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Hubble image of the dust disc and exoplanet (see fade-in at the bottom right) around the star Fomalhaut

An exoplanet, also known as an extrasolar planet, is a planetary celestial body outside ( Greek ἔξω ) of the predominant gravitational influence of the sun , but within the gravitational influence of another star or brown dwarf . Extrasolar planets do not belong to the solar system , but to other planetary systems . The largest objects are brown dwarfs themselves.

There are also celestial bodies similar to the planets that do not orbit any other celestial body and fall under the newly coined generic term Planemo (from English plane tary m ass o bject ), although as of the end of 2016 there was no consensus on whether and, if so, under what conditions can also be called exoplanets. Both exoplanets and these "free-flying or wandering planets" are among the objects of planetary mass .


First discoveries of exoplanets

The first exoplanets were discovered as early as the 1980s, but at that time either classified as brown dwarfs ( HD 114762 b ) or temporarily discarded due to the still inadequate measurement accuracy ( Gamma Cephei b ).

The first planets ever confirmed outside the solar system orbit the pulsar PSR 1257 + 12 . The pulsar was discovered in 1990 by the Polish astronomer Aleksander Wolszczan and Dale Frail . Accurate measurements of the return time of the beam reaching Earth from the pulsar revealed three planets with masses of 0.02, 4.3 and 3.9  Earth masses and orbital times of 25.262, 66.5419 and 98.2114 days in 1992 . In 1994 another planet was discovered around the pulsar PSR B1620-26 . Life as we know it from Earth is practically excluded on these planets .

The first definitive discovery of an exoplanet in orbit around a star similar to the Sun was made in 1995 by Michel Mayor from the Department of Astronomy at the University of Geneva and his colleague Didier Queloz using the radial velocity method. The planet 51 Pegasi b orbits every 4.2 days around the star 51 Pegasi (constellation: Pegasus ), which is approx. 40  light-years away from the earth, and has a mass of 0.46  Jupiter .

Further development until the start of the Kepler mission

In 1999, HD 209458 b , the first planet, was confirmed using the transit method . At the same planet first was in 2002 atmosphere of sodium are detected. The transit method proved to be extremely effective in the search for exoplanets in the following years and is now the most successful method in this special research area of ​​astronomy. Together with improvements in the radial velocity method, this led to an increasing number of exoplanets being discovered. In 2004 a planet was first discovered by direct observation in the orbit of the brown dwarf 2M1207 and confirmed in 2006 by follow-up measurements with the Hubble space telescope . For a long time, exoplanets in orbit around sun-like stars could not be observed directly with telescopes because they are very faint. They are outshone by the many times brighter star around which they circle. In 2005, Gliese 876 d was the first super-earth to be detected. Others were added later, with the Gliese 581 system being one of the first to gain a wider media coverage, as one or two of the super-earths in this system are in the habitable zone of the red dwarf . The discovery of these worlds led to an in-depth debate about the habitat of red dwarfs . In 2006, COROT, the first space telescope , started looking for exoplanets using the transit method. This mission discovered about 30 exoplanets; it ended in 2012. In 2008, water vapor was discovered at HD 189733 b . Other planets such as WASP-12b were added later.

Kepler mission and other discoveries

Image of the Kepler telescope in the assembly hall

In 2009 the extremely successful Kepler mission was started. The satellite took the constellations swan and lyre into the picture and focused mainly on faint red dwarfs. Up until 2013, over 2000 exoplanets were discovered during the primary mission. Because of this large amount of data, it was possible for the first time to narrow down estimates of the abundance of exoplanets in the Milky Way. The data also allowed conclusions to be drawn about the mass of a typical exoplanet. As it turns out, exoplanets with masses between that of Earth and around the mass of Neptune are probably the most common planets. In 2010, the first system with six (or more) exoplanets was discovered around HD 10180 using the radial velocity method .

After the alleged discovery of a planet around Alpha Centauri B in 2012, an exoplanet around our closest neighbor star Proxima Centauri was actually detected in 2016 . The planet Proxima b , which is comparable in terms of mass to the earth, orbits the parent star in a very narrow orbit. However, since this star is extremely faint, Proxima b is even within the habitable zone. However, due to the radiation bursts from Proxima Centauri and the bound rotation , the habitability of the planet has to be questioned. In the same year 2016, the first exoplanets were detected around Trappist-1 . A year later, the number of planets in the Trappist system increased to seven. The system is particularly interesting because all seven exoplanets have masses comparable to Earth. In addition, several of these planets are in the habitable zone, but the central star is again a faint red dwarf. In 2018, the successor to the successful Kepler mission was started with TESS . The main difference from TESS is that a much larger section of the sky is now to be examined. The focus is also on closer and brighter stars. This should simplify a subsequent investigation of the discovered planets compared to the Kepler planets.

Current status and future missions

In 2019 Michel Mayor and Didier Queloz were awarded the Nobel Prize in Physics for the discovery of 51 Pegasi b . Currently (2019) at least one exoplanet has been detected for many stars in the vicinity of the sun . An important future mission will be the Hubble successor, the James Webb Space Telescope . It should allow the more interesting exoplanets to be examined much more intensively than before. Scientists hope that this mission will also provide quantitatively and qualitatively significantly more meaningful information about the atmospheres of distant worlds, although some components could also suggest indications of possible life.

Detection methods

Indirect detection methods

Schematic representation of the orbits in planetary systems that can be discovered using the transit method (NASA)

So far, most exoplanets could only be detected indirectly. Several methods use the influence of the planets on the central star:

Transit method

If the planet's orbit is such that, from the point of view of the earth, it passes right in front of the star, these occultations produce periodic decreases in its brightness. They can be detected by high-precision photometry (brightness measurements of the star) while the exoplanet passes in front of its central star. This measurement can be carried out using terrestrial telescopes such as SuperWASP or much more precisely using satellites such as COROT , Kepler or ASTERIA . At the beginning of 2005, the Spitzer Space Telescope in infrared light also succeeded in detecting a secondary coverage of a hot planet by the central star. Light curves of the Hot Jupiter CoRoT-1 b also show fluctuations around 0.0001 mag, which are interpreted as the light phase of the planet.

In order to determine the masses of the planets, one of the other observation methods must also be used.

Radial velocity method

Schematic representation of the movement of the central star around the common center of gravity, the measurement of the movement of the star is the approach for the radial velocity method and for the astrometric method.

Star and planet (s) move under the influence of gravity around their common center of gravity. Because of its greater mass, the star moves much shorter distances than the planet. If one does not look exactly perpendicularly on this orbit from the earth, this periodic movement of the star has a component in the direction of sight ( radial velocity ), which by observing the alternating blue and red shift ( Doppler effect ) with the help of a frequency comb in very precise spectra of the star can be detected. Since the inclination of the orbit is unknown (if the planets have not been verified with the transit method at the same time), one cannot calculate the planetary mass itself, and certainly not verify it, if the star mass is known, but only calculate a lower limit for possibly existing planets.

Astrometric method

The movement of the star around the common center of gravity has components perpendicular to the viewing direction. They should be detectable relative to other stars by precisely measuring its star locations . If the star mass and distance are known, one could also enter the mass of the planet here, since the orbit inclination can be determined. Already in the middle of the 20th century the astrometric method was used to search for exoplanets, but the observations were still too imprecise and alleged discoveries later turned out to be incorrect. Even the astrometric satellite Hipparcos did not yet have the accuracy required to discover new exoplanets. Its successor Gaia has the potential to discover thousands of exoplanets using the astrometric method. By combining measurements from Gaia DR2 and the radial velocity method, Epsilon Indi A b has already achieved a much more precise determination of the planet discovered. In the future, the ground-based method should also have the potential to discover planets through interferometry , for example with the Very Large Telescope or its successor, the Extremely Large Telescope .

Gravitational microlensing method

This is another indirect method that uses the effect on background stars. Under microlensing refers to the amplification of the light of a background object by gravitational lens effect of a foreground star. The gain increases and decreases as the star moves past the background object. This course of brightness can be given a characteristic peak by a planet of the foreground star. A first such event was observed in 2003. Microlensing events are rare, but also allow observations of distant stars. However, it has not yet been proven with certainty whether planets of extremely distant systems can also be detected with it (e.g. extragalactic planets ).

Calculation after disturbed planetary orbit

Another indirect method is based on the observation of known exoplanets. Several planets in the same system attract each other through gravity, which slightly changes the planetary orbits. In January 2008, a Spanish-French team of researchers submitted a paper on computer simulations that suggest the existence of a planet GJ 436c based on disturbances in the orbit of the neighboring planet GJ 436b . The calculations suggest that this exoplanet has a mass of about five times the mass of the earth . So far there is no evidence for this hypothesis.

Time of flight method

The time of flight method is based on a strictly periodic signal from a central star or a central double star. Due to the influence of gravity, the center of gravity of the star system shifts on a revolving planet, which results in a time shift in the periodic signals. Sufficiently precise signals come from pulsar pulses, the maxima of some pulsation- variable stars and the minima of eclipsing stars . The time-of-flight method is distance-independent, but it is strongly influenced by the accuracy of the periodic signal. Therefore, with this method one could only detect exoplanets around pulsars so far.

Direct observation

2M1207 and the exoplanet 2M1207b (ESO / VLT)

On September 10, 2004, ESO announced that the first direct image of a planet at the brown dwarf 2M1207, 225 light-years away, may have been possible . Follow-up measurements with the Hubble space telescope in 2006 confirmed this.

On March 31, 2005, a working group from the astrophysical institute at the Jena University Observatory announced a planet with only one to two times the mass of the planet Jupiter at the star GQ, which is similar to the sun , but with an age of approx. 2 million years it is much younger To have observed Lupi , who is currently in the T-Tauri phase . This observation was also made with ESO's Very Large Telescope in the infrared spectral range .

In early 2008, British astronomers discovered an exoplanet in the development phase using the Very Large Array near the 520 light-years from Earth and still very young star HL Tau, which is around 100,000 years old .

A clear, direct evidence was published on November 14, 2008: Two images of the Hubble space telescope from 2004 and 2006 in the range of visible light show a moving point of light that describes a Kepler orbit . It is the object Dagon , which orbits the star Fomalhaut, 25 light-years away, at a distance of 113  AU on the inner edge of the dust belt surrounding it (twelve times the distance between the Sun and Saturn ). According to the discoverers, it is the coolest and smallest object to date that could be imaged outside the solar system. If it is indeed an exoplanet, it could have a mass about three times the mass of Jupiter. According to an April 2020 publication, the object could also be a cloud of dust resulting from a collision of two smaller bodies of about 200 km.

Also in November 2008, astronomers announced that the Gemini-North Observatory and the Keck Observatory had succeeded in imaging an entire planetary system around the 130 light-years distant star HR 8799 in the constellation Pegasus. Observations using adaptive optics in infrared light show three planets whose masses are given as seven to ten Jupiter masses. The exoplanets orbit their central star at a distance of 25, 40 and 70 astronomical units. With an estimated age of 60 million years, they are still young enough to give off thermal radiation themselves.

Well-known projects and instruments for the detection of exoplanets

Surname Type Method (noun) Discoveries (examples)
Kepler Mission Space telescope Transit Method , Orbital Brightness
, Transit Timing Variations
almost all planets of the Kepler and K2 stars
(e.g. Kepler-452b , Kepler-90 system)
CoRoT mission Space telescope Transit method CoRoT-9 b , CoRoT-7 b
Transiting Exoplanet Survey Satellite Space telescope Transit method Gliese 357 b, Pi Mensae c
HARPS ground based Radial velocity method Gliese 667 Cc , Ross 128b , Gliese 581 system
OGLE ground based Microlensing , transit method OGLE-2005-BLG-390L b
MOA ground based Microlensing MOA-2007-BLG-192Lb
SuperWASP ground based Transit method WASP-12b
HATNet ground based Transit method HAT-P-1b
Hubble Space Telescope Space telescope Imaging, transit method
Trappist ground based Transit method TRAPPIST-1 b to d
SPECULOOS ground based Transit method
Gaia Mission Space telescope Astrometric method
CHEOPS (space telescope) Space telescope Transit method


The rules for naming exoplanets are set by the International Astronomical Union (IAU). Then each exoplanet receives a "scientific name" ( "scientific designation") , consisting of the name or catalog name of the central star and an attached Latin is lowercase. The latter are assigned in the alphabetical order of the discovery, starting with "b". The IAU does not specify any regulations for planets around a central star discovered at the same time; Usually the letters are assigned here in the order of the distance to the central star. It is not regulated whether the lower case letter is to be separated from the star designation by a space ; the examples in the regulatory text itself are inconsistent. If the star name designates a multiple star system, the individual components of which are identified by Latin capital letters, the identification letter for an individually circled component must be placed immediately in front of the lower case letter (without spaces). If several components are circled, their identification letters are to be appended in brackets to the star name. Examples include: " 51 Pegasi b ", " CoRoT-7b ", " Alpha Centauri Bb ", " Kepler-34 (AB) b ".

In addition to these scientific names, the IAU also allows public names , with design rules analogous to the naming of asteroids . To this end, it organized a worldwide competition (NameExoWorlds) in 2015 to name 305 selected exoplanets. The results were published in December 2015.

Number of known exoplanets

As of July 11, 2020, 4281 exoplanets are known in 3163 systems, although some objects have masses in the range of brown dwarfs . Thus, the most massive object in the extra Planets Encyclopaedia 81 M J (Jupiter masses) , while in the NASA Exoplanet Archive an upper mass limit of 30 M J has been set. According to the current state of research, the minimum mass of brown dwarfs is 13 MJ . 652 multiplanetar systems have two to eight proven planets. Today, planetary systems in the immediate vicinity of the sun are considered to be a reliably proven, generally widespread phenomenon. Investigations and measurements by the Institut astrophysique de Paris have shown that a star in the Milky Way has one or two planets on average.

Number of exoplanets discovered per year
(as of July 11, 2020)
1988 1989 1990 1991 1992 1993 1994 1995 1996 1997
1 0 0 0 3 0 0 3 7th 0
1998 1999 2000 2001 2002 2003 2004 2005 2006 2007
8th 12 23 15th 32 26th 35 35 35 60
2008 2009 2010 2011 2012 2013 2014 2015 2016 2017
64 89 127 196 163 193 853 186 1472 169
2018 2019 2020
207 182 79
Discovery by Method (July 11, 2020)

Mass and radius of the discovered planets

Size comparison between Jupiter (left) and TrES-4 (right), one of the largest known exoplanets

While the initially discovered exoplanets were mainly Hot Jupiters , planets with a size between that of Earth and that of Neptune now make up the majority of the exoplanets discovered.

As of 2019 there are around 1000 known planets with less than twice the Earth's radius, of which around 150 are smaller than Earth. Since masses cannot be determined for all planets and tends to be more likely for larger planets, the number of planets with masses below three times the mass of the earth is still small at approx.

Classification according to radius (R E ). The radius of planets discovered using the transit method is basically known.

Small exoplanets

Mass of the exoplanets known up to the date shown in the diagram over the year of their discovery. Over the years, the mass spectrum widens, particularly downwards, i.e. with smaller masses. (Excluding controversial discoveries and planets around pulsars.)

Since 2000, increasingly smaller exoplanets have been discovered. In 2004 the lower limit of detectability with the radial velocity method was around 1 m / s. A planet orbiting its star at a distance of 1  AU therefore had to have a mass of approx. 11  Earth masses in order to be discovered at all. In the meantime, however, lower-mass and smaller exoplanets have also been discovered with the help of radial velocity and the microlensing and transit methods, with the greatest advances in the search for small exoplanets so far being achieved with the help of the Kepler telescope

One of the first small exoplanets to be found is the second companion of the star Gliese 581, discovered in April 2007 by astronomers at the European Southern Observatory (ESO) : Gliese 581 c at a distance of 20.45 light years. Its orbital period or annual length is only 13 earth days. The planet has a minimum mass of five times the mass of the earth. The planet was verified by a spectrograph operated in La Silla , Chile . Red and blue shifts were investigated, which are dependent on the orbit of the companion (radial velocity method).

Another planet of the same star that was only discovered in 2009 is Gliese 581 e . It is one of the lowest mass known exoplanets with a minimum mass of 1.9 Earth masses and an orbital period of just over 3 days.

Many other small exoplanets identified so far are so-called super - earths :

Gliese 876 d has about 7 times the mass of the earth. Since it orbits once around its star at a very short distance in just 47 hours, its surface temperature is around 200 ° C to 400 ° C.

OGLE-2005-BLG-390Lb was discovered in January 2006 by an international research group using the microlens effect . This exoplanet is about 25,000 to 28,000 light-years awayfrom Earth and has about five times the mass of Earth. It orbits the star OGLE-2005-BLG-390L (a red dwarf ) at a distance of 2.6  astronomical units once every ten earth years. Due to the small size and comparatively low radiation of the “mother star” as well as the great distance, the surface temperature of the planet is only around –220 ° C. The development of life forms is therefore extremely unlikely.

MOA-2007-BLG-192-Lb was discovered in June 2008 and is one of the smallest known exoplanets. It has 3.2 times the mass of the earth and is about 3000 light years away. However, recent evidence suggests that the mass of its parent star is significantly higher and that it is not a brown but a red dwarf . This results in a newly determined mass of only 1.4 earth masses for the exoplanet.

Kepler-37b was discovered in 2013 and is only slightly larger than the Earth's moon with a diameter of about 3900 km. It is currently the smallest known exoplanet (as of 2019) around a star comparable to the sun.

Types of exoplanets

There is still no internationally binding system for classifying extrasolar planets. So one tried a classification for the solar planets. This was then transferred to the extrasolar planets.

This classification was made into the following types:

Planets outside the Milky Way

It can be assumed that planets have also formed in other galaxies. Their reproducible detection, however, is clearly beyond the possibilities available today. Several microlens events have been observed that could possibly be due to exoplanets.

Exemplary exoplanets and systems

2M1207 b

The gas giant 2M1207 b was discovered in the orbit of the brown dwarf 2M1207 in 2004 and was the first exoplanet that could be directly perceived optically and thus offers the possibility of a direct spectroscopic investigation.


In this system, the very closely orbiting planet CVSO 30 b (large orbital half-axis approx. 0.00838 AU, orbital time 0.448413 days) was discovered using the transit method, as well as the very distant CVSO 30 c in 2016 (distance to the central star approx 660 AU) observed directly; the latter observation has meanwhile been questioned. If the latter observation is true, the system contains both a planet with a much narrower orbit than Mercury around the sun and a planet considerably further than Neptune from the sun and would thus be the system with the (as of end of 2017) largest ratio between the distance of the outermost known and innermost known planets.

Gliese 1214 b

GJ 1214 b (Gliese 1214 b) is an extrasolar super-earth discovered in 2009, which orbits the red dwarf GJ 1214 in the constellation Snake Bearer around 40  light-years from Earth in 38 hours , whose radiation is 200 times weaker than that of the sun . The exoplanet GJ 1214 b has an atmosphere that is composed primarily of water vapor.

HD 20782 b

The planet, with at least 2 Jupiter masses probably a gas giant , orbits its sun-like central star HD 20782 in 597 days on an extremely eccentric orbit ( eccentricity 0.96), in which the distance to the central star fluctuates between 0.06 and 2.5 AU .


In the course of a transit in front of the central star KELT-9 , gaseous iron and titanium could be detected in the atmosphere of its extremely hot gas planet KELT-9b .

Kepler-42 b / c / d

Planetary system of Kepler-42 and the Jupiter moon system

As part of the Kepler mission , NASA announced the discovery of the smallest planetary system to date (in terms of planet size) : The red dwarf Kepler-42 (then known as KOI-961 ), which is about 120 light years from Earth, has three rocky planets , all of which orbit the star closer than the habitable zone and thus have surfaces that are too hot for liquid water. Their radii are 0.78, 0.73 and 0.57 times the radius of the earth , making the smallest of these planets similar in size to Mars .


With the announcement of the discovery of the eighth planet in December 2017, the system at this stage is the one with the most known exoplanets.


Kepler-186f is a planet about the size of the earth (about 1.1 times the diameter of the earth ), discovered in 2012 , whose orbit lies in the outer area of ​​the habitable zone of its central star. Its mass is not known, but it is plausible to assume that it is an earth-like planet (rock planet).


Kepler-452b is a planet candidate discovered in 2015 with about 1.6 times the diameter of the earth , so it is probably an earth-like planet (rock planet) and is located in the habitable zone . If confirmed, it will be one of the first exoplanets to be discovered orbiting a sun-like star.

Kepler 1647 b

This approximately Jupiter-sized gas giant orbits a binary star consisting of two sun-like stars in a circumbinary manner at a distance of about 3700 light years with an orbital period of about three years. Since it lies in the habitable zone, it can be speculated that any moons that may be present could offer living conditions.

Proxima Centauri b

The star closest to the Sun, Proxima Centauri , is orbited in its habitable zone by a possibly Earth-like planet , the discovery of which was announced in August 2016.


The object Ssc2005-10c at the star HD 69830 fulfills a "shepherd dog function" for an asteroid belt discovered with the Spitzer space telescope of NASA, similar to Jupiter for the asteroid belt of the solar system . This belt has about 25 times its mass and is as close to the star as Venus is to the sun.

Titawin with Saffar, Samh and Majriti

The binary star system Titawin consists of the more luminous star Titawin A and the red dwarf Titawin B. The larger of the two stars, Titawin A, has at least three planets:

  • Saffar with a mass of 0.71 times Jupiter with a period of 4.617 days and an estimated temperature difference between day and night side of 1400 degrees,
  • Samh with 2.11 times the mass of Jupiter (241.2 days orbital period) - an exoplanet that is very warm, but could be on the inner edge of the life zone and
  • Majriti (4.61 times Jupiter's mass, 3.47 years orbital period), a planet that is rather cool, but could just be on the outer edge of the life zone.

Located in the constellation Andromeda , the system is 2.9-4.1 billion years old, 43.93 light-years away, and the orbital period of Titawin A and Titawin B is 20,000 years.


In the Trappist 1 system, discovered in 2016, 7 terrestrial planets have now been found, several of which are in the habitable zone. Thus, all the planets on earth are comparatively similar. The central star, however, is a faint red dwarf with only about 8% of the solar mass.

See also


  • Aleksandar Janjic: In search of the signatures of life. In astrobiology - the search for extraterrestrial life. Springer Berlin Heidelberg, 2019, ISBN 978-3-662-59492-6 .
  • Reto U. Schneider : Planet hunters. The exciting discovery of strange worlds. Birkhäuser, Basel a. a. 1997, ISBN 3-7643-5607-3 .
  • Geoffrey Marcy , R. Paul Butler , Debra Fischer , Steven Vogt , Jason T. Wright, Chris G. Tinney, Hugh RA Jones: Observed Properties of Exoplanets: Masses, Orbits, and Metallicities. In: Shin Mineshige, Shigeru Ida (eds.): Origins: From early universe to extrasolar planets. Proceedings of the 19th Nishinomiya-Yukawa memorial symposium. (November 1 and 2, 2004, Nishinomiya, Japan) (=  Progress of Theoretical Physics. Supplement. No. 158). Publishing Office Progress of Theoretical Physics - Kyoto University, Kyoto 2005, pp. 24–42, online (PDF; 629 kB).
  • Hans Deeg, Juan Antonio Belmonte, Antonio Aparicio (eds.): Extrasolar planets. Cambridge University Press, Cambridge 2008, ISBN 978-0-521-86808-2 .
  • Rudolf Dvorak (Ed.): Extrasolar planets. Formation, detection and dynamics. Wiley-VCH-Verlag, Weinheim 2008, ISBN 978-3-527-40671-5 .
  • John W. Mason (Ed.): Exoplanets. Detection, formation, properties, habitability. Springer u. a., Berlin a. a. 2008, ISBN 978-3-540-74007-0 .
  • Sven Piper: Exoplanets. The search for a second earth. Springer, Heidelberg a. a. 2011, ISBN 978-3-642-16469-9 .
  • Lisa Kaltenegger : The search for the second earth. In: Physik-Journal. Vol. 11, No. 2, 2012, ISSN  1617-9439 , pp. 25-29.
  • Mathias Scholz: Planetology of extrasolar planets. Springer, Heidelberg 2014, ISBN 978-3-642-41748-1 .

Web links

Commons : Exoplanets  - Collection of images, videos and audio files
Wiktionary: Exoplanet  - explanations of meanings, word origins, synonyms, translations


  1. Astronomical names and delimitations were often not clear and were changed. Examples: walking star versus fixed star - the walking star (planet) is no longer a star today (except for the sun) and the fixed star is no longer fixed (fixed). The first moons of Jupiter or asteroids were also called planets at that time. The best known case is the demarcation of the dwarf planets from the planets with the "sacrifice" Pluto .
  2. The discovery report for the exoplanet itself has meanwhile been withdrawn, so its use in the cited IAU document is only a (still valid) example of the designation scheme.

Individual evidence

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