Detecting Earth from distant star-based systems

Short description: Detecting Earth as an exoplanet

Pale Blue Dot, a photograph of Earth taken on February 14, 1990, by the Voyager 1 space probe from a distance of approximately 6 billion kilometers (3.7 billion miles, 40.5 AU). Earth is seen as a tiny dot within deep space: the blueish-white speck almost halfway up the rightmost band of light.

There are several methods currently used by astronomers to detect distant exoplanets from Earth.[1] Theoretically, some of these methods can be used to detect Earth as an exoplanet from distant star systems.

## Contents

* 1 History
* 2 Experiments
* 3 See also
* 4 References
* 5 External links

## History

Star 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.[2] 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.[2] In June 2021, astronomers reported identifying 29 planets in habitable zones that may be capable of observing the Earth.[3] 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.[4][5][6][7]

Transit method is the most popular tool used to detect exoplanets and the most common tool to spectroscopically analyze exoplanetary atmospheres.[4] 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.[4]

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.[8][9][10] 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.[11][12] Unlike the earliest and conventional radio- and television-broadcasting which has been claimed to be undetectable at short distances,[13][14] 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.[15]

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[16] if the location of Earth is known.[17][18][12] 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.[19][20][21]

## 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. It was launched in December 1990, and when it was 960 km (600 mi) 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.[22][23]

## See also

* Earliest known life forms
* Astronomy:Exoplanetology - study of planets outside the Solar System
* List of exoplanet search projects
* Lists of exoplanets
* Astronomy:MERMOZ - Way of remotely detecting living matter
* Astronomy:Methods of detecting exoplanets
* Organization:Nexus for Exoplanet System Science
* Astronomy:Planetary science - Science of planets and planetary systems
* Astronomy:Quiet and loud aliens - Concept in astrobiology

## References

1. ↑ Staff (2020). "5 Ways to Find a Planet". NASA. https://exoplanets.nasa.gov/alien-worlds/ways-to-find-a-planet/.
2. ↑ 2.0 2.1 Kaltenegger, L.; Faherty, J.K. (23 June 2021). "Past, present and future stars that can see Earth as a transiting exoplanet". Nature 594 (7864): 505–507. doi:10.1038/s41586-021-03596-y. PMID 34163055. Bibcode: 2021Natur.594..505K. https://www.nature.com/articles/s41586-021-03596-y. Retrieved 23 June 2021.
3. ↑ Sample, Ian (23 June 2021). "Scientists identify 29 planets where aliens could observe Earth – Astronomers estimate 29 habitable planets are positioned to see Earth transit and intercept human broadcasts". The Guardian. https://www.theguardian.com/science/2021/jun/23/scientists-identify-29-planets-where-aliens-could-observe-earth.
4. ↑ 4.0 4.1 4.2 Kaltenegger, L.; Pepper, J. (20 October 2020). "Which stars can see Earth as a transiting exoplanet?". Monthly Notices of the Royal Astronomical Society 499 (1): L111–L115. doi:10.1093/mnrasl/slaa161. https://academic.oup.com/mnrasl/article/499/1/L111/5931805. Retrieved 24 October 2020.
5. ↑ Letzer, Rafi (22 October 2020). "Aliens on 1,000 nearby stars could see us, new study suggests". Live Science. https://www.livescience.com/aliens-spot-earth-exoplanets.html.
6. ↑ Friedlander, Blaine (21 October 2020). "Smile, wave: Some exoplanets may be able to see us, too". Cornell University. https://news.cornell.edu/stories/2020/10/smile-wave-some-exoplanets-may-be-able-see-us-too.
7. ↑ Carter, Jamie (22 October 2020). "Are We Being Watched? There Are 509 Star Systems With A Great View Of Life On Earth, Say Scientists". Forbes. 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/.
8. ↑ "Pollution on other planets could help us find aliens, Nasa says" (in en). The Independent. 12 February 2021. https://www.independent.co.uk/life-style/gadgets-and-tech/alien-pollution-planets-nasa-b1801543.html.
9. ↑ "Can Alien Smog Lead Us to Extraterrestrial Civilizations?" (in en-us). Wired. https://www.wired.com/story/can-alien-smog-lead-us-to-extraterrestrial-civilizations/. Retrieved 6 March 2021.
10. ↑ Kopparapu, Ravi; Arney, Giada; Haqq-Misra, Jacob; Lustig-Yaeger, Jacob; Villanueva, Geronimo (22 February 2021). "Nitrogen Dioxide Pollution as a Signature of Extraterrestrial Technology" (in en). The Astrophysical Journal 908 (2): 164. doi:10.3847/1538-4357/abd7f7. ISSN 1538-4357. Bibcode: 2021ApJ...908..164K.
11. ↑ Haqq-Misra, Jacob; Busch, Michael W.; Som, Sanjoy M.; Baum, Seth D. (1 February 2013). "The benefits and harm of transmitting into space" (in en). Space Policy 29 (1): 40–48. doi:10.1016/j.spacepol.2012.11.006. ISSN 0265-9646. Bibcode: 2013SpPol..29...40H. https://www.sciencedirect.com/science/article/abs/pii/S0265964612001361. Retrieved 9 April 2021.
12. ↑ 12.0 12.1 Sullivan, W. T. III (1980). "Radio Leakage and Eavesdropping". Strategies for the Search for Life in the Universe. Astrophysics and Space Science Library. 83. pp. 227–239. doi:10.1007/978-94-009-9115-6_20. ISBN 978-90-277-1226-4. Bibcode: 1980ASSL...83..227S. https://ui.adsabs.harvard.edu/abs/1980ASSL...83..227S/abstract. Retrieved 9 April 2021.
13. ↑ "How far from Earth could aliens detect our radio signals?" (in en). 21 December 2019. https://www.sciencefocus.com/space/how-far-from-earth-could-aliens-detect-our-radio-signals/.
14. ↑ "This is how far human radio broadcasts have reached into the galaxy" (in en). https://www.planetary.org/articles/3390.
15. ↑ "XI. Planets and Life around Other Stars" (in en). International Geophysics (Academic Press) 87: 592–608. 1 January 2004. doi:10.1016/S0074-6142(04)80025-1. ISBN 978-0-12-446744-6. Bibcode: 2004InGeo..87..592.. https://www.sciencedirect.com/science/article/pii/S0074614204800251. Retrieved 5 April 2021.
16. ↑ "How Far Into Space Can Radio Telescopes Hear?" (in en). https://www.forbes.com/sites/quora/2017/01/27/how-far-into-space-can-radio-telescopes-hear/.
17. ↑ De Magalhães, João Pedro (1 November 2016). "A direct communication proposal to test the Zoo Hypothesis" (in en). Space Policy 38: 22–26. doi:10.1016/j.spacepol.2016.06.001. ISSN 0265-9646. Bibcode: 2016SpPol..38...22D. ""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"".
18. ↑ Loeb, Avi; Zaldarriaga, Matias (22 January 2007). "Eavesdropping on radio broadcasts from galactic civilizations with upcoming observatories for redshifted 21 cm radiation". Journal of Cosmology and Astroparticle Physics 2007 (1): 020. doi:10.1088/1475-7516/2007/01/020. Bibcode: 2007JCAP...01..020L. https://iopscience.iop.org/article/10.1088/1475-7516/2007/01/020/fulltext/. Retrieved 9 April 2021.
19. ↑ Patty, C.H.L. (2021). "Biosignatures of the Earth I. Airborne spectropolarimetric detection of photosynthetic life". Astronomy & Astrophysics A68: 651. doi:10.1051/0004-6361/202140845. Bibcode: 2021A&A...651A..68P. https://www.aanda.org/component/article?access=doi&doi=10.1051/0004-6361/202140845. Retrieved 21 June 2021.
20. ↑ Patty, C.H. Luca (1 June 2021). "Biosignatures of the Earth". Astronomy & Astrophysics 651: A68. doi:10.1051/0004-6361/202140845. Bibcode: 2021A&A...651A..68P.
21. ↑ University of Bern (20 June 2021). "Scientists Use New Technology to Detect Signatures of Life Remotely". SciTechDaily.com. https://scitechdaily.com/scientists-use-new-technology-to-detect-signatures-of-life-remotely/amp/.
22. ↑ Witze, Alexandra (16 October 2023). "How would we know whether there is life on Earth? This bold experiment found out" (in en). Nature 622 (7983): 451–452. doi:10.1038/d41586-023-03230-z. PMID 37845527. Bibcode: 2023Natur.622..451W.
23. ↑ Sagan, Carl; Thompson, W. Reid; Carlson, Robert; Gurnett, Donald; Hord, Charles (October 23, 1993). "A search for life on Earth from the Galileo spacecraft". Nature 365 (6448): 715–721. doi:10.1038/365715a0. PMID 11536539. Bibcode: 1993Natur.365..715S. https://www.ufrgs.br/leaa/arquivos/aulas/SERP06/1993_ASearchForLifeOnEarthFromTheGalileoSpacecraft_NATURE.pdf.

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* Extrasolar Planets Encyclopaedia by the Paris Observatory

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