Project Hephaistos I. Upper limits on partial Dyson spheres in the Milky Way (2201.11123v2)

Abstract: Dyson spheres are hypothetical megastructures built by advanced extraterrestrial civilizations to harvest radiation energy from stars. Here, we combine optical data from Gaia DR2 with mid-infrared data from AllWISE to set the strongest upper limits to date on the prevalence of partial Dyson spheres within the Milky Way, based on their expected waste-heat signatures. Conservative upper limits are presented on the fraction of stars at G $\leq$ 21 that may potentially host non-reflective Dyson spheres that absorb 1 - 90$\%$ of the bolometric luminosity of their host stars and emit thermal waste-heat in the 100 - 1000 K range. Based on a sample of $\approx$ $2.7\mathrm{e}\,5$ stars within 100 pc, we find that a fraction less than $\approx$ $2\mathrm{e}\,-5$ could potentially host $\sim$300 K Dyson spheres at 90$\%$ completion. These limits become progressively weaker for less complete Dyson spheres due to increased confusion with naturally occurring sources of strong mid-infrared radiation, and also at larger distances, due to the detection limits of WISE. For the $\sim2.9\mathrm{e}\,8$ stars within 5 kpc in our Milky Way sample, the corresponding upper limit on the fraction of stars that could potentially be $\sim$300 K Dyson spheres at 90$\%$ completion is $\leq$ $8\mathrm{e}\,-4$.

• The paper establishes unprecedented upper limits on the prevalence of partial Dyson spheres by identifying photometric anomalies using Gaia and AllWISE data.
• It employs color-magnitude diagram analysis to differentiate potential Dyson sphere signatures from normal stellar phenomena within a 100 pc to 5 kpc range.
• Results indicate that less than 0.002% of stars within 100 pc and around 0.08% beyond this distance could host such structures, refining SETI search strategies.

Upper Limits on Partial Dyson Spheres in the Milky Way

In the search for extraterrestrial civilizations, one of the most explored theoretical constructs is the Dyson sphere, a megastructure hypothesized to encapsulate stars for the purpose of energy harvesting. The paper by Suazo et al. positions itself within this quest by analyzing the prevalence of partial Dyson spheres in the Milky Way, utilizing a robust dataset comprised of optical data from Gaia DR2 and mid-infrared data from AllWISE. This paper leverages these data sources to set unprecedented upper limits on the presence of such structures, characterizing them through their distinctive waste-heat signatures.

Methodology

The methodology hinges on identifying Dyson spheres—structures that absorb star radiation and emit thermal waste—by examining their photometric anomalies. The authors employ color-magnitude diagrams (CMDs) using data from Gaia and WISE to distinguish normal stellar phenomena from those potentially signaling Dyson spheres. Their approach includes defining regions within these CMDs where such Dyson signatures, characterized by their temperature and covering factor, are expected to manifest.

For instance, the paper examines stars within 100 parsecs (pc) and extends to 5000 pc, allowing for the assessment of both local and more distant stellar populations. The analysis focuses on Dyson spheres that capture 10-90% of host stellar luminosity, emitting waste in the 100-1000 K range. The authors develop an exclusion map, presenting the fraction of stars that could harbor these structures concerning Dyson sphere parameters such as temperature and covering factor.

Results

The interstellar analysis reveals that less than 0.002% of stars within 100 pc could host Dyson spheres at 90% completion and ∼300 K, a finding that underscores the rarity of such megastructures. As the distance scope broadens to 5 kpc, this fraction slightly increases to 0.08%, reflecting weakened detection capabilities due to both the resolving power of WISE and the increasing number of interlopers such as young stellar objects (YSOs). The methodology is shown to be adept, with only about five stars out of a 100 pc subsample producing detections consistent with Dyson spheres, further emphasizing the absence of such megastructures in local stellar neighborhoods.

Implications and Future Work

Practically, these findings bear significant implications for the search for extraterrestrial intelligence (SETI). The constraints set on the prevalence of Dyson spheres, especially those operating efficiently at 300 K, limit the likelihood of their existence—or at least their detectability with current infrared technology—within the Milky Way. Theoretically, the paper reinforces the necessity of considering alternate signatures and phenomenological models for alien civilizations, advocating for expansion beyond thermal waste detection.

This paper not only delineates the boundaries of Dyson sphere prevalence but also provides a template for improving observational methodologies in future studies. As techniques advance and datasets expand, incorporating spectral analyses of candidates flagged in photometric surveys will enhance validation processes, reducing false positives from dusty interlopers like YSOs and evolved stars. The authors hint at upcoming Gaia releases with detailed stellar parameters, which will intrinsically refine this search paradigm, allowing even more refined constraints on megastructure populations.

In conclusion, Project Hephaistos sets a foundation for rigorous astroengineering searches, and this work represents a critical step towards understanding the potential presence of advanced technology in our galaxy. While the curtain remains drawn on the hypothesis of Dyson spheres in the Milky Way, the framework and results crafted in this paper significantly narrow the scope of this search, inviting subsequent investigations to build upon these calculated boundaries.