Kepler-442b (also known by its Kepler object of interest designation ''KOI-4742.01'') is a confirmed near-Earth-sized

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 ...

, likely

rocky ''Rocky'' is a 1976 American sports drama film directed by John G. Avildsen and written by and starring Sylvester Stallone. It is the first installment in the ''Rocky'' franchise and stars Talia Shire, Burt Young, Carl Weathers, and Burges ...

, orbiting 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. Kast ...

of the

K-type main-sequence star A K-type main-sequence star, also referred to as a K-type dwarf or an orange dwarf, is a main-sequence ( hydrogen-burning) star of spectral type K and luminosity class V. These stars are intermediate in size between red M-type main-sequence s ...

Kepler-442, about 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 sur ...

in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the earliest 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 ...

. The planet orbits its host star at a distance of about with an orbital period of roughly 112.3

days A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours, 1440 minutes, or 86,400 seconds. In everyday life, the word "day" often refers to a solar day, which is the length between two ...

. It has a mass of around 2.3 and has a radius of about 1.34 times that of Earth. It is one of the more promising candidates for potential habitability, as its parent star is at least 40% less massive than the Sun – thus, it can have a lifespan of about 30 billion years or so. The planet was 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, succeedin ...

's Kepler spacecraft using the transit method, in which it measures the dimming effect that a

planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...

causes as it crosses in front of its star. NASA announced the confirmation of the exoplanet on 6 January 2015.

Physical characteristics

Mass, radius, and temperature

Kepler-442b is a

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 ...

, an exoplanet with a mass and radius bigger than Earth's but smaller than the ice giants

Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus ( Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars), grandfather of Zeus (Jupiter) and father of ...

and

Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 time ...

. It has an equilibrium temperature of . It has a radius of 1.34 . Because of its radius, it is likely to be a rocky planet with a solid surface. The mass of the exoplanet is estimated to be 2.36 . The surface

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

on Kepler-442b would be 30% stronger than Earth, assuming a rocky composition similar to that of Earth.

Host star

The planet orbits a ( K-type)

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 ...

named Kepler-442. The star has a mass of 0.61 and a radius of 0.60 . It has a temperature of and is around 2.9 billion years old, with some uncertainty. In comparison, the Sun is 4.6 billion years old and has a temperature of . The star is somewhat metal-poor, with a

metallicity In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as ...

(Fe/H) of −0.37, or 43% of the solar amount. Its luminosity () is 12% that of the Sun. 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 ...

, or how bright it appears from Earth's perspective, is 14.76. Therefore, it is too dim to be seen with the naked eye.

Orbit

Kepler-442b orbits its host star with an orbital period of 112 days. It has an orbital radius of about (slightly larger than the distance of Mercury from the Sun, which is approximately ). It receives about 70% of Earth's

sunlight Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when ...

from the Sun.

Habitability

KeplerExoplanets-NearEarthSize-HabitableZone-20150106

The planet is in the

of its star, a region where liquid water could exist on the

's surface. It is one of the most Earth-like planets yet found in size and

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

. It is just outside the zone (around ) in which tidal forces from its host star would be enough to fully tidally lock it. As of July 2018, Kepler-442b was considered the most habitable non-tidally-locked exoplanet discovered.

Stellar factors

K-type main-sequence stars are smaller than the Sun and live longer, remaining on the

main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar He ...

for 18 to 34 billion years compared to the Sun's estimated lifespan of 10 billion years. Despite these properties, the small M-type and K-type stars can threaten life. Because of their high stellar activity at the beginning of their lives, they emit strong solar winds. The duration of this period is inversely linked to the size of the star. However, because of the uncertainty of the age of Kepler-442, it is likely it may have passed this stage, making Kepler-442b potentially more suitable for habitability.

Tidal effects and further reviews

Because Kepler-442b is closer to its star than Earth is to the Sun, the planet will probably rotate much more slowly than Earth; its day could be weeks or months long (see Tidal effects on rotation rate, axial tilt, and orbit). This is reflected in its orbital distance, just outside of the point where the tidal interactions from its star would be strong enough to tidally lock it. Kepler-442b's

axial tilt In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orb ...

(obliquity) is likely tiny, in which case it would not have tilt-induced seasons as Earth and Mars do. Its orbit is probably close to circular (eccentricity 0.04), so it will also lack eccentricity-induced seasonal changes like

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...

. One review essay in 2015 concluded that Kepler-442b, Kepler-186f, and Kepler-62f were likely the best candidates for being potentially habitable planets. Also, according to an index developed in 2015, Kepler-442b is even more likely to be habitable than a hypothetical "Earth twin" with physical and orbital parameters matching those of

. Going by this index, Earth has a rating of 0.829, but Kepler-442b has a rating of 0.836. The actual habitability is uncertain because Kepler-442b's atmosphere and surface are unknown. The paper introducing the habitability index clarifies that a higher-than-Earth value "does not mean these planets are “more habitable” than Earth".

Discovery and follow-up studies

In 2009,

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 o ...

spacecraft was completing observing stars on its

photometer A photometer is an instrument that measures the strength of electromagnetic radiation in the range from ultraviolet to infrared and including the visible spectrum. Most photometers convert light into an electric current using a photoresistor, ...

, the instrument it uses to detect

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 ...

events when a planet crosses in front of and dims its host star for a brief and roughly regular period. In this last test, Kepler observed 50,000 stars in the Kepler Input Catalog, including Kepler-442; the telescope sent the preliminary light curves to the Kepler science team for analysis, who chose prominent planetary companions from the bunch for follow-up at observatories. Observations for the potential exoplanet candidates took place between 13 May 2009 and 17 March 2012. After observing the respective transits, which for Kepler-442b occurred roughly every 113 days (its orbital period), the scientists eventually concluded that a planetary body was responsible for the periodic 113-day transits. The discovery, along with the unique planetary systems of the stars Kepler-438 and Kepler-440, was announced on 6 January 2015. At nearly distant, Kepler-442b is too remote and its star too far for current telescopes or the next generation of planned telescopes to determine its mass or whether it has an atmosphere. The Kepler spacecraft focused on a single small region of the sky. Still, next-generation planet-hunting space telescopes, such as

TESS Tess or TESS may refer to: Music * Tess (band), a Spanish pop band active from 2000 to 2005 * TESS (musician), a UK musician Film and theatre * ''Tess'' (1979 film), a 1979 film adaptation of '' Tess of the d'Urbervilles'' * ''Tess'' (2016 fil ...

and CHEOPS, will examine nearby stars throughout the sky. The

James Webb Space Telescope The James Webb Space Telescope (JWST) is a space telescope which conducts infrared astronomy. As the largest optical telescope in space, its high resolution and sensitivity allow it to view objects too old, distant, or faint for the Hubble Sp ...

and future large ground-based telescopes can then study nearby stars with planets to analyze atmospheres, determine masses, and infer compositions. Additionally, the Square Kilometer Array would significantly improve radio observations over the

Arecibo Observatory The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science ...

and Green Bank Telescope.

References

External links

NASA – Mission overview

NASA – Kepler Discoveries – Summary Table

NASA – Kepler-442b
at The NASA Exoplanet Archive.
NASA – Kepler-442b
at Extrasolar Planets Encyclopaedia.
Habitable Exolanets Catalog
at UPR-Arecibo. {{Use dmy dates, date=January 2015 442b Exoplanets discovered in 2015 Exoplanets in the habitable zone Kepler-442 Transiting exoplanets Near-Earth-sized exoplanets in the habitable zone Lyra (constellation) Super-Earths