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Kepler-442
Kepler-442
Kepler-442
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Kepler-442
Observation data
Epoch J2000      Equinox J2000
Constellation Lyra[1]
Right ascension 19h 01m 27.9743s[2]
Declination +39° 16′ 48.224″[2]
Apparent magnitude (V) 14.976[3]
Characteristics
Evolutionary stage Main sequence
Spectral type K5V[4][5]
Astrometry
Proper motion (μ) RA: 7.784(18) mas/yr[2]
Dec.: 1.882(19) mas/yr[2]
Parallax (π)2.7269±0.0165 mas[2]
Distance1,196 ± 7 ly
(367 ± 2 pc)
Absolute magnitude (MV)7.73+0.28
−0.25[3]
Details
Mass0.61 ± 0.03[3] M☉
Radius0.60 ± 0.02[3] R☉
Luminosity (bolometric)0.117[6] L☉
Luminosity (visual, LV)0.069[nb 1] L☉
Temperature4402 ± 100[3] K
Metallicity [Fe/H]−0.37 ± 0.10[3] dex
Age2.9+8.1
−0.2[3] Gyr
Other designations
Gaia DR2 2100258047339711488, KOI-4742, KIC 4138008, 2MASS J19012797+3916482[7]
Database references
SIMBADdata

Kepler-442 is a K-type main-sequence star approximately 1,196 light years from Earth in the constellation Lyra. It is located within the field of vision of the Kepler spacecraft, the satellite that NASA's Kepler Mission used to detect planets that may be transiting their stars. On January 6, 2015, along with the stars of Kepler-438 and Kepler-440, it was announced that the star has an extrasolar planet (a super-Earth) orbiting within the habitable zone, named Kepler-442b.[3]

Nomenclature and history

[edit]
The Kepler Space Telescope search volume, in the context of the Milky Way Galaxy.

Prior to Kepler observation, Kepler-442 had the 2MASS catalogue number 2MASS J19012797+3916482. In the Kepler Input Catalog it has the designation of KIC 4138008, and when it was found to have transiting planet candidates it was given the Kepler object of interest number of KOI-4742.

Planetary candidates were detected around the star by NASA's Kepler Mission, a mission tasked with discovering planets in transit 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, although other phenomenon can also be responsible which is why the term planetary candidate is used.[8]

Following the acceptance of the discovery paper, the Kepler team provided an additional moniker for the system of "Kepler-442".[9] The discoverers referred to the star as Kepler-442, which is the normal procedure for naming the exoplanets discovered by the spacecraft.[3] Hence, this is the name used by the public to refer to the star and its planet.

Candidate planets that are associated with stars studied by the Kepler Mission are assigned the designations ".01" etc. after the star's name, in the order of discovery.[10] If planet candidates are detected simultaneously, then the ordering follows the order of orbital periods from shortest to longest.[10] Following these rules, there was only candidate planet were detected, with an orbital period of 112.3053 days.

The designation b derive from the order of discovery. The designation of b is given to the first planet orbiting a given star, followed by the other lowercase letters of the alphabet.[11] In the case of Kepler-442, there was only one planet detected, so only the letter b is used. The name Kepler-442 derives directly from the fact that the star is the catalogued 442nd star discovered by Kepler to have confirmed planets.

Stellar characteristics

[edit]

Kepler-442 is a K-type main sequence star that is approximately 61% the mass of and 60% the radius of the Sun. It has a temperature of 4402 K and is about 2.9 billion years old, but the margin of error here is quite large.[3] In comparison, the Sun is about 4.6 billion years old[12] and has a temperature of 5778 K.[13]

The star is somewhat poor in metals, with a metallicity ([Fe/H]) of about –0.37, or about 43% of the amount of iron and other heavier metals found in the Sun.[3] The star's luminosity is a bit low for a star like Kepler-442, with a luminosity of around 12% of that of the solar luminosity.[6]

Kepler-442 orbits a star with an apparent magnitude of 14.976, rendering it too faint to be visible to the naked eye from Earth. This dimness, as well as its distance from Earth, poses a challenge for direct observation.

Planetary system

[edit]
The Kepler-442 planetary system[3]
Companion
(in order from star) 		Mass Semimajor axis
(AU) 		Orbital period
(days) 		Eccentricity Inclination Radius
b 2.3+5.9
−1.3 M🜨 		0.409+0.209
−0.060 		112.3053+0.0024
−0.0028 		0.04+0.08
−0.04 		89.94+0.06
−0.12° 		1.34+0.11
−0.18 R🜨

The only known planet transits the star; this means that the planet's orbit appear to cross in front of their star as viewed from the Earth's perspective. Its inclination relative to Earth's line of sight, or how far above or below the plane of sight it is, vary by less than one degree. This allows direct measurements of the planet's periods and relative diameters (compared to the host star) by monitoring the planet's transit of the star.

Kepler-442b is a super-Earth with a radius 1.34 times that of Earth, and orbits well within the habitable zone. It is likely a rocky planet due to its radius. According to NASA, it was described as being one of the most Earth-like planets, in terms of size and temperature, yet found.[14][15] It is just outside of the zone (around 0.362 AU) where tidal forces from its host star would be enough to tidally lock it.[16]

See also

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Notes

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References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Kepler-442

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from Grokipedia
Kepler-442 is a K5-type main-sequence star located approximately 1,206 light-years (370 parsecs) from in the constellation . It hosts the confirmed , a orbiting within the star's , where conditions may allow for liquid water on the surface under suitable atmospheric conditions. This system, discovered in 2015 by NASA's using the transit method, exemplifies the potential for Earth-like worlds around cooler, longer-lived K-type stars. The host star Kepler-442 has a mass of about 0.61 solar masses, a radius of 0.60 solar radii, and an of 4,402 K, making it an orange dwarf cooler and smaller than the Sun but with a stable lifespan of approximately 30 billion years. These properties contribute to a closer to the star compared to Sun-like systems, reducing the risk of stellar flares that could strip planetary atmospheres. Kepler-442b, with a radius of 1.34 radii and an estimated mass of 2.3 masses, completes an every 112.3 days at a semi-major axis of 0.409 AU, receiving about two-thirds the incident flux that does from the Sun. Its equilibrium temperature of approximately 260 K suggests a cool but potentially temperate environment, positioning it among the top candidates for rocky, habitable exoplanets identified by the Kepler mission.

Discovery and Nomenclature

Initial Detection

The Kepler mission, launched by in March 2009, employed the transit method to detect exoplanets by continuously monitoring the brightness of approximately 150,000 stars in the constellation Cygnus and . This technique identifies potential through periodic dips in a star's , caused when a planet passes in front of its host star from the observer's perspective, reducing the observed brightness by a fraction proportional to the planet's size relative to the star. The mission's primary phase, from May 2009 to May 2013, collected high-precision photometric data using a 0.95-meter to enable detection of Earth-sized planets around Sun-like stars. Kepler-442, initially designated as KOI-4742, was identified as a during the analysis of data from the mission's primary phase. The star's were first observed starting in Quarter 1 (Q1) of Kepler operations, beginning in May 2009, with no detections prior to the mission's launch. KOI-4742.01 achieved status in the Kepler Objects of Interest (KOI) catalog following the Q1-Q12 data release in February 2013, based on threshold crossing events (TCEs) processed by the Kepler Operations (SOC) pipeline. Initial photometric analysis revealed periodic transits with a depth of approximately 502 ppm, indicating a small relative to the host , a duration of about 5.87 hours per transit event, and an of roughly 112.3 days, corresponding to roughly three observed transits over the initial . These parameters, derived from fitting the with models assuming a planetary transit, established KOI-4742.01 as a promising candidate for further validation.

Confirmation and Naming

The planetary candidate KOI-4742.01, detected in Kepler's photometric data, underwent rigorous statistical validation to confirm its planetary nature. This process utilized the technique, which models various false positive scenarios—such as background eclipsing binaries or hierarchical triples—and compares them to the observed to estimate the probability of a genuine transiting . Ground-based follow-up observations provided supplementary evidence, including high-resolution with the NIRC2 instrument on the Keck II to resolve nearby companions, speckle at the Gemini North , and reconnaissance with the HIRES instrument on Keck I to assess radial velocities and stellar properties. Centroid analysis of the Kepler data further ruled out on-target false positives by checking for offsets in the photocenter during transits. The validation yielded a false positive probability of less than 0.11%, equivalent to a 99.89% level that the transit signal originates from a orbiting the target . announced the confirmation of on January 6, 2015, as part of a milestone batch that verified over 100 additional exoplanets from Kepler observations, bringing the mission's total to more than 1,000 confirmed worlds; this included eight small planets in their stars' habitable zones, with among the most promising due to its Earth-like size and orbital position. The detailed results were published in a peer-reviewed paper on February 18, 2015, in by Torres et al., which validated 12 such candidates (including ) at levels exceeding 99.73%. Following confirmation, the host star received the designation Kepler-442, adhering to the Kepler mission's numbering system for targets with validated planets, while the planet was named as the innermost (and only) confirmed member of the system, consistent with (IAU) guidelines for provisional . This naming convention assigns lowercase letters (starting with "b") to planets in order of discovery around a given host, without implying orbital sequence.

Host Star Characteristics

Physical Parameters

Kepler-442 is a K5V main-sequence star, characterized as an orange dwarf with properties that place it among the cooler and smaller stellar hosts in the Kepler field. The star's mass is estimated at 0.61 ± 0.03 M⊙, while its radius measures 0.60 ± 0.02 R⊙, making it roughly 61% and 60% of the Sun's respective values. Its is 4,402 ± 100 K, corresponding to a of 0.117 ± 0.003 L⊙ and a of log g = 4.7. These parameters indicate a stable, low-mass star with subdued energy output compared to solar-type stars.
ParameterValueUnit
0.61 ± 0.03M⊙
0.60 ± 0.02R⊙
4,402 ± 100K
0.117 ± 0.003L⊙
Surface Gravity4.7log g
The of Kepler-442 is [Fe/H] = -0.37, signifying a lower abundance of heavy elements relative to the Sun, which influences its evolutionary track and potential for formation. Located at a of 1,196 ± 7 light-years (367 ± 2 ) from Earth, as determined from DR3 measurements, the star appears faint with an of 14.976 in the Kepler band, rendering it invisible to the and requiring space-based or large ground-based telescopes for observation. These physical parameters were derived primarily through a combination of transit modeling of the Kepler photometry to constrain stellar density, spectroscopic analysis for and , and fitting to stellar evolution models such as the Dartmouth isochrones to obtain mass, radius, and luminosity.

Activity and Age

The age of Kepler-442 has been estimated at 2.9 +8.1/-0.2 Gyr through a combination of gyrochronology, which relates the star's to its age, and isochrone fitting using Dartmouth stellar evolution models. These methods account for the star's , , and to place it on evolutionary tracks, yielding consistent results within uncertainties dominated by the lack of a precisely measured rotation period. No definitive rotation period has been identified for Kepler-442 from Kepler photometry, with potential signals appearing as artifacts from quarter-length data gaps rather than true variability. Photometric variability is low, with a mean activity index of 736 ppm over 50-day light curves, consistent with expectations for an inactive K-type dwarf. Chromospheric activity indicators suggest subdued magnetic processes, and no significant flares were detected in the long-cadence Kepler data, with an upper limit on flare energy of 5.6 × 10^{32} erg. As a mid-main-sequence K dwarf with a mass of approximately 0.61 , Kepler-442 is in a stable evolutionary phase expected to persist for over 20 Gyr, far exceeding the Sun's main-sequence lifetime due to its lower mass and resulting slower core hydrogen fusion. This prolonged stability implies minimal changes in over billions of years, providing a favorable environment for long-term planetary habitability by reducing risks from rapid stellar evolution.

Planetary System

System Architecture

The Kepler-442 consists of a single confirmed , Kepler-442b, orbiting the K-type dwarf star Kepler-442, with no additional validated planets or significant candidates identified in subsequent analyses of Kepler data. Following the initial validation in 2015, reprocessing of the full Kepler dataset through Data Release 25 and later catalogs, including the 2023 updated Kepler planet candidate catalog, has not yielded evidence for other transiting or non-transiting companions around this host star. This sparse architecture contrasts with the compact, multi-planet configurations common in many Kepler systems, such as , which features six tightly packed planets within 0.5 AU of their host. As a single-planet , Kepler-442 exhibits no observed orbital resonances, resulting in a straightforward dynamical structure where the planet's orbit remains unperturbed by additional bodies. Long-term numerical simulations of candidates, including those like , indicate dynamical stability over gigayear timescales, facilitated by the planet's relatively wide orbit at approximately 0.41 AU from the star. This stability is consistent with validations from the original Kepler data, which modeled the 's configuration under nominal planetary masses and found no indications of instability. Key architectural features include the habitable zone's inner edge at roughly 0.3 AU, placing comfortably within the conservative boundaries for liquid water potential without close-in companions disrupting its orbit. Additionally, infrared observations from the (WISE) show no excess emission indicative of a around Kepler-442, suggesting a relatively cleared inner system lacking significant dust or populations.

Kepler-442b Properties

Kepler-442b orbits its host star at a semi-major axis of 0.409 ± 0.001 , completing one revolution every 112.3053 ± 0.0006 days. The orbit is nearly circular, with an eccentricity less than 0.1, often assumed to be zero for modeling purposes due to the lack of detectable deviations in transit timing. The planet has a radius of 1.34 +0.18/-0.11 times that of , classifying it as a . Its mass is estimated at 2.3 +5.9/-1.3 masses, derived from upper limits combined with mass-radius relationship models that favor terrestrial compositions. Given its size and mass estimates, Kepler-442b is likely rocky, with a in the range of approximately 5–8 g/cm³ under the assumption of an Earth-like composition. This suggests a structure potentially featuring an iron core and a mantle, consistent with models for low-mass rocky worlds. The equilibrium temperature of Kepler-442b, calculated as a blackbody without an atmosphere, is approximately 260 K. It receives an insolation flux of about 0.66 times that of , reflecting its position relative to the star's . Transit observations reveal a depth of 0.5%, a duration of roughly 5 hours, and an inclination near 90°, indicating an edge-on that facilitates detection.

Habitability Assessment

Habitable Zone Position

The (HZ) represents the annular region around a star where conditions might allow for the presence of liquid on a , assuming an Earth-like atmosphere and composition. For the K-type host star Kepler-442, with its lower of approximately 0.12 L_⊙ and of 4402 K, the conservative HZ—defined by limits where remains liquid under typical conditions—is calculated to extend from 0.385 to 0.728 AU. The optimistic HZ, which accounts for denser atmospheres capable of broader stability (e.g., high CO₂ scenarios), spans a wider range from 0.284 to 0.794 AU. These boundaries are derived from updated climate models that adjust limits for the star's spectral type, prioritizing the inner edge against runaway effects and the outer edge against CO₂ . Kepler-442b resides at a semi-major axis of 0.409 AU with an of 112.3 days, receiving about 66% of Earth's incident stellar radiation, equivalent to a time-averaged flux of 0.66 F_⊕. This placement situates the planet squarely in the middle of the conservative HZ, where the received energy supports surface temperatures potentially ranging from 260–290 K under Earth-analog atmospheric assumptions, well within liquid water viability. The planet's position benefits from the star's stable output, as K-type stars exhibit lower variability than hotter types, enhancing long-term prospects. Compared to G-type stars like the Sun, the HZ around K-type stars such as Kepler-442 is shifted inward due to reduced , resulting in closer orbital distances for equivalent levels, while also appearing wider in angular extent relative to the stellar radius. This configuration arises from the star's longer main-sequence lifetime (exceeding 30 billion years) and cooler spectrum, which minimizes UV-driven atmospheric erosion and allows more time for planetary . Kepler-442's HZ thus offers a more forgiving environment for retaining volatiles over cosmic timescales. Although Kepler-442b's is low at 0.04 ± 0.08, any minor variations could slightly modulate annual flux by less than 10%, an effect negligible for overall thermal stability. At 0.4 AU, the planet faces minimal risk of , as the equilibrium tide timescale exceeds the star's age of several billion years, permitting potential rotation and diverse climate regimes. These factors contribute to the planet's high of 0.84, ranking it among the top candidates for Earth-likeness based on radius, density, , and surface temperature metrics.

Life Potential and Recent Studies

Models suggest that Kepler-442b could support liquid on its surface under a range of atmospheric conditions. Equilibrium temperature calculations yield approximately 260 K, but the addition of a atmosphere could raise surface temperatures to 250–300 K, enabling stable liquid oceans if the planet is volatile-rich. Atmospheric modeling indicates that an Earth-like N2-O2 composition is plausible for Kepler-442b, given its size and stellar environment. The planet's host star, a quiet K-type dwarf with low flare activity, would result in reduced atmospheric escape rates, preserving volatiles over billions of years. Recent studies have refined habitability assessments for Kepler-442b. A 2024 modeling effort examined temperature forcing due to orbital eccentricity, finding variations of up to ±24 K for an eccentricity of ~0.1, which could still permit a stable climate with sufficient atmospheric buffering. In 2025, updated characterization tools like SEPHI 2.0 reaffirmed its potential habitability by incorporating improved internal structure estimates, while confirming the planet's low false alarm probability from archival data. A October 2025 reassessment using Gaia DR3 data further validated low false alarm probabilities for Kepler HZ exoplanets like Kepler-442b. Technosignature searches in 2025, focusing on eclipsing systems, highlighted Kepler-442b as a prime terrestrial candidate but yielded no detections of artificial signals. Additionally, analysis of photosynthetically active radiation (PAR) flux indicates that the planet receives slightly more than the threshold needed for an Earth-like biosphere, supporting efficient oxygenic photosynthesis. An October 2025 study in the Indonesian Journal of Astronomy also affirmed Kepler-442b's high habitability potential using updated Earth Similarity and Planetary Habitability Indices. Astrobiologically, Kepler-442b's estimated of ~1.3 g presents challenges for complex but is offset by a potentially stable climate and the of its K-type host star, which could allow up to 70 billion years for biological evolution—far exceeding the Sun's lifetime and enhancing superhabitable prospects compared to . Key limitations persist in evaluating life potential, including uncertainties in (estimated at 2.36 masses with significant ), which affect and composition inferences, and the absence of direct atmospheric to constrain effects or biosignatures.
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