Optical SETI At ESO In The 2040s
The searches for other life and for intelligence are fundamental problems that science faces today. Most searches so far have been focused on radio, but optical laser communication is an alternative, well suited for a ground-based observatory.
A project to search for artificial laser communications with the current and future extreme multiplexity spectroscopic facilities that ESO may develop by the 2040s is outlined. The monochromatic light is a clearly identifiable technosignature. The enormous corollary outreach potential of this initiative is underlined.
1 Signatures of life
Finding other life in the Universe is one of the most fascinating tasks of science. Even the details of the origin of life on our own planet are unclear and therefore a second example is needed to better understand this process. The basic building blocks of life – at least of life similar to ours – are commonly found across the cosmos [McGuire, 2021], but so far we have not securely identified life elsewhere [notwithstanding a few questioned claims: McKay et al., 1996, Greaves et al., 2021, Madhusudhan et al., 2023], including other planets in the Solar System where in situ exploration is possible.
However, the search space is widening. So far we know of over 6000 exoplanets and the orbits of ∼70 of them reside in the so-called habitable zone [HZ; Bohl et al., 2025] – a loosely defined region where the equilibrium temperature Teq of the exoplanet allows the existence of liquid water. HZ is a somewhat misleading term, because Teq also depends on other parameters, such as the planet mass, if there is greenhouse effect, orbital eccentricity, if the host star exhibits high-energy flares that could photoevaporate the planetary atmosphere, etc. The are two avenues to search for life:
(I) Biosignatures – spectral features in exoatmosphere from various volatile molecules that can originate from life (O2, O3, CH4, N2O, CH3Cl, etc.), or that indicate reflection from biological material (e.g., vegetation red edge); the seasonal variation of these signatures can also suggest the presence of life.
However, many of these species can also originate from geological processes, leaving the possibility for false positives. A safer strategy is to look for a complex of them. Biosignatures from our own life in the Earth’s atmosphere are a major obstacle, suggesting that these searches are likely to be more successful with space-based facilities – a path suggested early on by Burke [1992] and recently adopted by the Habitable Worlds Observatory [Gaudi et al., 2020].
(II) Technosigantures – indications of technology (either because of their nature or because of their information content) that cannot be produced by natural processes. The detection of radio signals was the first to be seriously considered and attempted [Cocconi and Morrison, 1959, Kardashev, 1964], together with thermal residual emission from Dyson spheres [Dyson, 1960, Timofeev et al., 2000, Carrigan, 2009] and later – transits from artificial megastructures such as Dyson swarms [Wright et al., 2016], among other examples of searches.
A common problem of radio and thermal technosignatures is that they rely on wasted radiation from the extraterrestrial civilizations. We already see in our own example the emerging tendency to reduce waste and to become more efficient. Ivanov et al. [2020] considered “quiet” (and energy-savvy) advanced civilizations that may co-exist with us, yet they would remain invisible to our searches.
The authors conclude that if this evolutionary trend toward more rational (and more economical) existence is common among civilizations, then our best detection opportunities are with the ones at technological stages of development close to our own, that also happened to be located nearby, or with the significantly more advanced civilizations that would set up beacons, purposely designed and optimized to be detectable by younger “cousins” like us [Benford et al., 2010b,a].
There was an early suggestion to use an optical communication channel, that does not rely on wasted energy. Carl Friedrich Gauss is said to have proposed in 1820 illuminated drawings of regular shapes in the deserts, intended to be visible from other planets in the Solar System. The optical signals became relevant for interstellar distances after Schwartz and Townes [1961] suggested using lasers for communication.
Howard et al. [2004] carried out a search, Maire et al. [2014] moved it to the infrared, minimizing the interstellar dust obscuration effects. Reines and Marcy [2002] begun a massive archival search for narrow unresolved emission lines in the high-resolution spectra used for radial velocity planet searches. The increase of resolution suppresses the contribution of the continuous spectrum in the spectral resolution element where the lased light falls.
Laser communications, unlike radio and thermal waste, are highly directional, so the senders must intentionally attempt to communicate with us, or at least they must have included the Solar System in their list of promising targets.
Valenitn D. Ivanov
Comments: This is an extended version of a white paper submitted in response to the ESO Expanding Horizons initiative; 5 pages
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Earth and Planetary Astrophysics (astro-ph.EP); Popular Physics (physics.pop-ph)
Cite as: arXiv:2512.18903 [astro-ph.IM](or arXiv:2512.18903v1 [astro-ph.IM] for this version)
https://doi.org/10.48550/arXiv.2512.18903
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Submission history
From: Valentin Dimitrov Ivanov
[v1] Sun, 21 Dec 2025 22:22:28 UTC (38 KB)
https://arxiv.org/abs/2512.18903
Astrobiology, SETI,
