In mathematics , the eccentricity of a conic section is a non-negative real number that uniquely characterizes its shape.
103-457: Proxima Centauri b (or Proxima b ), also referred to as Alpha Centauri Cb , is an exoplanet orbiting within the habitable zone of the red dwarf star Proxima Centauri , which is the closest star to the Sun and part of the larger triple star system Alpha Centauri . It is about 4.2 light-years (1.3 parsecs ) from Earth in the constellation Centaurus , making it and Proxima d , along with
206-400: A binary star system, and several circumbinary planets have been discovered which orbit both members of a binary star. A few planets in triple star systems are known and one in the quadruple system Kepler-64 . In 2013, the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue. Later that same year,
309-499: A pulsar planet in orbit around PSR 1829-10 , using pulsar timing variations. The claim briefly received intense attention, but Lyne and his team soon retracted it. As of 24 July 2024, a total of 5,787 confirmed exoplanets are listed in the NASA Exoplanet Archive, including a few that were confirmations of controversial claims from the late 1980s. The first published discovery to receive subsequent confirmation
412-402: A rectangular hyperbola is 2 {\displaystyle {\sqrt {2}}} . The eccentricity of a three-dimensional quadric is the eccentricity of a designated section of it. For example, on a triaxial ellipsoid, the meridional eccentricity is that of the ellipse formed by a section containing both the longest and the shortest axes (one of which will be the polar axis), and
515-416: A G2-type star. On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo. This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near
618-514: A bright star with an apparent magnitude of 0.40 in the constellation of Cassiopeia . The brightness of the Sun would be similar to that of Achernar or Procyon from Earth. Exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System . The first possible evidence of an exoplanet was noted in 1917 but was not then recognized as such. The first confirmation of
721-451: A distance of about 0.049 AU , over 20 times closer to Proxima Centauri than Earth is to the Sun. As of 2021, it is unclear whether it has an eccentricity but Proxima Centauri b is unlikely to have any obliquity . The age of the planet is unknown; Proxima Centauri itself may have been captured by Alpha Centauri and thus not necessarily of the same age as the latter pair of stars, which are about 5 billion years old. Proxima Centauri b
824-408: A gaseous protoplanetary disk , they accrete hydrogen / helium envelopes. These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space. This means that even terrestrial planets may start off with large radii if they form early enough. An example is Kepler-51b which has only about twice the mass of Earth but
927-426: A kilometre) ice cover that may be slushy in some places. Additional factors are: The stability of an atmosphere is a major issue for the habitability of Proxima Centauri b: Even if Proxima Centauri b lost its original atmosphere, volcanic activity could rebuild it after some time. A second atmosphere would likely contain carbon dioxide , which would make it more stable than an Earth-like atmosphere, particularly in
1030-503: A long time to travel interstellar distances ; Voyager 2 would take about 75,000 years to reach Proxima Centauri. Among the proposed technologies to reach Proxima Centauri b in human lifespans are solar sails that could reach speeds of 20% the speed of light ; problems would be how to decelerate a probe when it arrives in the Proxima Centauri system and collisions of the high-speed probes with interstellar particles . Among
1133-411: A mass equivalent to 0.122 M ☉ and a radius of 0.154 R ☉ that of the Sun. With an effective temperature of 3,050 ± 100 Kelvin , it has a spectral type of M5.5V . The magnetic field of Proxima Centauri is considerably stronger than that of the Sun, with an intensity of 600 ± 150 G ; it varies in a seven-year-long cycle. It is the closest star to
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#17327829478121236-527: A minimum mass of at least 1.07 M 🜨 and only a slightly larger radius than that of Earth. The planet orbits within the habitable zone of its parent star; but it is not known whether it has an atmosphere, which would impact the habitability probabilities. Proxima Centauri is a flare star with intense emission of electromagnetic radiation that could strip an atmosphere off the planet. The exoplanet's proximity to Earth offers an opportunity for robotic space exploration. Announced on 24 August 2016 by
1339-417: A number of related additional concepts (only for ellipses): The eccentricity of an ellipse is, most simply, the ratio of the linear eccentricity c (distance between the center of the ellipse and each focus) to the length of the semimajor axis a . The eccentricity is also the ratio of the semimajor axis a to the distance d from the center to the directrix: The eccentricity can be expressed in terms of
1442-413: A number of scenarios are possible: A planet partially or wholly covered with ice, planet-wide or small oceans or only dry land, combinations between these, scenarios with one or two "eyeballs" or lobster -shaped areas with liquid water (meaning near the equator, with two nearly identical areas on each hemisphere, sprouting from the equator like lobster claws), or a subsurface ocean with a thin (less than
1545-440: A planet around Alpha Centauri in 2012 was considered questionable. Despite its proximity to Earth, Proxima Centauri is too faint to be visible to the naked eye, except during superflares . Proxima Centauri b is located within the classical habitable zone of its star and receives about 65% of Earth's irradiation. Its equilibrium temperature is estimated to be about 234 K (−39 °C; −38 °F). Various factors, such as
1648-425: A planet may be able to be formed in their orbit. In the early 1990s, a group of astronomers led by Donald Backer , who were studying what they thought was a binary pulsar ( PSR B1620−26 b ), determined that a third object was needed to explain the observed Doppler shifts . Within a few years, the gravitational effects of the planet on the orbit of the pulsar and white dwarf had been measured, giving an estimate of
1751-560: A planet. As normally understood in the context of exoplanet life, liquid water on the surface and an atmosphere are prerequisites for habitability—any life limited to the subsurface of a planet, such as in a subsurface ocean , like those that reside in Europa in the Solar System, would be difficult to detect from afar although it may constitute a model for life in a cold ocean-covered Proxima Centauri b. The habitability of red dwarfs
1854-522: A position statement containing a working definition of "planet" in 2001 and which was modified in 2003. An exoplanet was defined by the following criteria: This working definition was amended by the IAU's Commission F2: Exoplanets and the Solar System in August 2018. The official working definition of an exoplanet is now as follows: The IAU's working definition is not always used. One alternate suggestion
1957-553: A separate category of planets, especially if they are gas giants , often counted as sub-brown dwarfs . The rogue planets in the Milky Way possibly number in the billions or more. The official definition of the term planet used by the International Astronomical Union (IAU) only covers the Solar System and thus does not apply to exoplanets. The IAU Working Group on Extrasolar Planets issued
2060-409: A significant effect. There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase -dependent, this is not always the case in the near infrared. Temperatures of gas giants reduce over time and with distance from their stars. Lowering the temperature increases optical albedo even without clouds. At
2163-406: A star's habitable zone (sometimes called "goldilocks zone"), where it is possible for liquid water, a prerequisite for life as we know it, to exist on the surface. However, the study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life. Rogue planets are those that do not orbit any star. Such objects are considered
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#17327829478122266-451: A statistical technique called "verification by multiplicity". Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they were more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth. On 23 July 2015, NASA announced Kepler-452b , a near-Earth-size planet orbiting the habitable zone of
2369-403: A sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures, ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths. Eccentricity (mathematics) One can think of the eccentricity as a measure of how much a conic section deviates from being circular. In particular: Two conic sections with
2472-467: A team of astronomers launched the Pale Red Dot project to confirm this hypothetical planet's existence. On 24 August 2016, the team led by Anglada-Escudé proposed that a terrestrial exoplanet in the habitable zone of Proxima Centauri could explain these anomalies and announced Proxima Centauri b's discovery. In 2022, another planet named Proxima Centauri d , which orbits even closer to the star,
2575-508: Is HR 2562 b , about 30 times the mass of Jupiter . However, according to some definitions of a planet (based on the nuclear fusion of deuterium ), it is too massive to be a planet and might be a brown dwarf . Known orbital times for exoplanets vary from less than an hour (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it. Almost all planets detected so far are within
2678-459: Is a controversial subject, with a number of considerations: On the other hand, red dwarfs like Proxima Centauri have a lifespan much longer than the Sun, exceeding the estimated age of the Universe , and thus give life plenty of time to develop. The radiation emitted by Proxima Centauri is ill-suited for oxygen-generating photosynthesis but sufficient for anoxygenic photosynthesis although it
2781-470: Is a member. As of 2023, Proxima Centauri b is believed to be the best-known exoplanet to the general public. Proxima Centauri had become a target for exoplanet searches already before the discovery of Proxima Centauri b, but initial studies in 2008 and 2009 ruled out the existence of larger-than-Earth exoplanets in the habitable zone. Planets are very common around dwarf stars, with on average 1–2 planets per star, and about 20–40% of all red dwarfs have one in
2884-529: Is almost the size of Saturn, which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old. 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 . Most known exoplanets orbit stars roughly similar to the Sun , i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars ( red dwarfs , of spectral category M) are less likely to have planets massive enough to be detected by
2987-415: Is likely to be tidally locked to the host star, which for a 1:1 orbit would mean that the same side of the planet would always face Proxima Centauri. It is unclear whether habitable conditions can arise under such circumstances as a 1:1 tidal lock would lead to an extreme climate with only part of the planet habitable. However, the planet may not be tidally locked. If the eccentricity of Proxima Centauri b
3090-820: Is monitored for the possible emission of technology-related radio signals by the Breakthrough Listen project which in April–May 2019 detected the BLC1 signal; later investigations, however, indicated it is probably of human origin. Future large ground-based telescopes and space-based observatories such as the James Webb Space Telescope and the Nancy Grace Roman Space Telescope could directly observe Proxima Centauri b, given its proximity to Earth, but disentangling
3193-422: Is not defined). It is worth to note that a parabola can be treated as an ellipse or a hyperbola, but with one focal point at infinity . The eccentricity is sometimes called the first eccentricity to distinguish it from the second eccentricity and third eccentricity defined for ellipses (see below). The eccentricity is also sometimes called the numerical eccentricity . In the case of ellipses and hyperbolas
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3296-707: Is not known why TrES-2b is so dark—it could be due to an unknown chemical compound. For gas giants , geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have
3399-510: Is now clear that hot Jupiters make up the minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets. Kepler-16 contains the first discovered planet that orbits a binary main-sequence star system. On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope . These exoplanets were checked using
3502-425: Is oriented with its axis vertical, the eccentricity is where β is the angle between the plane and the horizontal and α is the angle between the cone's slant generator and the horizontal. For β = 0 {\displaystyle \beta =0} the plane section is a circle, for β = α {\displaystyle \beta =\alpha } a parabola. (The plane must not meet
3605-421: Is strictly less than 1. When circles (which have eccentricity 0) are counted as ellipses, the eccentricity of an ellipse is greater than or equal to 0; if circles are given a special category and are excluded from the category of ellipses, then the eccentricity of an ellipse is strictly greater than 0. For any ellipse, let a be the length of its semi-major axis and b be the length of its semi-minor axis . In
3708-466: Is that planets should be distinguished from brown dwarfs on the basis of their formation. It is widely thought that giant planets form through core accretion , which may sometimes produce planets with masses above the deuterium fusion threshold; massive planets of that sort may have already been observed. Brown dwarfs form like stars from the direct gravitational collapse of clouds of gas, and this formation mechanism also produces objects that are below
3811-501: Is unclear how life depending on anoxygenic photosynthesis could be detected. One study in 2017 estimated that the productivity of a Proxima Centauri b ecosystem based on photosynthesis may be about 20% that of Earth's. As of 2021, Proxima Centauri b has not yet been directly imaged, as its separation from Proxima Centauri is too small. It is unlikely to transit Proxima Centauri from Earth's perspective; all surveys have failed to find evidence for any transits of Proxima Centauri b. The star
3914-411: Is unlikely to have stable orbits for moons . As of 2020, the estimated minimum mass of Proxima Centauri b is 1.173 ± 0.086 M 🜨 ; other estimates are similar, with the most recent estimate as of 2022 being at least 1.07 ± 0.06 M 🜨 , but all estimates are minimum because the inclination of the planet's orbit is not yet known. This makes it similar to Earth , but the radius of
4017-403: Is whether the core pressure is dominated by Coulomb pressure or electron degeneracy pressure with the dividing line at around 5 Jupiter masses. The convention for naming exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking
4120-549: The 13 M Jup limit and can be as low as 1 M Jup . Objects in this mass range that orbit their stars with wide separations of hundreds or thousands of Astronomical Units (AU) and have large star/object mass ratios likely formed as brown dwarfs; their atmospheres would likely have a composition more similar to their host star than accretion-formed planets, which would contain increased abundances of heavier elements. Most directly imaged planets as of April 2014 are massive and have wide orbits so probably represent
4223-572: The European Southern Observatory (ESO), Proxima Centauri b was confirmed via several years of using the method of studying the radial velocity of its parent star. Furthermore, the discovery of Proxima Centauri b, a planet at habitable distances from the closest star to the Solar System, was a major discovery in planetology , and has drawn interest to the Alpha Centauri star system as a whole, of which Proxima itself
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4326-600: The Milky Way , it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included. The least massive exoplanet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon . The most massive exoplanet listed on the NASA Exoplanet Archive
4429-401: The Milky Way galaxy . Planets are extremely faint compared to their parent stars. For example, a Sun-like star is about a billion times brighter than the reflected light from any exoplanet orbiting it. It is difficult to detect such a faint light source, and furthermore, the parent star causes a glare that tends to wash it out. It is necessary to block the light from the parent star to reduce
4532-546: The Mount Wilson Observatory , produced a spectrum of the star using Mount Wilson's 60-inch telescope . He interpreted the spectrum to be of an F-type main-sequence star , but it is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized by the gravity of the star, the resulting dust then falling onto the star. The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards,
4635-580: The Observatoire de Haute-Provence , ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics . Technological advances, most notably in high-resolution spectroscopy , led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing
4738-405: The equatorial eccentricity is the eccentricity of the ellipse formed by a section through the centre, perpendicular to the polar axis (i.e. in the equatorial plane). But: conic sections may occur on surfaces of higher order, too (see image). In celestial mechanics , for bound orbits in a spherical potential, the definition above is informally generalized. When the apocenter distance is close to
4841-458: The flattening f (defined as f = 1 − b / a {\displaystyle f=1-b/a} for semimajor axis a and semiminor axis b ): (Flattening may be denoted by g in some subject areas if f is linear eccentricity.) Define the maximum and minimum radii r max {\displaystyle r_{\text{max}}} and r min {\displaystyle r_{\text{min}}} as
4944-480: The pericenter distance, the orbit is said to have low eccentricity; when they are very different, the orbit is said be eccentric or having eccentricity near unity. This definition coincides with the mathematical definition of eccentricity for ellipses, in Keplerian, i.e., 1 / r {\displaystyle 1/r} potentials. A number of classifications in mathematics use derived terminology from
5047-543: The radial-velocity method . Despite this, several tens of planets around red dwarfs have been discovered by the Kepler space telescope , which uses the transit method to detect smaller planets. Using data from Kepler , a correlation has been found between the metallicity of a star and the probability that the star hosts a giant planet, similar to the size of Jupiter . Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity. Some planets orbit one member of
5150-413: The Milky Way. However, there is evidence that extragalactic planets , exoplanets located in other galaxies, may exist. The nearest exoplanets are located 4.2 light-years (1.3 parsecs ) from Earth and orbit Proxima Centauri , the closest star to the Sun. The discovery of exoplanets has intensified interest in the search for extraterrestrial life . There is special interest in planets that orbit in
5253-446: The Sun and are likewise accompanied by planets. In the eighteenth century, the same possibility was mentioned by Isaac Newton in the " General Scholium " that concludes his Principia . Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One ." In 1938, D.Belorizky demonstrated that it
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#17327829478125356-458: The Sun's energy. Despite this low radiation, due to its close orbit Proxima Centauri b still receives about 70% of the amount of infrared energy that the Earth receives from the Sun. That said, Proxima Centauri is also a flare star with its luminosity at times varying by a factor of 100 over a timespan of hours, its luminosity averaged at 0.155 ± 0.006 L ☉ . Proxima Centauri has
5459-433: The Sun, hence the name "Proxima", with a distance of 4.2426 ± 0.0020 light-years (1.3008 ± 0.0006 pc). Proxima Centauri is part of a multiple star system, whose other members are Alpha Centauri A and Alpha Centauri B which form a binary star subsystem. The dynamics of the multiple star system could have caused Proxima Centauri b to move closer to its host star over its history. The detection of
5562-497: The advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is physically unmotivated for planets with rocky cores, and observationally problematic due to the sin i ambiguity ." The NASA Exoplanet Archive includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses. Another criterion for separating planets and brown dwarfs, rather than deuterium fusion, formation process or location,
5665-405: The amount of water lost could be considerably larger and Airapetian et al. 2017 concluded that an atmosphere would be lost within ten million years. The estimates are strongly dependent on the initial mass of the atmosphere, however, and are thus highly uncertain. In the context of exoplanet research, "habitability" is usually defined as the possibility that liquid water exists on the surface of
5768-416: The atmosphere after the planet's early history. However, it is also possible that Proxima Centauri b had a primordial hydrogen atmosphere or formed farther away from its star, which would have reduced the escape of water. Thus, Proxima Centauri b may have kept its water beyond its early history. If an atmosphere exists, it is likely to contain oxygen-bearing gases such as oxygen and carbon dioxide. Together with
5871-420: The colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color, and Kappa Andromedae b , which if seen up close would appear reddish in color. Helium planets are expected to be white or grey in appearance. The apparent brightness ( apparent magnitude ) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light
5974-557: The composition of the object. As of 2011, the Extrasolar Planets Encyclopaedia included objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around 13 M Jup in the observed mass spectrum reinforces the choice to forget this mass limit". As of 2016, this limit was increased to 60 Jupiter masses based on a study of mass–density relationships. The Exoplanet Data Explorer includes objects up to 24 Jupiter masses with
6077-440: The conic section is not a parabola (which has eccentricity equal to 1), not a degenerate hyperbola or degenerate ellipse , and not an imaginary ellipse: where η = 1 {\displaystyle \eta =1} if the determinant of the 3×3 matrix is negative or η = − 1 {\displaystyle \eta =-1} if that determinant is positive. The eccentricity of an ellipse
6180-405: The coordinate system with origin at the ellipse's center and x -axis aligned with the major axis, points on the ellipse satisfy the equation with foci at coordinates ( ± c , 0 ) {\displaystyle (\pm c,0)} for c = a 2 − b 2 . {\textstyle c={\sqrt {a^{2}-b^{2}}}.} We define
6283-410: The currently disputed Proxima c , the closest known exoplanets to the Solar System . Proxima Centauri b orbits its parent star at a distance of about 0.04856 AU (7.264 million km; 4.514 million mi) with an orbital period of approximately 11.2 Earth days. Its other properties are only poorly understood as of 2024, but it is believed to be a potentially Earth-like planet with
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#17327829478126386-400: The designated or proper name of its parent star, and adding a lower case letter. Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to
6489-558: The detection occurred in 1992. A different planet, first detected in 1988, was confirmed in 2003. As of 7 November 2024, there are 5,787 confirmed exoplanets in 4,320 planetary systems , with 969 systems having more than one planet . The James Webb Space Telescope (JWST) is expected to discover more exoplanets, and to give more insight into their traits, such as their composition , environmental conditions , and potential for life . There are many methods of detecting exoplanets . Transit photometry and Doppler spectroscopy have found
6592-583: The existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars. However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable. During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star . Astronomers now generally regard all early reports of detection as erroneous. In 1991, Andrew Lyne , M. Bailes and S. L. Shemar claimed to have discovered
6695-410: The exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion." On 21 March 2022, the 5000th exoplanet beyond the Solar System was confirmed. On 11 January 2023, NASA scientists reported the detection of LHS 475 b , an Earth-like exoplanet – and
6798-414: The first confirmation of detection came in 1992 when Aleksander Wolszczan announced the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12 . The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi . Some exoplanets have been imaged directly by telescopes, but
6901-514: The first exoplanet discovered by the James Webb Space Telescope . This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own. In the sixteenth century, the Italian philosopher Giordano Bruno , an early supporter of the Copernican theory that Earth and other planets orbit the Sun ( heliocentrism ), put forward the view that fixed stars are similar to
7004-431: The glare while leaving the light from the planet detectable; doing so is a major technical challenge which requires extreme optothermal stability . All exoplanets that have been directly imaged are both large (more massive than Jupiter ) and widely separated from their parent stars. Specially designed direct-imaging instruments such as Gemini Planet Imager , VLT-SPHERE , and SCExAO will image dozens of gas giants, but
7107-572: The habitable zone, some around Sun-like stars. In September 2020, astronomers reported evidence, for the first time, of an extragalactic planet , M51-ULS-1b , detected by eclipsing a bright X-ray source (XRS), in the Whirlpool Galaxy (M51a). Also in September 2020, astronomers using microlensing techniques reported the detection , for the first time, of an Earth-mass rogue planet unbounded by any star, and free floating in
7210-523: The habitable zone. Additionally, red dwarfs are by far the most common type of stars. Before 2016, observations with instruments at the European Southern Observatory in Chile had identified anomalies in Proxima Centauri which could not be satisfactorily explained by flares or chromospheric activity of the star. This suggested that Proxima Centauri may be orbited by a planet. In January 2016,
7313-410: The linear eccentricity is sometimes called the half-focal separation . Three notational conventions are in common use: This article uses the first notation. Here, for the ellipse and the hyperbola, a is the length of the semi-major axis and b is the length of the semi-minor axis. When the conic section is given in the general quadratic form the following formula gives the eccentricity e if
7416-402: The low-mass end of a brown dwarf formation. One study suggests that objects above 10 M Jup formed through gravitational instability and should not be thought of as planets. Also, the 13-Jupiter-mass cutoff does not have a precise physical significance. Deuterium fusion can occur in some objects with a mass below that cutoff. The amount of deuterium fused depends to some extent on
7519-483: The mass of the third object that was too small for it to be a star. The conclusion that the third object was a planet was announced by Stephen Thorsett and his collaborators in 1993. On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi . This discovery, made at
7622-401: The maximum and minimum distances from either focus to the ellipse (that is, the distances from either focus to the two ends of the major axis). Then with semimajor axis a , the eccentricity is given by which is the distance between the foci divided by the length of the major axis. The eccentricity of a hyperbola can be any real number greater than 1, with no upper bound. The eccentricity of
7725-410: The most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone. In several cases, multiple planets have been observed around a star. About 1 in 5 Sun-like stars are estimated to have an " Earth -sized" planet in the habitable zone . Assuming there are 200 billion stars in
7828-579: The nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi . In 1855, William Stephen Jacob at the East India Company 's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system. In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved
7931-443: The orbital properties of Proxima Centauri b, the spectrum of radiation emitted by Proxima Centauri and the behaviour of clouds and hazes influence the climate of an atmosphere-bearing Proxima Centauri b. There are two likely scenarios for an atmosphere of Proxima Centauri b: in one case, the planet's water could have condensed and the hydrogen would have been lost to space, which would have only left oxygen and/or carbon dioxide in
8034-453: The planet consists of and the changes of these parameters over time. Proxima Centauri b likely developed under different conditions from Earth, with less water, stronger impacts and an overall faster development, assuming that it formed at its current distance from the star. Proxima Centauri b probably did not form at its current distance to Proxima Centauri, as the amount of material in the protoplanetary disk would be insufficient. Instead,
8137-443: The planet from its star would be difficult. Possible traits observable from Earth are the reflection of starlight from an ocean, the radiative patterns of atmospheric gases and hazes and of atmospheric heat transport. Efforts have been done to determine what Proxima Centauri b would look like to Earth if it has particular properties such as atmospheres of a particular composition. Even the fastest spacecraft built by humans would take
8240-433: The planet is poorly known and hard to determine—estimates based on possible composition give a range of 0.94 to 1.4 R 🜨 , and its mass may border on the cutoff between Earth-type and Neptune -type planets, if that value is lower than previously estimated. Depending on the composition, Proxima Centauri b could range from being a Mercury -like planet with a large core —which would require particular conditions early in
8343-556: The planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with a high albedo that is far from the star. The darkest known planet in terms of geometric albedo is TrES-2b , a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres, but it
8446-416: The planet's mantle , increasing volcanic activity and potentially shutting down a magnetic field-generating dynamo . The exact dynamics are strongly dependent on the internal structure of the planet and its evolution in response to tidal heating. Proxima b's parent star Proxima Centauri is a red dwarf , radiating only 0.005% of the amount of visible light that the Sun does and an average of about 0.17% of
8549-406: The planet's existence to be confirmed. On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12 . This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived
8652-481: The planet's history—to a very water-rich planet. Observations of the Fe – Si – Mg ratios of Proxima Centauri may allow a determination of the composition of the planet, since they are expected to roughly match the ratios of any planetary bodies in the Proxima Centauri system; various observations have found Solar System-like ratios of these elements. Little is known about Proxima Centauri b as of 2021—mainly its distance from
8755-422: The planet, or protoplanetary fragments, likely formed at larger distances and then migrated to the current orbit of Proxima Centauri b. Depending on the nature of the precursor material, it may be rich in volatiles. A number of different formation scenarios are possible, many of which depend on the existence of other planets around Proxima Centauri and which would result in different compositions. Proxima Centauri b
8858-549: The presence of an ocean, which, depending on its size, as well as the atmospheric mass and composition, may contribute to preventing atmospheric collapse. Additionally, impacts of exocomets could resupply water to Proxima Centauri b, if they are present. A number of mechanisms can deliver water to a developing planet; how much water Proxima Centauri b received is unknown. Modelling by Ribas et al. 2016 indicates that Proxima Centauri b would have lost no more than one Earth ocean 's equivalent of water but later research suggested that
8961-486: The projects of travelling to Proxima Centauri b are the Breakthrough Starshot project, which aims to develop instruments and power systems that can reach Proxima Centauri in the 21st century. From the surface of Proxima Centauri b, the binary stars Alpha Centauri would be considerably brighter than Venus is from Earth, with an apparent magnitude of −6.8 and −5.2, respectively. The Sun would appear as
9064-404: The same eccentricity are similar . Any conic section can be defined as the locus of points whose distances to a point (the focus) and a line (the directrix) are in a constant ratio. That ratio is called the eccentricity, commonly denoted as e . The eccentricity can also be defined in terms of the intersection of a plane and a double-napped cone associated with the conic section. If the cone
9167-574: The so-called small planet radius gap . The gap, sometimes called the Fulton gap, is the observation that it is unusual to find exoplanets with sizes between 1.5 and 2 times the radius of the Earth. In January 2020, scientists announced the discovery of TOI 700 d , the first Earth-sized planet in the habitable zone detected by TESS. As of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed, several of them being nearly Earth-sized and located in
9270-423: The star and its orbital period—but a number of simulations of its physical properties have been done. A number of simulations and models have been created that assume Earth-like compositions and include predictions of the galactic environment, internal heat generation from radioactive decay and magnetic induction heating , planetary rotation, the effects of stellar radiation, the amount of volatile species
9373-407: The star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets . A limited number of exoplanets have IAU-sanctioned proper names . Other naming systems exist. For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there
9476-398: The star's magnetic activity, they would give rise to auroras that could be observed from Earth if the planet has a magnetic field. Climate models including general circulation models used for Earth climate have been used to simulate the properties of Proxima Centauri b's atmosphere. Depending on its properties such as whether it is tidally locked, the amount of water and carbon dioxide
9579-460: The time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs , objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei, but subsequent work in 1992 again raised serious doubts. Finally, in 2003, improved techniques allowed
9682-405: The topic in the popular press. These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits. As pulsars are aggressive stars, it was considered unlikely at the time that
9785-440: The variation in a star's apparent luminosity as an orbiting planet transited in front of it. Initially, the most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these " hot Jupiters ", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it
9888-521: The vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method . In February 2018, researchers using the Chandra X-ray Observatory , combined with a planet detection technique called microlensing , found evidence of planets in a distant galaxy, stating, "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of
9991-555: The vast majority of known extrasolar planets have only been detected through indirect methods. Planets may form within a few to tens (or more) of millions of years of their star forming. The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming to planetary systems of over 10 Gyr old. When planets form in
10094-435: The vertex of the cone.) The linear eccentricity of an ellipse or hyperbola, denoted c (or sometimes f or e ), is the distance between its center and either of its two foci . The eccentricity can be defined as the ratio of the linear eccentricity to the semimajor axis a : that is, e = c a {\displaystyle e={\frac {c}{a}}} (lacking a center, the linear eccentricity for parabolas
10197-430: Was confirmed. Another planet candidate named Proxima Centauri c was reported in 2020, but its existence has since been disputed due to potential artifacts in the data, while the claimed existence of a dust belt around Proxima Centauri remains unconfirmed. Proxima Centauri b is the closest exoplanet to Earth, at a distance of about 4.2 ly (1.3 parsecs ). It orbits Proxima Centauri every 11.186 Earth days at
10300-432: Was higher than 0.1–0.06, it would tend to enter a Mercury-like 3:2 resonance or higher-order resonances such as 2:1. Additional planets around Proxima Centauri and interactions with Alpha Centauri could excite higher eccentricies. If the planet is not symmetrical ( triaxial ), a capture into a non-tidally locked orbit would be possible even with low eccentricity. A non-locked orbit, however, would result in tidal heating of
10403-473: Was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia . Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei . Partly because the observations were at the very limits of instrumental capabilities at
10506-417: Was no way of knowing whether they were real in fact, how common they were, or how similar they might be to the planets of the Solar System . Various detection claims made in the nineteenth century were rejected by astronomers. The first evidence of a possible exoplanet, orbiting Van Maanen 2 , was noted in 1917, but was not recognized as such. The astronomer Walter Sydney Adams , who later became director of
10609-469: Was realistic to search for exo-Jupiters by using transit photometry . In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason that planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits. Claims of exoplanet detections have been made since
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