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Main article: Astrobiology

The scientific search for extraterrestrial life is being carried out both directly and indirectly. As of September 2017, 3,667 exoplanets in 2,747 systems have been identified, and other planets and moons in the Solar System hold the potential for hosting primitive life such as microorganisms. As of 8 February 2021, an updated status of studies considering the possible detection of lifeforms on Venus (via phosphine) and Mars (via methane) was reported.[1]

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Lifeforms produce a variety of biosignatures that may be detectable by telescopes.[2][3]

Scientists search for biosignatures within the Solar System by studying planetary surfaces and examining meteorites.[4][5] Some claim to have identified evidence that microbial life has existed on Mars.[6][7][8][9] An experiment on the two Viking Mars landers reported gas emissions from heated Martian soil samples that some scientists argue are consistent with the presence of living microorganisms.[10] Lack of corroborating evidence from other experiments on the same samples suggests that a non-biological reaction is a more likely hypothesis.[10][11][12][13] In 1996, a controversial report stated that structures resembling nanobacteria were discovered in a meteorite, ALH84001, formed of rock ejected from Mars.[6][7]

Electron micrograph of Martian meteorite ALH84001 showing structures that some scientists think could be fossilized bacteria-like life forms

In February 2005 NASA scientists reported they may have found some evidence of extraterrestrial life on Mars.[14] The two scientists, Carol Stoker and Larry Lemke of NASA's Ames Research Center, based their claim on methane signatures found in Mars's atmosphere resembling the methane production of some forms of primitive life on Earth, as well as on their own study of primitive life near the Rio Tinto river in Spain. NASA officials soon distanced NASA from the scientists' claims, and Stoker herself backed off from her initial assertions.[15] Though such methane findings are still debated, support among some scientists for the existence of life on Mars exists.[16]

In November 2011 NASA launched the Mars Science Laboratory that landed the Curiosity rover on Mars. It is designed to assess the past and present habitability on Mars using a variety of scientific instruments. The rover landed on Mars at Gale Crater in August 2012.[17][18]

The Gaia hypothesis stipulates that any planet with a robust population of life will have an atmosphere in chemical disequilibrium, which is relatively easy to determine from a distance by spectroscopy. However, significant advances in the ability to find and resolve light from smaller rocky worlds near their stars are necessary before such spectroscopic methods can be used to analyze extrasolar planets. To that effect, the Carl Sagan Institute was founded in 2014 and is dedicated to the atmospheric characterization of exoplanets in circumstellar habitable zones.[19][20] Planetary spectroscopic data will be obtained from telescopes like WFIRST and ELT.[21]

The Green Bank Telescope is one of the radio telescopes used by the Breakthrough Listen project to search for alien communications.

In August 2011, findings by NASA, based on studies of meteorites found on Earth, suggest DNA and RNA components (adenine, guanine and related organic molecules), building blocks for life as we know it, may be formed extraterrestrially in outer space.[22][23][24] In October 2011, scientists reported that cosmic dust contains complex organic matter ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars.[25][26][27] One of the scientists suggested that these compounds may have been related to the development of life on Earth and said that, "If this is the case, life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life."[25]

In August 2012, and in a world first, astronomers at Copenhagen University reported the detection of a specific sugar molecule, glycolaldehyde, in a distant star system. The molecule was found around the protostellar binary IRAS 16293-2422, which is located 400 light years from Earth.[28][29] Glycolaldehyde is needed to form ribonucleic acid, or RNA, which is similar in function to DNA. This finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation.[30]

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Projects such as SETI are monitoring the galaxy for electromagnetic interstellar communications from civilizations on other worlds.[31][32] If there is an advanced extraterrestrial civilization, there is no guarantee that it is transmitting radio communications in the direction of Earth or that this information could be interpreted as such by humans. The length of time required for a signal to travel across the vastness of space means that any signal detected would come from the distant past.[33]

The presence of heavy elements in a star's light-spectrum is another potential biosignature; such elements would (in theory) be found if the star were being used as an incinerator/repository for nuclear waste products.[34]

Extrasolar planets

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Main article: Exoplanet
See also: List of multiplanetary systems
Artist's impression of Gliese 581 c, the first terrestrial extrasolar planet discovered within its star's habitable zone
Artist's impression of the Kepler telescope

Some astronomers search for extrasolar planets that may be conducive to life, narrowing the search to terrestrial planets within the habitable zones of their stars.[35][36] Since 1992, over four thousand exoplanets have been discovered (7,026 planets in 4,949 planetary systems including 1007 multiple planetary systems as of 24 July 2024).[37] The extrasolar planets so far discovered range in size from that of terrestrial planets similar to Earth's size to that of gas giants larger than Jupiter.[37] The number of observed exoplanets is expected to increase greatly in the coming years.[38]

The Kepler space telescope has also detected a few thousand[39][40] candidate planets,[41][42] of which about 11% may be false positives.[43]

There is at least one planet on average per star.[44] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone,[c] with the nearest expected to be within 12 light-years distance from Earth.[45][46] Assuming 200 billion stars in the Milky Way,[d] that would be 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if red dwarfs are included.[47] The rogue planets in the Milky Way possibly number in the trillions.[48]

The nearest known exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 pc) from Earth in the southern constellation of Centaurus.[49]

As of March 2014, the least massive exoplanet known is PSR B1257+12 A, which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is DENIS-P J082303.1-491201 b,[50][51] about 29 times the mass of Jupiter, although according to most definitions of a planet, it is too massive to be a planet and may be a brown dwarf instead. Almost all of the planets detected so far are within the Milky Way, but there have also been a few possible detections of extragalactic planets. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[52]

One sign that a planet probably already contains life is the presence of an atmosphere with significant amounts of oxygen, since that gas is highly reactive and generally would not last long without constant replenishment. This replenishment occurs on Earth through photosynthetic organisms. One way to analyze the atmosphere of an exoplanet is through spectrography when it transits its star, though this might only be feasible with dim stars like white dwarfs.[53]

Terrestrial analysis

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The science of astrobiology considers life on Earth as well, and in the broader astronomical context. In 2015, "remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia, when the young Earth was about 400 million years old.[54][55] According to one of the researchers, "If life arose relatively quickly on Earth, then it could be common in the universe."[54]

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