Characteristics

Mass, radius, and temperature

Kepler-442b is a super-Earth, an exoplanet with a mass and radius bigger than that of Earth, but smaller than that of the ice giants Uranus and Neptune. It has an equilibrium temperature of {{convert|233|K|C F}}.^[5] It has a radius of 1.34 {{earth radius}}. 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 2.34 {{Earth mass}}.^[16] The surface gravity on Kepler-442b would be 30% stronger than that of Earth, assuming a rocky composition similar to that of Earth.^[6]

Host star

The planet orbits a (K-type) star named Kepler-442. The star has a mass of 0.61 {{solar mass}} and a radius of 0.60 {{solar radius}}. It has a temperature of 4402 K and is around 2.9 billion years old, with some uncertainty. In comparison, the Sun is 4.6 billion years old^[7] and has a temperature of 5778 K.^[8] The star is somewhat metal-poor, with a metallicity (Fe/H) of −0.37, or 42% of the solar amount. Its luminosity ({{solar luminosity|link=y}}) is 11% that of the Sun.

The star's apparent magnitude, or how bright it appears from Earth's perspective, is 14.97. Therefore, it is too dim to be seen with the naked eye.

Orbit

Kepler-442b orbits its host star with an orbital period of 112.3 days and an orbital radius of about 0.4 times that of Earth's (a little larger than the distance of Mercury from the Sun, which is about 0.38 AU).^[2][3] It receives about 70% of the sunlight that Earth receives from the Sun.

Habitability

The planet was announced as being located within the habitable zone of its star, a region where liquid water could exist on the surface of the planet. It was described as being one of the most Earth-like planets, in terms of size and temperature, yet found.^[2][3] It is outside of the zone (around 0.02 AU) where tidal forces from its host star would be enough to tidally lock it.^[9] As of July, 2018, Kepler-442b was considered the most-habitable non-tidally locked exoplanet discovered.^[10]

K-type main-sequence stars are smaller than the Sun and live longer, remaining on the main sequence 15 to 30 billion years compared to the Sun's estimated 10 billion.^[11] Despite these properties, the small M-type and K-type stars can pose a threat to life.^[12] 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.^[13] 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. Because it 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 (obliquity) is likely very small, in which case it would not have tilt-induced seasons as Earth and Mars do. Its orbit is probably close to circular so it will also lack eccentricity-induced seasonal changes like those of Mars.

One review essay in 2015 concluded that Kepler-442b, along with the exoplanets Kepler-186f and Kepler-62f, were likely the best candidates for being potentially habitable planets.^[14]

Discovery and follow-up studies

In 2009, NASA's Kepler spacecraft was completing observing stars on its photometer, the instrument it uses to detect transit events, in which a planet crosses in front of and dims its host star for a brief and roughly regular period of time. In this last test, Kepler observed {{val|50,000}} stars in the Kepler Input Catalog, including Kepler-442; the preliminary light curves were sent to the Kepler science team for analysis, who chose obvious 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), it was eventually concluded that a planetary body was responsible for the periodic 113-day transits. The discovery, along with the notable planetary systems of the stars Kepler-438 and Kepler-440, were announced on 6 January 2015.

At nearly {{convert|342|pc|ly|disp=flip}} 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 but next-generation planet-hunting space telescopes, such as TESS and CHEOPS, will examine nearby stars throughout the sky.

Nearby stars with planets can then be studied by the upcoming James Webb Space Telescope and future large ground-based telescopes to analyze atmospheres, determine masses and infer compositions. Additionally the Square Kilometer Array would significantly improve radio observations over the Arecibo Observatory and Green Bank Telescope.^[15]

See also

• List of potentially habitable exoplanets
• Kepler-62f
• Kepler-186f
• Kepler-452b
• Kepler-440b
• Kepler-438b

References