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Kepler-138, also known as KOI-314, is a
red dwarf ''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave since 2009, gaining a cult following. T ...
located in the constellation
Lyra Lyra (; Latin for lyre, from Greek ''λύρα'') is a small constellation. It is one of the 48 listed by the 2nd century astronomer Ptolemy, and is one of the modern 88 constellations recognized by the International Astronomical Union. Lyra was ...
, 219
light years A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...
from
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
. It is located within the field of vision of the
Kepler spacecraft The Kepler space telescope is a disused space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocentric orb ...
, the satellite that
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...
's
Kepler Mission The Kepler space telescope is a disused space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocentric orb ...
used to detect planets transiting their stars. The star hosts three confirmed planets and a likely fourth, including the lowest-mass
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 recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
with a measured mass and size discovered to date, Kepler-138b, with a mass comparable to that of
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
. Kepler-138d is remarkable for its low density; initially thought likely to be a
gas dwarf A Mini-Neptune (sometimes known as a gas dwarf or transitional planet) is a planet less massive than Neptune but resembling Neptune in that it has a thick hydrogen–helium atmosphere, probably with deep layers of ice, rock or liquid oceans (mad ...
, more recent observations as of 2022 show that it, as well as planet c, are likely to be
ocean world An ocean world, ocean planet, panthalassic planet, maritime world, water world or aquaplanet, is a type of planet that contains a substantial amount of water in form of oceans, either beneath the surface, as subsurface oceans, or on the surfa ...
s.


Nomenclature and history

Prior to Kepler observation, KOI-314 had the
2MASS The Two Micron All-Sky Survey, or 2MASS, was an astronomical survey of the whole sky in infrared light. It took place between 1997 and 2001, in two different locations: at the U.S. Fred Lawrence Whipple Observatory on Mount Hopkins, Arizona, and ...
catalogue number 2MASS J19213157+4317347. In the
Kepler Input Catalog The Kepler Input Catalog (or KIC) is a publicly searchable database of roughly 13.2 million targets used for the Kepler Spectral Classification Program (SCP) and ''Kepler''. Overview The Kepler SCP targets were observed by the 2MASS project ...
it has the designation of KIC 7603200, and when it was found to have transiting planet candidates it was given the Kepler object of interest number of KOI-314. Planetary candidates were detected around the star by
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...
's
Kepler Mission The Kepler space telescope is a disused space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocentric orb ...
, a mission tasked with discovering planets in
transit Transit may refer to: Arts and entertainment Film * ''Transit'' (1979 film), a 1979 Israeli film * ''Transit'' (2005 film), a film produced by MTV and Staying-Alive about four people in countries in the world * ''Transit'' (2006 film), a 2006 ...
around their stars. The transit method that Kepler uses involves detecting dips in brightness in stars. These dips in brightness can be interpreted as planets whose orbits pass in front of their stars from the perspective of
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
, although other phenomena can also be responsible which is why the term planetary candidate is used. By timing these dips, gravitational interactions were detected between two of the candidates, allowing for a measurement of their masses and confirmation as real planets given that the masses were significantly below the deuterium burning limits. Following the acceptance of the discovery paper, the Kepler team provided an additional moniker for the system of "Kepler-138". However, the planets were discovered by scientists outside of the Kepler team who referred to the star as KOI-314, as the Kepler designation had not been assigned yet. Candidate planets that are associated with stars studied by the Kepler Mission are assigned the designations ".01", ".02", ".03", etc. after the star's name, in the order of discovery. If planet candidates are detected simultaneously, then the ordering follows the order of orbital periods from shortest to longest. Following these rules, the first two candidate planets were detected simultaneously and assigned the names KOI-314.01 and KOI-314.02, with respective orbital periods of 13.8 and 23.1 days. Over a year later, a much smaller planet candidate was detected and assigned the name KOI-314.03, despite being the shortest orbital period planet (period of 10.3 days) found in the system. Confirmed planets are conventionally assigned the designations ''b'', ''c'', ''d'', etc. after the star's name. The labels are assigned alphabetically in the order of discovery starting from ''b''. Since KOI-314.01 and KOI-314.02 were confirmed as planets simultaneously, the alphabetical names were assigned in order of orbital period by the discoverers, and thus became KOI-314b and KOI-314c respectively. Since no gravitational interactions were detected due to KOI-314.03, this planetary candidate remained unconfirmed as 6 January 2014 and thus kept the same name. In the following weeks, on 28 February 2014, a new paper validated KOI-314.03 as being a real planet with a false alarm probability of less than 1%. The new paper used different names for the planets, going from KOI-314b to Kepler-138c, KOI-314c to Kepler-138d and KOI-314.03 to Kepler-138b. These designations have been used by subsequent studies, and by databases such as the
NASA Exoplanet Archive The NASA Exoplanet Archive is an online astronomical exoplanet catalog and data service that collects and serves public data that support the search for and characterization of extra-solar planets (exoplanets) and their host stars. It is part of ...
. This situation is similar to that of some other planetary systems such as
Mu Arae Mu Arae (μ Arae, abbreviated Mu Ara, μ Ara), often designated HD 160691, officially named Cervantes , is a main sequence G V star, G-type star approximately 50 light-years away from the Sun in the constellation of Ara (constellation), A ...
, where different designations have been used for the same planets in the literature.


Stellar characteristics

Kepler-138 is a
red dwarf ''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave since 2009, gaining a cult following. T ...
with approximately 54% the mass of and 54% the radius of the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
. It has a surface temperature of . In comparison, the Sun has a surface temperature of 5778 K. Kepler-138's
apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...
(how bright it appears from Earth's perspective) is 13.04, too dim to be seen with the naked eye.


Planetary system

The three inner known planets of Kepler-138
transit Transit may refer to: Arts and entertainment Film * ''Transit'' (1979 film), a 1979 Israeli film * ''Transit'' (2005 film), a film produced by MTV and Staying-Alive about four people in countries in the world * ''Transit'' (2006 film), a 2006 ...
the star; this means that all three planets' orbits appear to cross in front of their star as viewed from the Earth's perspective. Their
inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a Plane of reference, reference plane and the orbital plane or Axis of rotation, axis of direction of the orbiting object ...
s relative to Earth's line of sight, or how far above or below the plane of sight they are, vary by less than one degree. This allows direct measurements of the planets' orbital periods and relative diameters (compared to the host star) by monitoring each planet's transit of the star. There is also a likely fourth non-transiting planet, Kepler-138e, detected through
transit-timing variation Transit-timing variation is a method for detecting exoplanets by observing variations in the timing of a transit. This provides an extremely sensitive method capable of detecting additional planets in the system with masses potentially as small a ...
s. Although the innermost planet has a size similar to
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
, Kepler-138c and d both have a radius of about 1.5
Earth radii Earth radius (denoted as ''R''🜨 or R_E) is the distance from the center of Earth to a point on or near its surface. Approximating the figure of Earth by an Earth spheroid, the radius ranges from a maximum of nearly (equatorial radius, deno ...
(revised from earlier estimates of 1.2 Earth radii). Although Kepler-138c and d have similar radii, their masses and densities were initially thought to vary greatly. Of these two, the inner planet was thought to be consistent with a rocky
super-Earth A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to ...
, whereas the outer planet's low density implies it may have a substantial proportion of water ice or a significant gas envelope, resembling a miniaturized
gas giant A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...
(a
gas dwarf A Mini-Neptune (sometimes known as a gas dwarf or transitional planet) is a planet less massive than Neptune but resembling Neptune in that it has a thick hydrogen–helium atmosphere, probably with deep layers of ice, rock or liquid oceans (mad ...
). The striking differences between these two planets have been hypothesized to be due
photoevaporation Photoevaporation denotes the process where energetic radiation ionises gas and causes it to disperse away from the ionising source. This typically refers to an astrophysical context where ultraviolet radiation from hot stars acts on clouds of mat ...
. However, more recent observations as of 2022 have found similarly low densities for both planets c and d, suggesting that they are likely to be
ocean world An ocean world, ocean planet, panthalassic planet, maritime world, water world or aquaplanet, is a type of planet that contains a substantial amount of water in form of oceans, either beneath the surface, as subsurface oceans, or on the surfa ...
s. The mass of candidate Kepler-138e would be intermediate of Mars and
Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
. While a radius could not be estimated for planet e, it is likely smaller than c and d and larger than b, which is consistent with an Earth-like composition. The three inner planets are too close to their star to be considered within the
habitable zone In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.J. F. Kas ...
, while the likely planet Kepler-138e orbits near the inner edge of the habitable zone.


See also

*
Hunt for Exomoons with Kepler The Hunt for Exomoons with Kepler (HEK) is a project whose aim is to search for exomoons, natural satellites of exoplanets, using data collected by the Kepler (spacecraft), Kepler space telescope. Founded by British exomoonologist David Kipping ( ...
*
List of multiplanetary systems From the total of stars known to have exoplanets (as of ), there are a total of known multiplanetary systems, or stars with at least two confirmed planets, beyond the Solar System. This list includes systems with at least three confirmed planet ...


References


External links


The Hunt for Exomoons with Kepler (HEK) project
- HEK.
Kepler Mission
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...
.
Kepler – Discoveries – Summary Table
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...
.
Kepler – Tally of Planets/interactive (2013)
NYT.
Video (10:47) - David Kipping - Press Briefing on First Earth-Mass Transiting Planet, KOI-314c (01/06/2014).
{{Sky, 19, 21, 31.563, +, 43, 17, 34.76 Planetary systems with three confirmed planets M-type main-sequence stars Planetary transit variables Lyra (constellation) 0314 J19213157+4317347 159376971