K2-72e (also known by its EPIC designation ''EPIC 206209135.04''), is a confirmed

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

, likely rocky, orbiting within the

habitable zone In astronomy and astrobiology, the habitable zone (HZ), or more precisely the circumstellar habitable zone (CHZ), is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressu ...

of the

red dwarf A red dwarf is the smallest kind of star on the main sequence. Red dwarfs are by far the most common type of fusing star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs are ...

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

K2-72 K2-72 (also designated EPIC 206209135) is a cool red dwarf star of spectral class M2V located about away from the Earth in the constellation of Aquarius. It is known to host four planets, all similar in size to Earth, with one of them res ...

, the outermost of four such planets discovered in the system by

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

's ''

Kepler Johannes Kepler (27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws of p ...

'' spacecraft on its "Second Light" mission. It is located about 217.1

light-year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...

s (66.56

parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (AU), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...

s, or nearly km) away from Earth in the constellation of Aquarius. The exoplanet was found by using the

transit method Methods of detecting exoplanets usually rely on indirect strategies – that is, they do not directly image the planet but deduce its existence from another signal. Any planet is an extremely faint light source compared to its parent star. For e ...

, in which the dimming effect that a planet causes as it crosses in front of its star is measured.

Characteristics

Mass, radius, and temperature

K2-72e is an Earth-sized exoplanet, meaning it has a mass and radius close to that of Earth. It has an equilibrium temperature of . It has a radius of 1.29 ., and a mass of 2.2 , depending on its composition.Orbit graphic with a table of simplified figures
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Host star

The planet orbits a ( M-type)

named

, orbited by a total of four planets, of which K2-72e has the longest orbital period. The star has a mass of 0.27 and a radius of 0.33 . It has a temperature of 3360 K and its age is unknown. In comparison, the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

is 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 Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...

, 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.

Orbit

K2-72e orbits its host star with an orbital period of 24 days and an orbital radius of about 0.1 times that of Earth's (compared to the distance of Mercury from the Sun, which is about 0.38 AU).

Habitability

The exoplanet, along with K2-72c, were announced to be orbiting in the habitable zone of its parent star, the region where, with the correct conditions and atmospheric properties, liquid water may exist on the surface of the planet. K2-72e has a radius of 1.29 , so it is likely rocky. Its host star is a

, with about a fifth as much mass than the Sun does. As a result, stars like K2-72 have the ability to live up to 1–2 trillion years, hundreds of times longer than the Sun will live. The planet is very likely tidally locked, with one side of its hemisphere permanently facing towards the star, while the opposite side shrouded in eternal darkness. It receives 11% more

insolation Solar irradiance is the power per unit area ( surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre ...

than Earth does, so any oceans present are potentially vulnerable to boiling away, therefore possibly reducing the chance of habitability.

Discovery

The planet, along with the other three planets in the K2-72 system, were announced in mid-July 2016 as part of the new results from the second mission of the

spacecraft.

References

{{DEFAULTSORT:K2-72e Exoplanets discovered in 2016 Exoplanets in the habitable zone Transiting exoplanets Near-Earth-sized exoplanets in the habitable zone Aquarius (constellation) 72