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If extraterrestrial life exists, it could range from simple microorganisms and multicellular organisms similar to animals or plants, to complex alien intelligences akin to humans. When scientists talk about extraterrestrial life, they consider all those types.
- Overview
- Life beyond the solar system
- The Drake equation and extrasolar life
For thousands of years humans have wondered whether they were alone in the universe or whether other worlds populated by more or less humanlike creatures might exist. In ancient times and throughout the Middle Ages, the common view was that Earth was the only “world” in the universe. Many mythologies populated the sky with divine beings, certainly a kind of extraterrestrial life. Some philosophers held that life was not unique to Earth. Metrodorus, an Epicurean in the 3rd and 4th centuries bce, argued that
to consider the Earth the only populated world in infinite space is as absurd as to assert that in an entire field sown with millet, only one grain will grow.
For thousands of years humans have wondered whether they were alone in the universe or whether other worlds populated by more or less humanlike creatures might exist. In ancient times and throughout the Middle Ages, the common view was that Earth was the only “world” in the universe. Many mythologies populated the sky with divine beings, certainly a kind of extraterrestrial life. Some philosophers held that life was not unique to Earth. Metrodorus, an Epicurean in the 3rd and 4th centuries bce, argued that
to consider the Earth the only populated world in infinite space is as absurd as to assert that in an entire field sown with millet, only one grain will grow.
American astrophysicist Frank D. Drake devised a simple approach that illuminates the uncertainties involved in determining whether extraterrestrial intelligence is possible. The number of extant technical civilizations in the Milky Way Galaxy is estimated by the following equation (the so-called Drake equation, or Green Bank formula): N = R*fpneflfifcL where R* is the average rate of star formation over the lifetime of the Milky Way Galaxy, fp is the fraction of stars with planetary systems, ne is the mean number of planets per star that are ecologically suitable for the origin and evolution of life, fl is the fraction of such planets on which life arises, fi is the fraction of such planets on which intelligent life evolves, fc is the fraction of such planets on which a technical civilization develops, and L is the mean lifetime of a technical civilization. A consideration of the factors involved in the choice of numerical values for each parameter follows. These estimates are little better than informed guesses; no great reliability should be pretended for them.
There are about 200 billion stars in the Milky Way Galaxy. The age of the Milky Way Galaxy is about 10 billion years. A value of R* = 10 stars per year is probably fairly reliable. While most contemporary theories of star formation imply that the origin of planets accompanies the origin of stars, such theories are not developed well enough to merit much confidence. More than 250 extrasolar planets have been confirmed. They have been observed via several different means: by “wobble,” which detects the changing wavelength of a star’s light as the star gets closer and then farther away from Earth as a massive planet tugs it from the centre of the system; by transit, which detects the dimming of a star as a planet crosses between it and Earth, much like a solar eclipse; and by infrared observation, which observes a planet directly.
Owing to limitations of current detection methods, most of the planets discovered so far have masses at least as great as the solar system’s largest planets, Jupiter and Saturn. Most of these planets are also very close to their stars, much closer than Earth is to the Sun, so life systems similar to Earth’s could not exist on them. This difference has led scientists to consider new models for planetary formation. For example, Gliese 876, a red dwarf star one-third the mass of the Sun and 15 light-years away in Aquarius, has three planets: a gas giant half the mass of Jupiter that orbits Gliese 876 every 30 Earth days, another twice the mass of Jupiter that orbits exactly once for every two orbits of its neighbour, and a third six times the mass of Earth that orbits every 2 Earth days. HD 168443, lying 123 light-years away in Serpens, has one planet 8 times Jupiter’s mass and another 18 times the mass of Jupiter, which is beyond the scale that had been considered possible for a planet; this giant may be a brown dwarf. Several planets even smaller than Saturn have been found. Gliese 581, at 20 light-years away, has three planets, one of which is only five times the mass of Earth. NASA’s Kepler mission, launched in 2009, used space-based telescopes to observe Sun-like stars that host Earth-sized planets and discovered thousands of extrasolar planets.
Because the wobble method can detect only a planet that has been observed for a significant portion of its orbit, finding a planet like Jupiter with an orbital period of 12 Earth years requires several years of observations. Nevertheless, some planetary systems similar to the solar system have been found. HD 190360A, at 52 light-years away, is very similar to the Sun and has a detectable planet similar in size and orbital distance to Jupiter. Systems like HD 190360A could also have smaller planets arranged like those in the solar system. The existence of large planets around so many nearby stars demonstrates that a significant fraction of stars do indeed have planets in orbit around them.
Another indication that planetary formation is a general process throughout the universe is the satellite systems of the major planets of the solar system. Jupiter with 79 satellites, Saturn with 82, and Uranus with 27 resemble miniature solar systems. Considering the wide range of temperatures that seem to be compatible with life, it can be tentatively concluded that fpne is about 1. However, since liquid water is considered to be crucial to life’s origin and evolution, fpne probably has a significantly smaller value.
Because of the short time it took for life to arise on Earth, as implied by the fossil record, and because of the ease with which relevant organic molecules are produced in experiments that simulate the early Earth, the likelihood of life’s arising during a period of billions of years may be high. Some scientists believe that the appropriate value of fl, the fraction of planets with life, is about 1. For the quantities of fi, the fraction of planets with intelligent life, the parameters are even more uncertain. The evolutionary path leading to mammals depends on a great many specific circumstances and historical accidents; it is therefore highly unlikely that such a path will ever repeat. However, intelligence clearly has a great selective advantage and is not necessarily restricted to the single evolutionary path that occurred on Earth.
Sep 5, 2024 · Extraterrestrial life refers to life that may exist or may have existed in the universe outside of Earth. The search for extraterrestrial life encompasses many fundamental scientific questions, including ‘What are the basic requirements for life?’ and ‘Are there other planets like Earth?’
Feb 1, 2023 · 19 min read. The Search for Extraterrestrial Life as We Don’t Know It. Scientists are abandoning conventional thinking to search for extraterrestrial creatures that bear little resemblance...
Mar 18, 2021 · We marvel that humans, dolphins and octopuses evolved some degree of intelligence, for example, but the three animals have a common ancestry that goes back hundreds of millions of years before they split off from one another.
Sep 5, 2024 · The search for extraterrestrial life is most clearly grasped by imagining the reverse situation. For example, if humans were on Mars , examination of Earth for life with the full armory of contemporary scientific instrumentation and knowledge would be illuminating .
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The branch of biology concerned with extraterrestrial life, from microscopic organisms to intelligent beings, is called exobiology or astrobiology. Scientists in this field consider the conditions necessary for life and how it evolves.
