Detecting Earth from distant star-based systems: Difference between revisions

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{{use dmy dates |date=October 2020}}

{{short description|Detecting Earth as an exoplanet}}

[[File:Pale Blue Dot.png|thumb|upright=1.3|alt=Dark grey and black static with coloured vertical rays of sunlight over part of the image. A small pale blue point of light is barely visible.|[[Pale Blue Dot]], a photograph of [[Earth]] taken on February 14, 1990, by the ''[[Voyager 1]]'' [[space probe]] from a distance of approximately {{Nowrap|6 billion}} kilometers ({{nowrap|3.7 billion}} miles, 40.5 [[Astronomical unit|AU]]). Earth is seen as a tiny dot within deep space: the blueish-white speck almost halfway up the rightmost band of light.]]

[[File:Dopspec-inline.gif|thumb|right|150px|[[Star]] dims due to [[Transit (astronomy)|transiting]] [[exoplanet]]]]

There are several methods currently used by astronomers to detect distant [[exoplanet]]s from [[Earth]].<ref name="NASA-2020">{{cite web |author=Staff |title=5 Ways to Find a Planet |url=https://exoplanets.nasa.gov/alien-worlds/ways-to-find-a-planet/ |date=2020 |work=[[NASA]] |accessdateaccess-date=24 October 2020 }}</ref> Theoretically, some of these same methods maycan be used to detect the Earth as an exoplanet from distant star systems.

==History==

In June 2021, astronomers identified 1,715 stars (with likely related [[Planetary system|exoplanetary systems]]) within 326 [[light-year]]s (100 [[parsec]]s) that have a favorable positional vantage point—in relation to the Earth Transit Zone (ETZ)—of detecting [[Earth]] as an [[exoplanet]] [[Transit (astronomy)|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.<ref name="NAT-20210623">{{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 (journal)|Nature]] |volume=594 |issue=7864 |pages=505–507 |doi=10.1038/s41586-021-03596-y |pmid=34163055 |bibcode=2021Natur.594..505K |accessdate=23 June 2021 |arxiv=2107.07936 |s2cid=235626242 }}</ref> 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.<ref name="NAT-20210623" /> In June 2021, astronomers reported identifying 29 planets in [[habitable zone]]s that may be capable of observing the Earth.<ref name="TG20210623">{{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]] |accessdate=23 June 2021 }}</ref> Earlier, in October 2020, astronomers had initially identified 508 such stars within 326 [[light-year]]s (100 [[parsec]]s) that would have a favorable positional vantage point—in relation to the Earth Transit Zone (ETZ)—of detecting Earth as an [[exoplanet]] [[Transit (astronomy)|transiting]] the Sun.<ref name="RAS-20201020">{{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 |accessdate=24 October 2020 |doi-access=free }}</ref><ref name="LS-20201022">{{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]] |accessdate=24 October 2020 }}</ref><ref name="COR-20201021">{{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]] |accessdate=24 October 2020 }}</ref><ref name="FRBS-20201022">{{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]] |accessdate=24 October 2020 }}</ref>

[[File:Dopspec-inline.gif|thumb|right|150px|[[Star]] dims due to [[Transit (astronomy)|transiting]] [[exoplanet]]]]

In June 2021, astronomers identified 1,715 stars (with likely related [[Planetary system|exoplanetary systems]]) within 326 [[light-year]]s (100 [[parsec]]s) that have a favorable positional vantage point—in relation to the Earth Transit Zone (ETZ)—of detecting [[Earth]] as an [[exoplanet]] [[Transit (astronomy)|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.<ref name="NAT-20210623">{{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 (journal)|Nature]] |volume=594 |issue=7864 |pages=505–507 |doi=10.1038/s41586-021-03596-y |pmid=34163055 |bibcode=2021Natur.594..505K |accessdateaccess-date=23 June 2021 |arxiv=2107.07936 |s2cid=235626242 }}</ref> 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.<ref name="NAT-20210623" /> In June 2021, astronomers reported identifying 29 planets in [[habitable zone]]s that may be capable of observing the Earth.<ref name="TG20210623">{{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]] |accessdateaccess-date=23 June 2021 }}</ref> Earlier, in October 2020, astronomers had initially identified 508 such stars within 326 [[light-year]]s (100 [[parsec]]s) that would have a favorable positional vantage point—in relation to the Earth Transit Zone (ETZ)—of detecting Earth as an [[exoplanet]] [[Transit (astronomy)|transiting]] the Sun.<ref name="RAS-20201020">{{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 |accessdateaccess-date=24 October 2020 |doi-access=free }}</ref><ref name="LS-20201022">{{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]] |accessdateaccess-date=24 October 2020 }}</ref><ref name="COR-20201021">{{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]] |accessdateaccess-date=24 October 2020 }}</ref><ref name="FRBS-20201022">{{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]] |accessdateaccess-date=24 October 2020 }}</ref>

Transit method is the most popular tool used to detect exoplanets and the most common tool to [[Astronomical spectroscopy|spectroscopically]] analyze [[Extraterrestrial atmosphere#Exoplanets|exoplanetary atmospheres]].<ref name="RAS-20201020" /> As a result, such studies, based on the transit method, will be useful in the [[Extraterrestrial life|search for life on exoplanets]] beyond the [[Solar System]] by the [[Search for extraterrestrial intelligence|SETI program]], [[Breakthrough Listen|Breakthrough Listen Initiative]], as well as upcoming exoplanetary [[Transiting Exoplanet Survey Satellite|TESS mission]] searches.<ref name="RAS-20201020" />

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 "[[Technosignature#Atmospheric analysis|atmospheric SETI]]" for the detection of atmospheric compositions explainable only by use of (artificial) technology like [[air pollution]] containing [[Nitrogen dioxide#Human-caused sources and exposure|nitrogen dioxide]] from e.g. transportation technologies.<ref>{{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}}</ref><ref>{{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}}</ref><ref>{{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 |pagespage=164 |doi=10.3847/1538-4357/abd7f7 |url=https://iopscience.iop.org/article/10.3847/1538-4357/abd7f7 |access-date=6 March 2021 |language=en |issn=1538-4357|arxiv=2102.05027|bibcode=2021ApJ...908..164K |s2cid=231855390 |doi-access=free }}</ref> 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.<ref>{{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 }}</ref><ref name="10.1007/978-94-009-9115-6_20">{{cite journalbook |last1=Sullivan |first1=W. T., III |titlechapter=Radio leakageLeakage and eavesdroppingEavesdropping |journaltitle=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}}</ref> Unlike the earliest and conventional radio- and television-broadcasting which has been claimed to be undetectable at short distances,<ref>{{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}}</ref><ref>{{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}}</ref> 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.<ref>{{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=9780124467446978-0-12-446744-6 |language=en|url-access=subscription }}</ref><!--https://www.sciencedirect.com/topics/earth-and-planetary-sciences/radar-transmission-->

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]]<ref>{{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}}</ref> if the ___location of Earth is known.<ref>{{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 }}</ref><ref name="JCAP-2007">{{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 }}</ref><ref name="10.1007/978-94-009-9115-6_20"/> 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 [[biosignature]]s, including [[MERMOZ|MERMOZ instruments]] that may be capable of remotely detecting living matter on Earth.<ref name="AA-2021">{{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 |accessdateaccess-date=21 June 2021 |arxiv=2106.00493 |bibcode=2021A&A...651A..68P |s2cid=235265876 }}</ref><ref name="ARX-20210601">{{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 }}</ref><ref name="STD-20210620">{{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 |accessdateaccess-date=21 June 2021 }}</ref>

== Experiments ==

In 1980s, astronomer [[Carl Sagan]] persuaded NASA to perform an experiment of detecting life and civilization on Earth using instruments of the ''[[Galileo (spacecraft)|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.<ref>{{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 }}</ref><ref>{{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 }}</ref>

==See also==

{{div col|colwidth=30em}}

* [[Earliest known life forms]]

* {{annotated link|Exoplanetology}}

* [[List of exoplanet search projects]]

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* {{annotated link|Nexus for Exoplanet System Science}}

* {{annotated link|Planetary science}}

* {{annotated link|Quiet and loud aliens}}

{{div col end}}

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==External links==

* [httphttps://exoplanet.eu/home/ Extrasolar Planets Encyclopaedia] by the [[Paris Observatory]]

{{Astrobiology}}

{{Exoplanet}}

{{Exoplanet search projects}}

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[[Category:Astrobiology]]