Optical SETI at ESO in the 2040s

Abstract

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 (McKay et al., 1996; Greaves et al., 2021; Madhusudhan et al., 2023, notwithstanding a few questioned
claims:),
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 (Bohl et al., 2025, HZ;) – 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; Benford et al., 2010a).

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.

2 The Solar System as a target of directional signaling

Tarter (2001) points out that SETI (Search for
Extraterrestrial Intelligence) programs are easier to get approved and
carried out, and perhaps even more likely to succeed, if they are
performed in the course of regular astronomical observations, as
opposed to dedicated SETI campaigns. Given the vast parametric space
that SETI needs to cover and the poorly constrained properties of
extraterrestrial communication, the astronomical surveys stand out as
the most obvious types of programs to look for synergies with, due to
of their capability to capture diverse signals: potentially from wide
areas on the sky and in broad wavelength ranges.

One can arrive at the idea of using the established channel of the
astronomical observations to signal other civilizations from a game
theory concept -- that of communication without
messaging. Despite its complicated name, this is something most people
apply in everyday life, without even realizing it, when they ask
themselves the question what is expected of them by others. Card games
where one pair of players opposes another pair are a typical example
of cooperation without message exchange where the rules and goals help
players coordinate. In daily life we have the laws, social, ethical,
and religious norms, and the common sense as analogs of the game rules.

The only assumption necessary in the context of SETI is that the other
civilizations are interested in the Cosmos to the extent of exploring
it by means of astronomical observations; the homogeneity of the
Universe “unifies” the communication channels of distant
civilizations. For example, for a radioastronomer – human or alien –
the radio noise in the Milky Way will be the lowest at 0.3-30 cm
(Kardashev, 1964, fig. 1).

Following similar reasoning Filippova et al. (1991); Heller and Pudritz (2016)
arrived at the idea that astronomical observations can reveal the
Earth’s habitability via transmission spectroscopy observable by
extraterrestrial astronomer located in out ecliptic plane
(Charbonneau et al., 2002, e.g.,). This is possible via emission
spectroscopy (Deming et al., 2005, e.g.,) for observers away
from the ecliptic plane as well, but it is a more challenging
observation for cool exoplanets in the HZ.

3 Technosignature search opportunities at ESO with surveys from

today to the 2040s

ESO has a long and successful tradition in astronomical surveys, and
is entering a new era of high-multiplexity spectroscopic surveys with
the 4-metre Multi-Object Spectroscopic Telescope
(de Jong et al., 2019, 4MOST, to begin operations in 2026;).
Furthermore, a new 10-m class Wide-field Spectroscopic Telescope
(Mainieri et al., 2024, WST;) with an extremely high multiplexity
spectrograph (~30,000 optical fibers) is being proposed for the
2040s.
4MOST and WST are especially well suited for optical SETI, searching for
laser communications that will appear as unresolved emission lines in
spectra of stars that do not show natural features like those. The
program discussed here relies on the synergy with other surveys – it
can be carried out in parallel with any stellar survey.

4 Timeline, facilities and requirements

The search for laser communications requires:

(I) High multiplexity to maximize the number of observed stars.

(II) High cadence to increase the signal-to-noise ratio (SNR), if
the senders have adopted an energy-efficient strategy that would minimize
the energy expenditure – the host star emits continuously, while the
laser pulses are expected to be brief, so shorter integrations improve
detection significance.

(III) High spectral resolution, again to increase the SNR, because
the laser emits monochromatic light superimposed ob the continuous stellar
spectrum, so the monochromatic laser pulses will be confined to a single
spectral resolution element, and the narrower this element is, the higher
the SNR.

(IV) High processing speed to allow rapid discovery and scheduling
of follow-up observations.

Meeting the scientific goals requires more than high-cadence spectroscopy:
a data flow system capable of processing data, detecting signals and
interpreting them in real time is needed. The delays can undermine the
follow-up. No existing facility offers these capabilities and the planned
WST will be a giant step for the optical SETI.

5 Outreach potential

A long-term future SETI project at ESO can become an excellent vehicle
for education, for explaining to the public the most basic scientific
concepts like the scientific method, for introducing the newest
discoveries – and reaching far more people than usual channels
(Ivanov, 2021; Ivanov, 2023a; Ivanov, 2023b).

The interstellar comet 3I/ATLAS helped to demonstrate the significant
public interest in the search for life and its implications. This
object attracted an enormous attention, because of the possibility –
however remote – that it may be a piece of alien technology
(Hibberd et al., 2025, e.g.,). An opportunity to study a comet
originating from a different planetary system can be a treasure trove
for new discoveries, but the public reaction is important on its own
because it demonstrated how much the imagination and curiosity of
people can be aroused by the unanswered questions of the Universe.

6 Summary and conclusions

A SETI program at ESO in the next decade will allow addressing a
question of both scientific and public interest – if intelligent
life exists elsewhere in the Universe. We consider here a program
that can be executed in parallel with the new large scale
spectroscopic surveys that will be carried out in the next coming
years, and with the future WST facility that has been proposed as
ESO’s next big telescope project.

Acknowledgements

The author is grateful to Claudio Cáceres Acevedo, Dante Minniti,
and Juan Carlos Beamín for the fruitful discussions.

References

• Benford et al. (2010a)
  G. Benford, J. Benford, and D. Benford

Searching for Cost-Optimized Interstellar Beacons.

Astrobiology 10 (5), pp. 491–498.

External Links: Document

Cited by: §1.

• Benford et al. (2010b)
  J. Benford, G. Benford, and D. Benford

Messaging with Cost-Optimized Interstellar Beacons.

Astrobiology 10 (5), pp. 475–490.

External Links: Document

Cited by: §1.

• Bohl et al. (2025)
  A. Bohl, L. Lawrence, G. Lowry, and L. Kaltenegger

Probing the Limits of Habitability: A Catalog of Rocky Exoplanets in the Habitable Zone.

arXiv e-prints, pp. arXiv:2501.14054.

External Links: Document,
2501.14054

Cited by: §1.

• Burke (1992)
  B. F. Burke

Searching for exoplanets..

In Targets for Space-Based Interferometry, C. Mattok (Ed.),

ESA Special Publication, Vol. 354, pp. 81–83.

Cited by: §1.

• Carrigan (2009)
  R. A. Carrigan

IRAS-Based Whole-Sky Upper Limit on Dyson Spheres.

ApJ 698 (2), pp. 2075–2086.

External Links: Document,
0811.2376

Cited by: §1.

• Charbonneau et al. (2002)
  D. Charbonneau, T. M. Brown, R. W. Noyes, and R. L. Gilliland

Detection of an Extrasolar Planet Atmosphere.

ApJ 568 (1), pp. 377–384.

External Links: Document,
astro-ph/0111544

Cited by: §2.

• Cocconi and Morrison (1959)
  G. Cocconi and P. Morrison

Searching for Interstellar Communications.

Nat. 184 (4690), pp. 844–846.

External Links: Document

Cited by: §1.

• de Jong et al. (2019)
  R. S. de Jong, O. Agertz, A. A. Berbel, J. Aird, D. A. Alexander, A. Amarsi, F. Anders, R. Andrae, B. Ansarinejad, W. Ansorge, and et al.

4MOST: Project overview and information for the First Call for Proposals.

The Messenger 175, pp. 3–11.

External Links: Document,
1903.02464

Cited by: §3.

• Deming et al. (2005)
  D. Deming, S. Seager, L. J. Richardson, and J. Harrington

Infrared radiation from an extrasolar planet.

Nat. 434 (7034), pp. 740–743.

External Links: Document,
astro-ph/0503554

Cited by: §2.

• Dyson (1960)
  F. J. Dyson

Search for Artificial Stellar Sources of Infrared Radiation.

Science 131 (3414), pp. 1667–1668.

External Links: Document

Cited by: §1.

• Filippova et al. (1991)
  L. N. Filippova, N. S. Kardashev, S. F. Likhachev, and V. S. Strelnitskij

On the Strategy of SETI.

In Bioastronomy: The Search for Extraterrestial Life — The Exploration Broadens, J. Heidmann and M. J. Klein (Eds.),

Vol. 390, pp. 254–258.

External Links: Document

Cited by: §2.

• Gaudi et al. (2020)
  B. S. Gaudi, S. Seager, B. Mennesson, A. Kiessling, K. Warfield, K. Cahoy, J. T. Clarke, S. Domagal-Goldman, L. Feinberg, O. Guyon, and et al.

The Habitable Exoplanet Observatory (HabEx) Mission Concept Study Final Report.

arXiv e-prints, pp. arXiv:2001.06683.

External Links: Document,
2001.06683

Cited by: §1.

• Greaves et al. (2021)
  J. S. Greaves, A. M. S. Richards, W. Bains, P. B. Rimmer, H. Sagawa, D. L. Clements, S. Seager, J. J. Petkowski, C. Sousa-Silva, S. Ranjan, and et al.

Phosphine gas in the cloud decks of Venus.

Nature Astronomy 5, pp. 655–664.

External Links: Document,
2009.06593

Cited by: §1.

• Heller and Pudritz (2016)
  R. Heller and R. E. Pudritz

The Search for Extraterrestrial Intelligence in Earth’s Solar Transit Zone.

Astrobiology 16 (4), pp. 259–270.

External Links: Document,
1603.00776

Cited by: §2.

• Hibberd et al. (2025)
  A. Hibberd, A. Crowl, and A. Loeb

Is the Interstellar Object 3I/ATLAS Alien Technology?.

arXiv e-prints, pp. arXiv:2507.12213.

External Links: Document,
2507.12213

Cited by: §5.

• Howard et al. (2004)
  A. W. Howard, P. Horowitz, D. T. Wilkinson, C. M. Coldwell, E. J. Groth, N. Jarosik, D. W. Latham, R. P. Stefanik, A. J. Willman, J. Wolff, and et al.

Search for Nanosecond Optical Pulses from Nearby Solar-Type Stars.

ApJ 613 (2), pp. 1270–1284.

External Links: Document

Cited by: §1.

• Ivanov et al. (2020)
  V. D. Ivanov, J. C. Beamín, C. Cáceres, and D. Minniti

A qualitative classification of extraterrestrial civilizations.

A&A 639, pp. A94.

External Links: Document,
2005.13221

Cited by: §1.

• Ivanov (2021)
  V. D. Ivanov

Boosting the public engagement with astronomy through arts.

arXiv e-prints, pp. arXiv:2112.14702.

External Links: Document,
2112.14702

Cited by: §5.

• Ivanov (2023a)
  V. D. Ivanov

Popular astronomy and other science articles in glossy magazines – outreaching to those who do not care to be reached.

arXiv e-prints, pp. arXiv:2312.15797.

External Links: Document,
2312.15797

Cited by: §5.

• Ivanov (2023b)
  V. D. Ivanov

Through the eyes of a reader and science communicator: science in the mainstream and in the genre literature of yesterday and today.

arXiv e-prints, pp. arXiv:2312.16254.

External Links: Document,
2312.16254

Cited by: §5.

• Kardashev (1964)
  N. S. Kardashev

Transmission of Information by Extraterrestrial Civilizations..

Sov.Astro. 8, pp. 217.

Cited by: §1,
§2.

• Madhusudhan et al. (2023)
  N. Madhusudhan, S. Sarkar, S. Constantinou, M. Holmberg, A. A. A. Piette, and J. I. Moses

Carbon-bearing Molecules in a Possible Hycean Atmosphere.

ApJL 956 (1), pp. L13.

External Links: Document,
2309.05566

Cited by: §1.

• Mainieri et al. (2024)
  V. Mainieri, R. I. Anderson, J. Brinchmann, A. Cimatti, R. S. Ellis, V. Hill, J. Kneib, A. F. McLeod, C. Opitom, M. M. Roth, and et al.

The Wide-field Spectroscopic Telescope (WST) Science White Paper.

arXiv e-prints, pp. arXiv:2403.05398.

External Links: Document,
2403.05398

Cited by: §3.

• Maire et al. (2014)
  J. Maire, S. A. Wright, D. Werthimer, R. R. Treffers, G. W. Marcy, R. P. S. Stone, F. Drake, and A. Siemion

A near-infrared SETI experiment: probability distribution of false coincidences.

In Ground-based and Airborne Instrumentation for Astronomy V, S. K. Ramsay, I. S. McLean, and H. Takami (Eds.),

Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 9147, pp. 91474K.

External Links: Document,
1407.2618

Cited by: §1.

• McGuire (2021)
  B. A. McGuire

The 2021 Census of Interstellar, Circumstellar, Extragalactic, Protoplanetary Disk, and Exoplanetary Molecules: what are the Pressing Spectroscopic Needs from the Laboratory?.

In 2021 International Symposium on Molecular Spectroscopy,

External Links: Document

Cited by: §1.

• McKay et al. (1996)
  D. S. McKay, E. K. Gibson, K. L. Thomas-Keprta, H. Vali, C. S. Romanek, S. J. Clemett, X. D. F. Chillier, C. R. Maechling, and R. N. Zare

Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001.

Science 273 (5277), pp. 924–930.

External Links: Document

Cited by: §1.

• Reines and Marcy (2002)
  A. E. Reines and G. W. Marcy

Optical Search for Extraterrestrial Intelligence: A Spectroscopic Search for Laser Emission from Nearby Stars.

PASP 114 (794), pp. 416–426.

External Links: Document,
astro-ph/0112479

Cited by: §1.

• Schwartz and Townes (1961)
  R. N. Schwartz and C. H. Townes

Interstellar and Interplanetary Communication by Optical Masers.

Nat. 190 (4772), pp. 205–208.

External Links: Document

Cited by: §1.

• Tarter (2001)
  J. Tarter

The Search for Extraterrestrial Intelligence (SETI).

A&AAR 39, pp. 511–548.

External Links: Document

Cited by: §2.

• Timofeev et al. (2000)
  M. Y. Timofeev, N. S. Kardashev, and V. G. Promyslov

A search of the IRAS database for evidence of Dyson Spheres.

Acta Astronautica 46 (10), pp. 655–659.

External Links: Document

Cited by: §1.

• Wright et al. (2016)
  J. T. Wright, K. M. S. Cartier, M. Zhao, D. Jontof-Hutter, and E. B. Ford

The Search for Extraterrestrial Civilizations with Large Energy Supplies. IV. The Signatures and Information Content of Transiting Megastructures.

ApJ 816 (1), pp. 17.

External Links: Document,
1510.04606

Cited by: §1.

Where this page came from

This page was imported from arXiv. “Optical SETI at ESO in the 2040s” by Valenitn D. Ivanov, arXiv:2512.18903 (2025), published under CC BY 4.0. Changed here: set as a page from arXiv's HTML version; footnotes and author notes left out; figures the paper marks as under other terms left out.

Nobody has written it yet — it is the source material at a new address, which is why search engines are asked to skip it and why no one earns from it. It is up for grabs: take it on, and it is yours to rewrite and to earn from.