K2-72 (also designated EPIC 206209135) is a cool

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 ...

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

of spectral class M2.7V located about away from the Earth in the constellation of

Aquarius Aquarius may refer to: Astrology * Aquarius (astrology), an astrological sign * Age of Aquarius, a time period in the cycle of astrological ages Astronomy * Aquarius (constellation) * Aquarius in Chinese astronomy Arts and entertainme ...

. It is known to host four planets, all similar in size to Earth, with one of them residing 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 ...

Nomenclature and history

K2-72 also has 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 J22182923-0936444. Its EPIC (Ecliptic Plane Input Catalog) number is 206209135. The star's planetary companions were discovered 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 ...

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

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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 move in front of their stars from the perspective of

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. The name K2-72 derives directly from the fact that the star is the catalogued 72nd star discovered by the ''K2'' mission to have confirmed planets. The designation ''b'', ''c'', ''d'', and ''e'' derives from the order of discovery. The designation of ''b'' is given to the first planet orbiting a given star, and ''e'' to the last. In the case of K2-72, there were four planets, so only letters ''b'' to ''e'' are used. At first the planets were all thought to be smaller than Earth. However, in 2017, new analysis by Martinez ''et al.'' and Courtney Dressing found that K2-72 was significantly larger than previous estimates, and found that the planets were all larger than Earth, although all are still expected to be rocky.

Stellar characteristics

K2-72 is a M-type star that is approximately 27% the mass of and 33% 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 ...

, according to the analysis done by Dressing ''et al.'' The results found by Martinez ''et al.'' suggest a larger star, with about 36% the radius and mass of the Sun. Both give a luminosity estimate between 0.013 and 0.015 solar luminosities. It has a surface temperature of between 3360 and 3370 K and its age is unknown. In comparison, the Sun is about 4.6 billion years old and has a surface temperature of 5778 K. The star'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 ...

, or how bright it appears from Earth's perspective, is 15.309. Therefore, it is too dim to be seen with the naked eye and can only be observed with a telescope.

Planetary system

The star is known to host four planets, all likely to be rocky. Only one (K2-72e) is currently known to reside inside the habitable zone, although K2-72c may straddle the inner edge.

References

{{DEFAULTSORT:K2-72 Planetary systems with four confirmed planets Planetary transit variables M-type main-sequence stars J22182923-0936444 Aquarius (constellation)