounds, while likely not significantly interfere with the shape of its optical spectrum. This is likely to lead to a significantly overestimated spectrophotometric. The Gaia space mission, on the other hand, can measure distances through its parallax, which is not expected to be affected by the presence of a Dyson sphere. Hence, Dyson spheres candidates with a sufficiently high covering fraction should exhibit discrepancies between their parallax and spectrophotometric distances.

In this paper, we combine data from Gaia DR1 (which provides the parallax) and RAVE DR5 (which provides spectrophotometric distances using spectroscopy from a groundbased telescope) to look for objects with discrepant distance estimates. While there are 230 000 objects which feature in both these datasets, only about 8000 stars have sufficiently small errors to make the comparison meaningful. We single out a handful of stars for which the distances deviate in the way expected for a near-complete Dyson sphere, and discuss one of these, TYC-6111-1162-1, in more detail. While our follow-up observations bascially confirm the spectrophotometric distance from RAVE, temporal changes in its radial velocity suggest the presence of an unseen companion which could have gotten Gaia confused about its parallax (Update: later Gaia data releases essentially confirm this). Hence, TYC-6111-1162-1 is clearly not a good Dyson sphere candidate.

Paper link:

SETI with Gaia: The observational signatures of nearly complete Dyson spheres [pdf]
Zackrisson, E., Korn, A.J., Wehrhahn, A., Ritter, A. 2018, Astrophysical Journal, 862, 21

By combining data from large astronomical catalogs at optical and infrared wavelengths, it may be possible to identify partial Dyson spheres in the Milky Way, as these are expected as turn up in such data sets as objects with anomalously low optical brightness yet very high mid-infrared fluxes. In this paper, we set the stage for the largest Dyson sphere search to date, covering on the order of one hundred million objects with distances measured by the Gaia space mission.

Scanning the data for objects with characteristics similar to those expected for Dyson spheres constructed around main-sequence stars does however reveal plenty of interlopers, primarily in the form of young stellar objects, which sometimes display spectral energy distributions very similar to those expected for near-complete Dyson spheres. Even without weeding out such interlopers, it is nonetheless possible to set conservative upper limits on the fraction of Milky Way stars that may be hosting Dyson spheres. Here, we present such upper limits for Dyson spheres as a function of their covering fraction, temperature and distance. For example, less than 1 in 50000 stars within 100 parsec could potentially host Dyson spheres that are 90 percent complete and are operating at 300 K.

Paper link:

Project Hephaistos - I. Upper limits on partial Dyson spheres in the Milky Way [pdf]
Suazo, M., Zackrisson, E., Wright, J.T., Korn, A., Huston, M. 2022, Monthly Notices of the Royal Astronomical Society 512, 2988

When scanning large databases of optical and infrared photometry data, it turns out that there are many objects in the Milky Way with spectral energy distributions that at first glance appear to be consistent with stars partially covered by Dyson spheres. The vast majority of these are, however, just young, dust-enshrouded stars, since objects of this type can display similar characteristics. Here, we use data from Gaia, 2MASS and WISE to attempt to weed out such interlopers. Using a number of methods, including convolutional neural networks, we start from a sample of 5 million stars within 300 parsec and work our way down to a handful of Dyson sphere candidates for which there are no obvious signs of youth. Are these objects truly Dyson spheres then? It's way too early to tell, but they are certainly the most Dyson sphere-like objects detected so far, and certainly interesting targets for follow-up studies with telescopes like the James Webb Space Telescope in coming years.

Paper link:

Project Hephaistos - II. Dyson sphere candidates from Gaia DR3, 2MASS, and WISE [pdf]
Suazo, M., Zackrisson, E., Mahto, P.K., Lundell, F., Nettelblad, C., Korn, A.J., Wright, J.T., Majumdar, S. 2024, Monthly Notices of the Royal Astronomical Society 531, 695