Technosignature

Shkadov thrusters, with the hypothetical ability to change the orbital paths of stars in order to avoid various dangers to life such as cold molecular clouds or cometary impacts, would also be detectable in a similar fashion to the transiting extrasolar planets searched by Kepler. Unlike planets, though, the thrusters would appear to abruptly stop over the surface of a star rather than crossing it completely, revealing their technological origin.[12] In addition, evidence of targeted extrasolar asteroid mining may also reveal extraterrestrial intelligence (ETI).[13] Furthermore, it has been suggested that information could be hidden within the transit signatures of other planets.[14] Advanced civilizations could "cloak their presence, or deliberately broadcast it, through controlled laser emission".[15] Other characteristics proposed as potential technosignatures (or starting points for detection of clearer signatures) include peculiar orbital periods such as arranging planets in prime number patterns.[16][17][18] Coronal and chromospheric activity on stars might be altered.[19] Extraterrestrial civilizations may use free-floating planets (rogue planets) for interstellar transportation with a number of proposed possible technosignatures.[20]

A study suggests that if ETs exist, they may have established communications network(s) and may already have probes in the solar system whose communication may be detectable.[21] Studies by John Gertz suggest flyby (scout)[22] probes might intermittently surveil nascent solar systems and permanent probes would communicate with a home base, potentially using triggers and conditions such as detection of electromagnetic leakage or biosignatures.[23] They also suggest several strategies to detecting local ET probes[24] such as detecting emitted optical messages.[25] He also finds that due to interstellar networks of communications nodes, the search for deliberate interstellar signals – as is common in SETI[26] – may be futile.[27] The architecture may consist of nodes separated by sub-light-year distances and strung out between neighboring stars.[28] It may also contain pulsars as beacons[29] or nodes whose beams are modulated by mechanisms that could be searched for.[30] Moreover, a study suggests prior searches wouldn't have detected cost-effective electromagnetic signal beacons.[31]

Lights from cities and infrastructure on Earth at night from space

Various astronomers, including Avi Loeb of the Harvard-Smithsonian Center for Astrophysics and Edwin L. Turner of Princeton University have proposed that artificial light from extraterrestrial planets, such as that originating from cities, industries, and transport networks, could be detected and signal the presence of an advanced civilization. Such approaches, though, make the assumption that the radiant energy generated by civilization would be relatively clustered and can therefore be detected easily.[32][33]

Light and heat detected from planets must be distinguished from natural sources to conclusively prove the existence of intelligent life on a planet.[4] For example, NASA's 2012 Black Marble experiment showed that significant stable light and heat sources on Earth, such as chronic wildfires in arid Western Australia, originate from uninhabited areas and are naturally occurring.[34] The proposed LUVOIR A may be able to detect city lights twelve times those of Earth on Proxima b in 300 hours.[35]

Artist's illustration of an advanced ET civilization with industrial pollution[36]

Atmospheric analysis of planetary atmospheres, as is already done on various Solar System bodies and in a rudimentary fashion on several hot Jupiter extrasolar planets, may reveal the presence of chemicals produced by technological civilizations.[37][38] For example, atmospheric emissions from human technology use on Earth, including nitrogen dioxide and chlorofluorocarbons, are detectable from space.[39] Artificial air pollution may therefore be detectable on extrasolar planets and on Earth via "atmospheric SETI" – including NO₂ pollution levels and with telescopic technology close to today.[40][41][42][43] Such technosignatures may consist not of the detection of the level of one specific chemical but simultaneous detections of levels of multiple specific chemicals in atmospheres.[44]

However, there remains a possibility of mis-detection; for example, the atmosphere of Titan has detectable signatures of complex chemicals that are similar to what on Earth are industrial pollutants, though not the byproduct of civilisation.[45] Some SETI scientists have proposed searching for artificial atmospheres created by planetary engineering to produce habitable environments for colonisation by an ETI.[38]

The IKAROS light sail of 2010

Interstellar spacecraft may be detectable from hundreds to thousands of light-years away through various forms of radiation, such as the photons emitted by an antimatter rocket or cyclotron radiation from the interaction of a magnetic sail with the interstellar medium. Such a signal would be easily distinguishable from a natural signal and could hence firmly establish the existence of extraterrestrial life, were it to be detected.[46] In addition, smaller Bracewell probes within the Solar System itself may also be detectable by means of optical or radio searches.[47][48] Self-replicating spacecraft or their communications networks could potentially be detectable within our Solar system or in nearby star-based systems,[49] if they are located there.[50] Such technologies or their footprints could be in Earth's orbit, on the Moon or on the Earth.

A less advanced technology, and one closer to humanity's current technological level, is the Clarke Exobelt proposed by Astrophysicist Hector Socas-Navarro of the Instituto de Astrofisica de Canarias.[51] This hypothetical belt would be formed by all the artificial satellites occupying geostationary/geosynchronous orbits around an exoplanet. From early simulations it appeared that a very dense satellite belt, requiring only a moderately more-advanced civilization than ours, would be detectable with existing technology in the light curves from transiting exoplanets,[52] but subsequent analysis has questioned this result, suggesting that exobelts detectable by current and upcoming missions will be very rare.[53]

It has been suggested that once extraterrestrials arrive "at a new home, such life will almost certainly create technosignatures (because it used technology to get there), and some fraction of them may also eventually give rise to a new biosphere".[54] Microorganism DNA may have been used for self-replicating messages.[55] See also: DNA digital data storage

Low- or high-albedo installations such as solar panels may also be detectable, albeit distinguishing artificial megastructures from high- and low-albedo natural environments (e.g., bright ice caps) may make it unfeasible.[26]

In 2016, it was proposed that vanishing stars are a plausible technosignature.[70] A pilot project searching for vanishing stars was carried out, finding one candidate object. In 2019, the Vanishing & Appearing Sources during a Century of Observations (VASCO) project[71] began more general searches for vanishing and appearing stars, and other astrophysical transients[70] They identified 100 red transients of "most likely natural origin", while analyzing 15% of the image data. In 2020, the VASCO collaboration started up a citizen science project, vetting through images of many thousands of candidate objects.[72] The citizen science project is carried out in close collaboration with schools and amateur associations mainly in African countries.[73] The VASCO project has been referred to as "Perhaps the most general artefact search to date".[74] In 2021, VASCO's principal investigator Beatriz Villarroel received a L'Oreal-Unesco prize in Sweden for the project.[75] In June 2021, the collaboration published the discovery of nine light sources seemingly appearing and vanishing simultaneously in the sky.[76] No natural phenomena can explain the presence of the objects in an old photographic plate from 1950. The group carefully indicated that either nuclear fallout from unlisted atomic bombs contaminated the plates or that a new celestial phenomenon might be behind. For example, the high spatial density of transients is caused by the presence of artificial, reflective objects at high orbits around Earth in 1950. Continued studies, are bringing more support for the authenticity of the phenomenon with multiple transients.[77] See also: Diminished Reality (the reverse of augmented reality).

Methods and ancillary benefits of the search for various technosignatures.[56]

In June 2020, NASA was awarded their first SETI-specific grant in three decades. The grant funds the first NASA-funded search for technosignatures from advanced extraterrestrial civilizations other than radio waves, including the creation and population of an online technosignature library.[78][79][80] A 2021 scientific review produced by the i.a. NASA-sponsored online workshop TechnoClimes 2020 classified possible optimal mission concepts for the search of technosignatures. It evaluates signatures based on a metric about the distance of humanity to the capacity of developing the signature's required technology – a comparison to contemporary human technology footprints, associated methods of detection and ancillary benefits of their search for other astronomy. The study's conclusions include a robust rationale for organizing missions for searching artifacts – including probes – within the Solar system.[81][56]

In 2021, astronomers proposed a sequence of "verification checks for narrowband technosignature signals" after concluding that technosignature candidate BLC1 could be the result of a form of local radiofrequency interference.[82]

Capabilities for detecting technosignatures with recent, ongoing, and future missions and facilities. Cells colored green indicate that at least such a signature could be detectable for at least one stellar system and there being at least one peer-reviewed publication that has evaluated detectability of that signature.

It has been suggested that observatories on the Moon could be more successful.[83][84] In 2022, scientists provided an overview of the capabilities of ongoing, recent, past, planned and proposed missions and observatories for detecting various alien technosignatures.[85][86]