Project Hephaistos I. Upper limits on partial Dyson spheres in the Milky Way

ABSTRACT Dyson spheres are hypothetical megastructures built by advanced extraterrestrial civilizations to harvest radiation energy from stars. Here, we combine optical data Gaia DR2 with mid-infrared AllWISE set the strongest upper limits date on prevalence of partial within Milky Way, based their expected waste-heat signatures. Conservative presented fraction stars at G ≤ 21 that may potentially host non-reflective absorb 1–90 per cent bolometric luminosity and emit thermal in 100–1000 K range. Based a sample ≈2.7 × 105 100 pc, find less than ≈2 10−5 could ∼300 90 completion. These become progressively weaker for complete due increased confusion naturally occurring sources strong radiation, also larger distances, detection WISE. For ∼2.9 108 5 kpc our Way sample, corresponding limit be completion is ≲8 10−4.