K2-136 is aK-type main-sequence star located in thezodiacconstellationTaurus. It is a member of theHyades open cluster, which provides a well-constrained young age on the system. The star is known to host threetransiting exoplanets, discovered in 2017 from lightcurves gathered by theKepler space telescope during the K2 extension mission. The innermost planet is approximatelyEarth-sized, the first such planet found in a young open cluster.[10]
The star is likelyaccompanied by a faintred dwarf of spectral type M7/8V located at aprojected separation of~40 AU.[4][8]
K2-136 was identified as a high-proper-motion star during the Luyten Palomar Survey in the 1970s and was included in theLuyten-Palomar proper motion catalogue, where it received the designationLP 358‑348. 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 byphotometric measurements by Edward W. Weis in 1982.[11]
A candidateplanetary transit signal was already proposed in theWASP telescope photometry as part of the SEAWOLF survey in 2013, however the ground observations at the time found the proposed3.169 d signal to be inconclusive.[12] 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 theKepler space telescope by seven different guest observers.[5][4][13] The star, designated asEPIC 247589423 in theK2 input catalog, was observed from 8 March 2017 to 27 May 2017 during Campaign 13, with the calibrated data publicly released on 28 August 2017.[5] Of the independent teams, two (led by Andrew Mann and byDavid Ciardi) have submitted their discovery papers on 29 September 2017,[5][4] with a third one (led by John Livingston) following on 16 October 2017,[13] making the planetary system a case of a triple co-discovery. All three discovery papers were published inThe Astronomical Journal in January and March 2018.
X-ray observations of the star were carried out byXMM-Newton on 11 September 2018, obtaining a good quality spectrum in the0.2–12.0 keV energy band.[8]
A series ofradial velocity measurements was carried out in the course of a planetary mass characterization study by Andrew W. Mayo et al. between 11 August 2018 and 31 October 2020, 93 withHARPS-N and 22 withESPRESSO.[8]
The star was also observed byTESS during Sectors 43 and 44, 2021 through 6 November 2021,[8] receiving a TESS object of interest designationTOI‑5087.
K2-136 is relatively quiet for its age, with its lightcurve showing a coherent variability of ~1%[4] and minor flares.[5] Theperiodogram of brightness variations recorded by theKepler space telescope shows the strongest variation at periods of 13-15 days,[5][4] which can be identified with therotation period of the star. The differing rates of rotation reported by different studies is possibly explained bydifferential rotation, similar tothe Sun, with the equator likely rotating faster than higher latitudes by ~1 day.[4][8]
The star's membership in the Hyades is confirmed by itskinematic,photometric data, with its motion and position on thecolor-magnitude diagram aligning with the rest of the Hyades cluster members.[5][4]
X-ray observations withXMM-Newton measured the star's X-rayluminosity as(1.26±0.19)×1028 erg/s, or a fraction of(1.97±0.30)×10−5 of the totalbolometric luminosity. 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.[5]
| Companion (in order from star) | Mass | Semimajor axis (AU) | Orbital period (days) | Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| b | <4.3 M🜨 | 0.0707±0.0012 | 7.97525±0.00073 | 0.14+0.12 −0.11 | 89.3+0.5 −0.7° | 1.014+0.050 −0.049 R🜨 |
| c | 18.1+1.9 −1.8 M🜨 | 0.1185+0.0020 −0.0021 | 17.307081+0.000014 −0.000013 | 0.047+0.062 −0.034 | 89.6±0.3° | 3.00±0.13 R🜨 |
| d | <3.0 M🜨 | 0.1538+0.0026 −0.0027 | 25.5750+0.0022 −0.0021 | 0.071+0.063 −0.049 | 89.4+0.4 −0.2° | 1.565+0.077 −0.076 R🜨 |
K2-136 b, the innermost planet of the system, is approximately Earth-sized, with a radius of1.014+0.050
−0.049 R🜨. It is likely aterrestrial planet, though as of 2025[update], no composition is ruled out yet based on the upper limit of mass determined fromradial velocity observations.[8] 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 likelyterrestrial.[14] The planet's estimatedequilibrium temperature is560–610 K.[b][8]
K2-136 c, the largest planet of the system, can be classified as asub-Neptune. The measured value of its mass18.1+1.9
−1.8 M🜨 is comparable to that ofNeptune, but the radius of3.00±0.13 R🜨 is significantly smaller. This is in contrast with the general trend that youngNeptunes are puffier, implying a heavier composition. The calculated density of3.69+0.67
−0.56 g⋅cm−3 is consistent with both a water-dominatedocean world 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 theSubaru Telescope during the transit on 2020.[15]e two extremes.[8] However, anice-rich composition is unlikely and would imply that the planet initially formed much further away from the star.[14] The planet's estimatedequilibrium temperature is440–470 K.[b][8]
The planet is large enough that it is expected to retain most of its primordial atmosphere of hydrogen and helium, and that the contemporaryatmospheric escape is minimal.[8][14] 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.[15]
K2-136 d is the outermost of the known planets of the system. Its radius of1.565+0.077
−0.076 R🜨 places it at the lower edge of theFulton gap. The planet's mass and density remain uncertain, as radial velocity observations have not yet conclusively detected its signal as of 2025[update]. The upper limit of3.0 M🜨 with95% confidence corresponds to an upper limit for density of4.3 g⋅cm−3 suggesting a bulk composition lighter than Earth.[8] The planet's estimatedequilibrium temperature is380–420 K.[b][8]
Based on plausible evolutionary models, the planet most likely has a mass of at least2 M🜨 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 ofice, both of which is unlikely. The planet is likely still experiencingatmospheric escape at a non-negligible rate and may lose the remaining gaseous envelope within 2 billion years.[14]
