K2-136 is a

K-type main-sequence star A K-type main-sequence star, also referred to as a K-type dwarf, or 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 stars ...

located in the

zodiac The zodiac is a belt-shaped region of the sky that extends approximately 8° north and south celestial latitude of the ecliptic – the apparent path of the Sun across the celestial sphere over the course of the year. Within this zodiac ...

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

Taurus Taurus is Latin for 'bull' and may refer to: * Taurus (astrology), the astrological sign ** Vṛṣabha, in vedic astrology * Taurus (constellation), one of the constellations of the zodiac * Taurus (mythology), one of two Greek mythological ch ...

. It is a member of the Hyades open cluster, which provides a well-constrained young age on the system. The star is known to host three transiting exoplanets, discovered in 2017 from lightcurves gathered by the

Kepler space telescope The Kepler space telescope is a defunct 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 the K2 extension mission. The innermost planet is approximately Earth-sized, the first such planet found in a young open cluster. The star is likely accompanied by a faint

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 ...

of spectral type M7/8V located at a

projected separation This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outs ...

of .

Observational history

K2-136 was identified as a high- proper-motion star during the Luyten Palomar Survey in the 1970s and was included in the Luyten-Palomar proper motion catalogue, where it received the designation LP 358348. Its possible membership in the Hyades based on its proper motion was pointed out by Natalia M. Artyukhina and Pavel N. Kholopov who published a catalog of proper motions of stars in the region of the open cluster in 1975–76, which was later confirmed by photometric measurements by Edward W. Weis in 1982. A candidate planetary transit signal was already proposed in the WASP telescope photometry as part of the SEAWOLF survey in 2013, however the ground observations at the time found the proposed signal to be inconclusive. The proposed period and depth do not match any of the now known planets. Due to its known membership in the Hyades cluster and relative brightness, the star was proposed for observation by the

by seven different guest observers. The star, designated as EPIC 247589423 in the K2 input catalog, was observed from to during Campaign 13, with the calibrated data publicly released on . Of the independent teams, two (led by Andrew Mann and by David Ciardi) have submitted their discovery papers on , with a third one (led by John Livingston) following on , making the planetary system a case of a triple co-discovery. All three discovery papers were published in ''

The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the p ...

'' in January and March 2018. X-ray observations of the star were carried out by

XMM-Newton ''XMM-Newton'', also known as the High Throughput X-ray Spectroscopy Mission and the X-ray Multi-Mirror Mission, is an X-ray space observatory launched by the European Space Agency in December 1999 on an Ariane 5 rocket. It is the second corners ...

on , obtaining a good quality spectrum in the energy band. A series of radial velocity measurements was carried out in the course of a planetary mass characterization study by Andrew W. Mayo et al. between and , 93 with

HARPS-N __NOTOC__ HARPS-N, the ''High Accuracy Radial velocity Planet Searcher for the Northern hemisphere'' is a high-precision radial-velocity spectrograph, installed at the Italian Telescopio Nazionale Galileo, a 3.58-metre telescope located at the Roqu ...

and 22 with

ESPRESSO Espresso (, ) is a concentrated form of coffee produced by forcing hot water under high pressure through finely ground coffee beans. Originating in Italy, espresso has become one of the most popular coffee-brewing methods worldwide. It is cha ...

. The star was also observed by

TESS Tess or TESS may refer to: Film and theatre * Tess (1979 film), ''Tess'' (1979 film), a 1979 film adaptation of ''Tess of the d'Urbervilles'' * Tess (2016 film), ''Tess'' (2016 film), a South African production Music * Tess (band), a Spanish pop ...

during Sectors 43 and 44, through , receiving a TESS object of interest designation TOI5087.

Stellar characteristics

K2-136 is relatively quiet for its age, with its lightcurve showing a coherent variability of ~1% and minor flares. The

periodogram In signal processing, a periodogram is an estimate of the spectral density of a signal. The term was coined by Arthur Schuster in 1898. Today, the periodogram is a component of more sophisticated methods (see spectral estimation). It is the most ...

of brightness variations recorded by the

shows the strongest variation at periods of 13-15 days, which can be identified with the

rotation Rotation or rotational/rotary motion is the circular movement of an object around a central line, known as an ''axis of rotation''. A plane figure can rotate in either a clockwise or counterclockwise sense around a perpendicular axis intersect ...

period of the star. The differing rates of rotation reported by different studies is possibly explained by

differential rotation Differential rotation is seen when different parts of a rotating object move with different angular velocities (or rates of rotation) at different latitudes and/or depths of the body and/or in time. This indicates that the object is not rigi ...

, similar to

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

, with the equator likely rotating faster than higher latitudes by ~1 day. The star's membership in the Hyades is confirmed by its

kinematic In physics, kinematics studies the geometrical aspects of motion of physical objects independent of forces that set them in motion. Constrained motion such as linked machine parts are also described as kinematics. Kinematics is concerned with s ...

, photometric data, with its motion and position on the color-magnitude diagram aligning with the rest of the Hyades cluster members. X-ray observations with

measured the star's X-ray

luminosity Luminosity is an absolute measure of radiated electromagnetic radiation, electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electroma ...

as , or a fraction of of the total

bolometric luminosity Luminosity is an absolute measure of radiated electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electromagnetic energy emitted per ...

. These values indicate that K2-136 is somewhat less luminous in X-rays compared to other K dwarfs in the Hyades cluster, which is also consistent with a slightly slower than average rotation, but lies within the typical range for late-K dwarfs.

Planetary system

K2-136 b

K2-136 b, the innermost planet of the system, is approximately Earth-sized, with a radius of . It is likely a

terrestrial planet A terrestrial planet, tellurian planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate, rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to ...

, though , no composition is ruled out yet based on the upper limit of mass determined from radial velocity observations. However, based on evolution modelling, it is expected that the young star's intense X-ray radiation would have stripped the planet of its original gas envelope during the initial ten million years, meaning that the planet is most likely

terrestrial Terrestrial refers to things related to land or the planet Earth, as opposed to extraterrestrial. Terrestrial may also refer to: * Terrestrial animal, an animal that lives on land opposed to living in water, or sometimes an animal that lives on o ...

. The planet's estimated

equilibrium temperature The planetary equilibrium temperature is a theoretical temperature that a planet would be if it were in radiative equilibrium, typically under the assumption that it radiates as a black body being heated only by its parent star. In this model, th ...

is .

K2-136 c

K2-136 c, the largest planet of the system, can be classified as a

sub-Neptune The term sub-Neptune can refer to a planet with a smaller radius than Neptune even though it may have a larger mass or to a planet with a smaller mass than Neptune even though it may have a larger radius like a super-puff and both meanings can ev ...

. The measured value of its mass is comparable to that of

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

, but the radius of is significantly smaller. This is in contrast with the general trend that young

Neptunes The Neptunes were an American record production duo composed of Pharrell Williams and Chad Hugo, formed in Virginia Beach, Virginia, in 1992. Williams often provided backing vocals and music video appearances on the duo's productions, while H ...

are puffier, implying a heavier composition. The calculated density of is consistent with both a water-dominated

ocean world An ocean world, ocean planet or water world is a type of planet or natural satellite that contains a substantial amount of water in the form of oceans, as part of its hydrosphere, either beneath the surface, as subsurface oceans, or on the s ...

composition and a large Earth-like core and a H/He envelope with a mass fraction of ~5%, or any combination of the non-detection of excess absorption by neutral helium in the spectrum of the starlight recorded by the

Subaru Telescope is the telescope of the National Astronomical Observatory of Japan, located at the Mauna Kea Observatory on Hawaii. It is named after the open star cluster known in English as the Pleiades. It had the largest monolithic primary mirror in the ...

during the transit on .e two extremes. However, an

ice Ice is water that is frozen into a solid state, typically forming at or below temperatures of 0 ° C, 32 ° F, or 273.15 K. It occurs naturally on Earth, on other planets, in Oort cloud objects, and as interstellar ice. As a naturally oc ...

-rich composition is unlikely and would imply that the planet initially formed much further away from the star. The planet's estimated

is . The planet is large enough that it is expected to retain most of its primordial atmosphere of hydrogen and helium, and that the contemporary

atmospheric escape Atmospheric escape is the loss of planetary atmospheric gases to outer space. A number of different mechanisms can be responsible for atmospheric escape; these processes can be divided into thermal escape, non-thermal (or suprathermal) escape, and ...

is minimal. This is also implied by the non-detection of excess absorption by neutral helium in the spectrum of the starlight recorded by the Subaru Telescope during the transit on 2020.

K2-136 d

K2-136 d is the outermost of the known planets of the system. Its radius of places it at the lower edge of the

Fulton gap The small planet radius gap (also called the Fulton gap, photoevaporation valley, or Sub-Neptune Desert) is an observed scarcity of planets with radii between 1.5 and 2 times Earth's radius, likely due to photoevaporation-driven mass loss. A bimo ...

. The planet's mass and density remain uncertain, as radial velocity observations have not yet conclusively detected its signal . The upper limit of with 95% confidence corresponds to an upper limit for density of suggesting a bulk composition lighter than Earth. The planet's estimated

is . Based on plausible evolutionary models, the planet most likely has a mass of at least that still retains a fraction of its gaseous envelope to maintain the current low bulk density. It is likely that it formed with a similar envelope mass fraction as the planet c, but has lost most of it during the early evolution stages. An even lower current mass would imply either that the planet's initial gas envelope heavier than the core mass, or that the planet has already lost the gas envelope but is composed of a high fraction of

, both of which is unlikely. The planet is likely still experiencing

at a non-negligible rate and may lose the remaining gaseous envelope within 2 billion years.

Notes

References

{{Sky, 04, 29, 38.9939, +, 22, 52, 57.794, 192 K-type main-sequence stars Planetary systems with three confirmed planets Binary stars Taurus (constellation) Hyades (star cluster) 5087 018310799 J04293897+2252579 J042939.05+225257.5