Detecting Earth from distant star-based systems

File:Dopspec-inline.gif dims due to transiting exoplanet]]

In June 2021, astronomers identified 1,715 stars (with likely related exoplanetary systems) within 326 light-years (100 parsecs) that have a favorable positional vantage point—in relation to the Earth Transit Zone (ETZ)—of detecting Earth as an exoplanet transiting the Sun since the beginnings of human civilization (about 5,000 years ago); an additional 319 stars are expected to arrive at this special vantage point in the next 5,000 years.{{cite journal |last1=Kaltenegger |first1=L. | author-link = Lisa Kaltenegger | last2=Faherty |first2=J.K.|author2-link=Jackie Faherty|title=Past, present and future stars that can see Earth as a transiting exoplanet |url=https://www.nature.com/articles/s41586-021-03596-y |date=23 June 2021 |journal=Nature |volume=594 |issue=7864 |pages=505–507 |doi=10.1038/s41586-021-03596-y |pmid=34163055 |bibcode=2021Natur.594..505K |access-date=23 June 2021 |arxiv=2107.07936 |s2cid=235626242 }} Seven known exoplanet hosts, including Ross 128, may be among these stars. Teegarden's Star and Trappist-1 may be expected to see the Earth in 29 and 1,642 years, respectively. Radio waves, emitted by humans, have reached over 75 of the closest stars that were studied. In June 2021, astronomers reported identifying 29 planets in habitable zones that may be capable of observing the Earth.{{cite news |last=Sample |first=Ian |title=Scientists identify 29 planets where aliens could observe Earth - Astronomers estimate 29 habitable planets are positioned to see Earth transit and intercept human broadcasts |url=https://www.theguardian.com/science/2021/jun/23/scientists-identify-29-planets-where-aliens-could-observe-earth |date=23 June 2021 |work=The Guardian |access-date=23 June 2021 }} Earlier, in October 2020, astronomers had initially identified 508 such stars within 326 light-years (100 parsecs) that would have a favorable positional vantage point—in relation to the Earth Transit Zone (ETZ)—of detecting Earth as an exoplanet transiting the Sun.{{cite journal |last1=Kaltenegger |first1=L. |last2=Pepper |first2=J. |title=Which stars can see Earth as a transiting exoplanet? |url=https://academic.oup.com/mnrasl/article/499/1/L111/5931805 |date=20 October 2020 |journal=Monthly Notices of the Royal Astronomical Society |volume=499 |issue=1 |pages=L111–L115 |doi=10.1093/mnrasl/slaa161 |arxiv=2010.09766 |access-date=24 October 2020 |doi-access=free }}{{cite news |last=Letzer |first=Rafi |title=Aliens on 1,000 nearby stars could see us, new study suggests |url=https://www.livescience.com/aliens-spot-earth-exoplanets.html |date=22 October 2020 |work=Live Science |access-date=24 October 2020 }}{{cite news |last=Friedlander |first=Blaine |title=Smile, wave: Some exoplanets may be able to see us, too |url=https://news.cornell.edu/stories/2020/10/smile-wave-some-exoplanets-may-be-able-see-us-too |date=21 October 2020 |work=Cornell University |access-date=24 October 2020 }}{{cite news |last=Carter |first=Jamie |title=Are We Being Watched? There Are 509 Star Systems With A Great View Of Life On Earth, Say Scientists |url=https://www.forbes.com/sites/jamiecartereurope/2020/10/22/are-we-being-watched-there-are-509-star-systems-with-a-great-view-of-life-on-earth-say-scientists/amp/ |date=22 October 2020 |work=Forbes |access-date=24 October 2020 }}{{Excessive citations inline|date=March 2025}}

Transit method is the most popular tool used to detect exoplanets and the most common tool to spectroscopically analyze exoplanetary atmospheres. As a result, such studies, based on the transit method, will be useful in the search for life on exoplanets beyond the Solar System by the SETI program, Breakthrough Listen Initiative, as well as upcoming exoplanetary TESS mission searches.

Detectability of Earth from distant star-based systems may allow for the detectability of humanity and/or analysis of Earth from distant vantage points such as via "atmospheric SETI" for the detection of atmospheric compositions explainable only by use of (artificial) technology like air pollution containing nitrogen dioxide from e.g. transportation technologies.{{cite news |title=Pollution on other planets could help us find aliens, Nasa says |url=https://www.independent.co.uk/life-style/gadgets-and-tech/alien-pollution-planets-nasa-b1801543.html |access-date=6 March 2021 | work = The Independent |date=12 February 2021 |language=en}}{{cite magazine |title=Can Alien Smog Lead Us to Extraterrestrial Civilizations? |url=https://www.wired.com/story/can-alien-smog-lead-us-to-extraterrestrial-civilizations/ |access-date=6 March 2021 |magazine=Wired |language=en-us}}{{cite journal |last1=Kopparapu |first1=Ravi |last2=Arney |first2=Giada |last3=Haqq-Misra |first3=Jacob |last4=Lustig-Yaeger |first4=Jacob |last5=Villanueva |first5=Geronimo |title=Nitrogen Dioxide Pollution as a Signature of Extraterrestrial Technology |journal=The Astrophysical Journal |date=22 February 2021 |volume=908 |issue=2 |page=164 |doi=10.3847/1538-4357/abd7f7 |language=en |issn=1538-4357|arxiv=2102.05027|bibcode=2021ApJ...908..164K |s2cid=231855390 |doi-access=free }} The easiest or most likely artificial signals from Earth to be detectable are brief pulses transmitted by anti-ballistic missile (ABM) early-warning and space-surveillance radars during the Cold War and later astronomical and military radars.{{cite journal |title=The benefits and harm of transmitting into space |journal=Space Policy |date=1 February 2013 |volume=29 |issue=1 |pages=40–48 |doi=10.1016/j.spacepol.2012.11.006 |url=https://www.sciencedirect.com/science/article/abs/pii/S0265964612001361 |access-date=9 April 2021 |language=en |issn=0265-9646|arxiv=1207.5540 |last1=Haqq-Misra |first1=Jacob |last2=Busch |first2=Michael W. |last3=Som |first3=Sanjoy M. |last4=Baum |first4=Seth D. |bibcode=2013SpPol..29...40H |s2cid=7070311 }}{{cite book |last1=Sullivan |first1=W. T. III |chapter=Radio Leakage and Eavesdropping |title=Strategies for the Search for Life in the Universe |series=Astrophysics and Space Science Library |date=1980 |volume=83 |pages=227–239 |doi=10.1007/978-94-009-9115-6_20 |bibcode=1980ASSL...83..227S |isbn=978-90-277-1226-4 |chapter-url=https://ui.adsabs.harvard.edu/abs/1980ASSL...83..227S/abstract |access-date=9 April 2021}} Unlike the earliest and conventional radio- and television-broadcasting which has been claimed to be undetectable at short distances,{{cite web |title=How far from Earth could aliens detect our radio signals? |url=https://www.sciencefocus.com/space/how-far-from-earth-could-aliens-detect-our-radio-signals/ |website=BBC Science Focus Magazine |date=21 December 2019 |access-date=9 April 2021 |language=en}}{{cite web |title=This is how far human radio broadcasts have reached into the galaxy |url=https://www.planetary.org/articles/3390 |website=The Planetary Society |access-date=9 April 2021 |language=en}} such signals could be detected from very distant, possibly star-based, receiver stations – any single of which would detect brief episodes of powerful pulses repeating with intervals of one Earth day – and could be used to detect both Earth as well as the presence of a radar-utilizing civilization on it.{{cite journal |title=XI. - Planets and Life around Other Stars |journal=International Geophysics |date=1 January 2004 |volume=87 |pages=592–608 |url=https://www.sciencedirect.com/science/article/pii/S0074614204800251 |access-date=5 April 2021 |publisher=Academic Press |doi=10.1016/S0074-6142(04)80025-1 |bibcode=2004InGeo..87..592. |isbn=978-0-12-446744-6 |language=en}}

Studies have suggested that radio broadcast leakage – with the program material likely not being detectable – may be a technosignature detectable at distances of up to a hundred light years with technology equivalent to the Square Kilometer Array{{cite web |title=How Far Into Space Can Radio Telescopes Hear? |url=https://www.forbes.com/sites/quora/2017/01/27/how-far-into-space-can-radio-telescopes-hear/ |website=Forbes |access-date=9 April 2021 |language=en}} if the location of Earth is known.{{cite journal |title=A direct communication proposal to test the Zoo Hypothesis |journal=Space Policy |date=1 November 2016 |volume=38 |pages=22–26 |doi=10.1016/j.spacepol.2016.06.001 |language=en |issn=0265-9646 |quote="While the limits of detection of Earth's radio transmissions are a subject of debate (Sullivan argues ~25 light-years, Atri et al. (2011) and Baum et al. (2011) up to 100 light years), as they largely depend on the size of the receiving antenna"|doi-access=free |last1=De Magalhães |first1=João Pedro |arxiv=1509.03652 |bibcode=2016SpPol..38...22D }}{{cite journal |last1=Loeb |first1=Avi |last2=Zaldarriaga|first2=Matias |title=Eavesdropping on radio broadcasts from galactic civilizations with upcoming observatories for redshifted 21 cm radiation |url=https://iopscience.iop.org/article/10.1088/1475-7516/2007/01/020/fulltext/ |journal=Journal of Cosmology and Astroparticle Physics |date=22 January 2007 |volume=2007 |issue=1 |page=020 |doi=10.1088/1475-7516/2007/01/020 |access-date=9 April 2021|arxiv=astro-ph/0610377 |bibcode=2007JCAP...01..020L |s2cid=119398714 }} Likewise, if Earth's location can be and is known, it may be possible to use atmospheric analysis to detect life or favorable conditions for it on Earth via biosignatures, including MERMOZ instruments that may be capable of remotely detecting living matter on Earth.{{cite journal |author=Patty, C.H.L. |display-authors=et al. |title=Biosignatures of the Earth I. Airborne spectropolarimetric detection of photosynthetic life |url=https://www.aanda.org/component/article?access=doi&doi=10.1051/0004-6361/202140845 |date=2021 |journal=Astronomy & Astrophysics |volume=A68 |page=651 |doi=10.1051/0004-6361/202140845 |access-date=21 June 2021 |arxiv=2106.00493 |bibcode=2021A&A...651A..68P |s2cid=235265876 }}{{cite journal|author=Patty, C.H. Luca |display-authors=et al. |title=Biosignatures of the Earth |journal=Astronomy & Astrophysics |date=1 June 2021 |volume=651 |pages=A68 |doi=10.1051/0004-6361/202140845 |arxiv=2106.00493v1 |bibcode=2021A&A...651A..68P |s2cid=235265876 }}{{cite news |author=University of Bern |title=Scientists Use New Technology to Detect Signatures of Life Remotely |url=https://scitechdaily.com/scientists-use-new-technology-to-detect-signatures-of-life-remotely/amp/ |date=20 June 2021 |work=SciTechDaily.com |access-date=21 June 2021 }}

In 1980s, astronomer Carl Sagan persuaded NASA to perform an experiment of detecting life and civilization on Earth using instruments of the Galileo spacecraft. It was launched in December 1990, and when it was {{convert|960|km|mi|abbr=on}} from the planet's surface, Galileo turned its instruments to observe Earth. Sagan's paper was titled "A search for life on Earth from the Galileo spacecraft"; he wrote that "high-resolution images of Australia and Antarctica obtained as Galileo flew overhead did not yield signs of civilization"; other measurements showed the presence of vegetation and detected radio transmissions.{{cite journal |last1=Witze |first1=Alexandra |title=How would we know whether there is life on Earth? This bold experiment found out |journal=Nature |date=16 October 2023 |volume=622 |issue=7983 |pages=451–452 |doi=10.1038/d41586-023-03230-z |bibcode=2023Natur.622..451W |language=en |doi-access=free |pmid=37845527 }}{{Cite journal|url=https://www.ufrgs.br/leaa/arquivos/aulas/SERP06/1993_ASearchForLifeOnEarthFromTheGalileoSpacecraft_NATURE.pdf|title=A search for life on Earth from the Galileo spacecraft|first1=Carl|last1=Sagan|first2=W. Reid|last2=Thompson|first3=Robert|last3=Carlson|first4=Donald|last4=Gurnett|first5=Charles|last5=Hord|date=October 23, 1993|journal=Nature|volume=365|issue=6448|pages=715–721|doi=10.1038/365715a0|pmid=11536539 |bibcode=1993Natur.365..715S |s2cid=4269717 }}

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• Earliest known life forms
• {{annotated link|Exoplanetology}}
• List of exoplanet search projects
• Lists of exoplanets
• {{annotated link|MERMOZ}}
• {{annotated link|Methods of detecting exoplanets}}
• {{annotated link|Nexus for Exoplanet System Science}}
• {{annotated link|Planetary science}}
• {{annotated link|Quiet and loud aliens}}

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{{reflist|30em}}

• [https://exoplanet.eu/home/ Extrasolar Planets Encyclopaedia] by the Paris Observatory

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{{Exoplanet search projects}}

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