K2-141b (also designated EPIC 246393474.01) is a massive

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 Burgess M ...

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 ...

orbiting extremely close to a K Type orange main-sequence star K2-141. The planet was first discovered by the

Kepler space telescope 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 ...

during its K2 “Second Light” mission and later observed by the HARPS-N spectrograph. It is classified as an Ultra-short Period (USP) and is confirmed to be terrestrial in nature. Its high density implies a massive iron core taking up between 30% and 50% of the planet's total mass.

Characteristics

Mass and radius

Like the majority of known exoplanets, K2-141b was detected using the transit method, where a planet blocks a tiny fraction of its star's light as it passed between our line of the sight and its host. This method is only able to determine the radius of the planet, not its mass. However, K2-141b was also detected by the radial velocity method using the HARPS-N spectrograph. Therefore, its mass could also be determined along with its radius. The planet is classified as a Super-Earth, being significantly larger and more massive than

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 surfa ...

but not as large as 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 (mythology), Uranus (Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars (mythology), Mars), grandfather ...

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 times ...

. K2-141b has a radius of 1.51 , below the 1.6 threshold where most planets are expected to accumulate thick hydrogen and helium atmospheres, transforming them into

mini-Neptune A Mini-Neptune (sometimes known as a gas dwarf or transitional planet) is a planet less massive than Neptune but resembling Neptune in that it has a thick hydrogen–helium atmosphere, probably with deep layers of ice, rock or liquid oceans (made ...

s. The planet's mass confirms that it is rocky. It has a mass of 5.08 , which gives K2-141b a high density of 8.2 g/cm³, about 1.48 times the density of Earth. This high density implies a composition with a large iron core taking up about 30% to 50% of the planet's total mass.

Orbit

K2-141b has one of the shortest known orbital periods of any confirmed exoplanet. With an orbital period of only 6.7 hours, it is the shortest-period planet known to date with a precisely determined mass. Only a few planets, including those around

Kepler-70 Kepler-70, also known as KIC 5807616 and formerly as KOI-55, is a star in the constellation Cygnus with an apparent visual magnitude of 14.87, and is 4200 light-years away. This is too faint to be seen with the naked eye; viewing it requ ...

, have shorter orbital periods. At this proximity, K2-141b is most likely

tidally locked Tidal locking between a pair of co-orbiting astronomical body, astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where ...

with its host star, meaning that the same side of the planet always faces the star. It has a semi-major axis of 0.00716 AU. For a comparison, Mercury's perihelion is 0.307499 AU, more than 41 times farther away from the Sun.

Atmosphere and climate

Despite its terrestrial nature, K2-141b is far from habitable. Its extremely close proximity to its host star has resulted in an equilibrium temperature of about . However, the actual temperature is probably much higher. About two-thirds of K2-141b faces perpetual daylight. The night side experiences frigid temperatures of below . The day side of the exoplanet, at an estimated , is hot enough to not only melt rocks but vaporize them as well. A low albedo of around 0.3 means most light which hits the planet is absorbed, adding to the heat of the dayside. K2-141b is believed to have both an atmosphere and oceans, which are magma and likely tens of kilometers deep. The makeup of the atmosphere is unknown but likely consists of vaporized metals which are common in solid form on Earth. The atmosphere is believed to have extreme wind speeds of over 1.75 kilometers per second. Temperatures are high enough that the magma in the oceans can vaporize into the atmosphere. The mineral vapor formed by evaporated rock is swept to the frigid night side by supersonic winds and rocks "rain" back down into a magma ocean. The resulting currents flow back to the hot day side of the exoplanet, where rock evaporates once more. If the planet's atmosphere has high levels of sodium, then solid sodium might slowly slide towards the planet's oceans, similarly to how

glacier A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its Ablation#Glaciology, ablation over many years, often Century, centuries. It acquires dis ...

s move on Earth.

Host star

K2-141 is an K5 main-sequence star about 61 parsecs (202 light years) away, in the direction of the constellation

Pisces Pisces may refer to: * Pisces, an obsolete (because of land vertebrates) taxonomic superclass including all fish * Pisces (astrology), an astrological sign * Pisces (constellation), a constellation **Pisces Overdensity, an overdensity of stars in ...

. Based on the spectral type (K5/6 D) of the star, the star's colour is orange. It has a radius of 0.681 ±0.018 and a mass of 0.708 ±0.028 . It has a temperature of 4,599 ±79 K and is between 1.6 and 12.9 billion years old. For comparison, the Sun has a temperature of 5778 K and is 4.5 billion years old.

References