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Register a new userThree small planets transiting the bright young field star K2-233
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We report the detection of three small transiting planets around the young K3 dwarf K2-233 (2MASS J15215519-2013539) from observations during Campaign 15 of the K2 mission. The star is relatively nearby ($d$ = 69 pc) and bright ($V$ = 10.7 mag, $K_s$ = 8.4 mag), making the planetary system an attractive target for radial velocity follow-up and atmospheric characterization with the James Webb Space Telescope. The inner two planets are hot super-Earths ($R_b$ = 1.40 $pm$ 0.06 R$_oplus$, $R_c$ = 1.34 $pm$ 0.08 R$_oplus$), while the outer planet is a warm sub-Neptune ($R_d$ = 2.6 $pm$ 0.1 R$_oplus$). We estimate the stellar age to be 360$^{+490}_{-140}$ Myr based on rotation, activity, and kinematic indicators. The K2-233 system is particularly interesting given recent evidence for inflated radii in planets around similarly-aged stars, a trend potentially related to photo-evaporation, core-cooling, or both mechanisms.
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We present the discovery of three small planets transiting K2-136 (LP 358 348, EPIC 247589423), a late K dwarf in the Hyades. The planets have orbital periods of $7.9757 pm 0.0011$, $17.30681^{+0.00034}_{-0.00036}$, and $25.5715^{+0.0038}_{-0.0040}$ days, and radii of $1.05 pm 0.16$, $3.14 pm 0.36$, and $1.55^{+0.24}_{-0.21}$ $R_oplus$, respectively. With an age of 600-800 Myr, these planets are some of the smallest and youngest transiting planets known. Due to the relatively bright (J=9.1) host star, the planets are compelling targets for future characterization via radial velocity mass measurements and transmission spectroscopy. As the first known star with multiple transiting planets in a cluster, the system should be helpful for testing theories of planet formation and migration.
High-precision planetary densities are key to derive robust atmospheric properties for extrasolar planets. Measuring precise masses is the most challenging part, especially in multi-planetary systems. We measure the masses and densities of a four-planet near resonant chain system (K2-32), and a young ($sim400$ Myr old) planetary system consisting of three close-in small planets (K2-233). We obtained 199 new HARPS observations for K2-32 and 124 for K2-233 covering a more than three year baseline. We find that K2-32 is a compact scaled-down version of the Solar Systems architecture, with a small rocky inner planet (M$_e=2.1^{+1.3}_{-1.1}$~M$_{oplus}$, P$_esim4.35$~days) followed by an inflated Neptune-mass planet (M$_b=15.0^{+1.8}_{-1.7}$~M$_{oplus}$, P$_bsim8.99$~days) and two external sub-Neptunes (M$_c=8.1pm2.4$~M$_{oplus}$, P$_csim20.66$~days; M$_d=6.7pm2.5$~M$_{oplus}$, P$_dsim31.72$~days). K2-32 becomes one of the few multi-planetary systems with four or more planets known with measured masses and radii. Additionally, we constrain the masses of the three planets in K2-233. For the two inner Earth-size planets we constrain their masses to be smaller than M$_b<11.3$ M$_{oplus}$ (P$_bsim2.47$~days), M$_c<12.8$ M$_{oplus}$ (P$_csim7.06$~days). The outer planet is a sub-Neptune size planet with an inferred mass of M$_d=8.3^{+5.2}_{-4.7}$ M$_{oplus}$ (M$_d<21.1$ M$_{oplus}$, P$_dsim24.36$~days). Our observations of these two planetary systems confirm for the first time the rocky nature of two planets orbiting a young star, with relatively short orbital periods ($<7$ days). They provide key information for planet formation and evolution models of telluric planets. Additionally, the Neptune-like derived masses of the three planets K2-32 b, c, d puts them in a relatively unexplored regime of incident flux and planet mass, key for transmission spectroscopy studies.
We report on the discovery of three transiting super-Earths around K2-155 (EPIC 210897587), a relatively bright early M dwarf ($V=12.81$ mag) observed during Campaign 13 of the NASA K2 mission. To characterize the system and validate the planet candidates, we conducted speckle imaging and high-dispersion optical spectroscopy, including radial velocity measurements. Based on the K2 light curve and the spectroscopic characterization of the host star, the planet sizes and orbital periods are $1.55_{-0.17}^{+0.20},R_oplus$ and $6.34365pm 0.00028$ days for the inner planet; $1.95_{-0.22}^{+0.27},R_oplus$ and $13.85402pm 0.00088$ days for the middle planet; and $1.64_{-0.17}^{+0.18},R_oplus$ and $40.6835pm 0.0031$ days for the outer planet. The outer planet (K2-155d) is near the habitable zone, with an insolation $1.67pm 0.38$ times that of the Earth. The planets radius falls within the range between that of smaller rocky planets and larger gas-rich planets. To assess the habitability of this planet, we present a series of 3D global climate simulations assuming that K2-155d is tidally locked and has an Earth-like composition and atmosphere. We find that the planet can maintain a moderate surface temperature if the insolation proves to be smaller than $sim 1.5$ times that of the Earth. Doppler mass measurements, transit spectroscopy, and other follow-up observations should be rewarding, since K2-155 is one of the optically brightest M dwarfs known to harbor transiting planets.
The prime Kepler mission revealed that small planets (<4 R_earth) are common, especially around low-mass M dwarfs. K2, the re-purposed Kepler mission, continues this exploration of small planets around small stars. Here we combine K2 photometry with spectroscopy, adaptive optics imaging, and archival survey images to analyze two small planets orbiting the nearby, field age, M dwarfs K2-26 (EPIC 202083828) and K2-9. K2-26 is an M1.0 +/- 0.5 dwarf at 93 +/- 7 pc from K2 Campaign 0. We validate its 14.5665 d period planet and estimate a radius of 2.67^+0.46_-0.42 R_earth. K2-9 is an M2.5 +/- 0.5 dwarf at 110 +/- 12 pc from K2 Campaign 1. K2-9b was first identified by Montet et al. 2015; here we present spectra and adaptive optics imaging of the host star and independently validate and characterize the planet. Our analyses indicate K2-9b is a 2.25^+0.53_-0.96 R_earth planet with a 18.4498 d period. K2-26b exhibits a transit duration that is too long to be consistent with a circular orbit given the measured stellar radius. Thus, the long transits are likely due to the photoeccentric effect and our transit fits hint at an eccentric orbit. Both planets receive low incident flux from their host stars and have estimated equilibrium temperatures <500 K. K2-9b may receive approximately Earth-like insolation. However, its host star exhibits strong GALEX UV emission which could affect any atmosphere it harbors. K2-26b and K2-9b are representatives of a poorly studied class of small planets with cool temperatures that have radii intermediate to Earth and Neptune. Future study of these systems can provide key insight into trends in bulk composition and atmospheric properties at the transition from silicate dominated to volatile rich bodies.
We report on the discovery of three transiting planets around GJ~9827. The planets have radii of 1.75$_{-0.12}^{+0.11 }$, 1.36$_{- 0.09 }^{+ 0.09}$, and 2.10$_{- 0.15 }^{+ 0.15 }$~R$_{oplus}$, and periods of 1.20896, 3.6480, and 6.2014 days, respectively. The detection was made in Campaign 12 observations as part of our K2 survey of nearby stars. GJ~9827 is a $V = 10.39$~mag K6V star at distance of 30.3 parsecs and the nearest star to be found hosting planets by Kepler and K2. The radial velocity follow-up, high resolution imaging, and detection of multiple transiting objects near commensurability drastically reduce the false positive probability. The orbital periods of GJ~9827~b, c and d planets are very close to the 1:3:5 mean motion resonance. Our preliminary analysis shows that GJ~9827 planets are excellent candidates for atmospheric observations. Besides, the planetary radii span both sides of the rocky and gaseous divide, hence the system will be an asset in expanding our understanding of the threshold.
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