page_title: Extraterrestrial life
section_title: Extrasolar planets
hierarchical_section_title: Extraterrestrial life / Scientific search / Extrasolar planets
caption_attribution_description: English: Kepler Mission Overview NASA's Planet-hunting Kepler and K2 Missions Take Your Questions on Reddit October 27, 2014 http://www.nasa.gov/ames/kepler/nasas-planet-hunting-kepler-and-k2-missions-take-your-questions-on-reddit IMAGE: Artist’s conception of the Kepler space telescope observing planets transiting a distant star. DISCUSSION: Scientists and engineers from NASA's planet-hunting Kepler and K2 missions will answer questions about the missions on Reddit.com on Monday, Oct. 27 at 10 a.m. PDT. Reddit, a popular online community where users vote on content they find interesting, has a sub-forum for interviews with volunteers who answer questions about their specific experiences. This “I Am A ______, Ask Me Anything,” template attracts people from all walks of life, including high profile ones, like the president of the United States, and the Mars Curiosity team. Launched in 2009, Kepler is NASA's first mission to find and confirm small Earth-size planets around other stars in the habitable zone, the range of distance from a star where liquid water might pool on the surface of an orbiting planet. To date Kepler has identified more than 4,200 exoplanet candidates and verified nearly 1,000 as bonafide planets. Through Kepler discoveries, planets are now known to be common and diverse, showing the universe hosts a vast range of environments. After the failure of two of its four reaction wheels following the completion of data collection in its primary mission, the Kepler spacecraft was resuscitated this year and reborn as K2, a mission that extends the Kepler legacy to observations in the ecliptic – the part of the sky that is home to the familiar constellations of the zodiac. The K2 mission will continue exoplanet discovery, and introduces new scientific observation opportunities to study notable star clusters, young and old stars, active galaxies and supernovae. Those interested in posting questions about the missions will be able to do so starting at approximately 7:00 a.m. Kepler/K2 team members will answer questions for about two hours, beginning at 10 a.m. on the Reddit Science forum: https://www.reddit.com/r/science/
context_page_description: Extraterrestrial life is hypothetical life which may occur outside of Earth and which did not originate on Earth. Such life might range from simple prokaryotes to beings with civilizations far more advanced than humanity. The Drake equation speculates about the existence of intelligent life elsewhere in the universe. The science of extraterrestrial life in all its forms is known as astrobiology.
  Since the mid-20th century, active ongoing research has taken place to look for signs of extraterrestrial life. This encompasses a search for current and historic extraterrestrial life, and a narrower search for extraterrestrial intelligent life. Depending on the category of search, methods range from the analysis of telescope and specimen data to radios used to detect and send communication signals.
  The concept of extraterrestrial life, and particularly extraterrestrial intelligence, has had a major cultural impact, chiefly in works of science fiction. Over the years, science fiction has introduced a number of theoretical ideas, each having a wide range of possibilities. Many have piqued public interest in the possibilities of extraterrestrial life.
context_section_description: Some astronomers search for extrasolar planets that may be conducive to life, narrowing the search to terrestrial planets within the habitable zone of their star. Since 1992 over four thousand exoplanets have been discovered (4,301 planets in 3,176 planetary systems including 703 multiple planetary systems as of 1 August 2020).
  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. The number of observed exoplanets is expected to increase greatly in the coming years.
  The Kepler space telescope has also detected a few thousand candidate planets, of which about 11% may be false positives.
  There is at least one planet on average per star. About 1 in 5 Sun-like stars have an "Earth-sized" planet in the habitable zone, with the nearest expected to be within 12 light-years distance from Earth. Assuming 200 billion stars in the Milky Way, that would be 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if red dwarfs are included. The rogue planets in the Milky Way possibly number in the trillions.
  The nearest known exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 pc) from Earth in the southern constellation of Centaurus.
  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, 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.
  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.
caption: Artist's impression of the Kepler telescope
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