K2 News and Results

Disproval of the validated planets K2-78b, K2-82b, and K2-92b
https://arxiv.org/abs/1707.08007

Quote :

Transiting super-Earths orbiting bright stars in short orbital periods are interesting targets for the study of planetary atmospheres. While selecting super-Earths suitable for further characterization from the ground among a list of confirmed and validated exoplanets detected by K2, we found some suspicious cases that led to us re-assessing the nature of the detected transiting signal. We did a photometric analysis of the K2 light curves and centroid motions of the photometric barycenters. Our study shows that the validated planets K2-78b, K2-82b, and K2-92b are actually not planets but background eclipsing binaries. The eclipsing binaries are inside the Kepler photometric aperture, but outside the ground-based high resolution images used for validation. We advise extreme care on the validation of candidate planets discovered by space missions. It is important that all the assumptions in the validation process are carefully checked. An independent confirmation is mandatory in order to avoid wasting valuable resources on further characterization of non-existent targets.

TRAPPIST-1 has received the K2 designation K2-112.
EPIC 220504338 has received the K2 designation K2-113.

EPIC 211418729b and EPIC 211442297b: Two Transiting Warm Jupiters
https://arxiv.org/abs/1708.07128

Quote :

We report the first results from a search for transiting warm Jupiter exoplanets - gas giant planets receiving stellar irradiation below about 108 erg s−1 cm−2, equivalent to orbital periods beyond about 10 days around Sun-like stars. We have discovered two transiting warm Jupiter exoplanets initially identified as transiting candidates in K2 photometry. EPIC 211418729b has a mass of 1.85+0.23−0.22 MJ, a radius of 0.942+0.032−0.020 RJ, and an orbital period of 11.4 days. EPIC 211442297b has a mass of 0.84+0.18−0.20 MJ, a radius of 1.115+0.057−0.061 RJ, and an orbital period of 20.3 days. Both planets are among the longest period transiting gas giant planets with a measured mass, and they are orbiting relatively old host stars. Both planets are not inflated as their radii are consistent with theoretical expectations. Their position in the planet radius - stellar irradiation diagram is consistent with the scenario where the radius - irradiation correlation levels off below about 108 erg s−1 cm−2, suggesting that for warm Jupiters the stellar irradiation does not play a significant role in determining the planet radius. We also report our identification of another K2 transiting warm Jupiter candidate, EPIC 212504617, as a false positive.

K2-51, K2-67 and K2-76 are low-mass stellar systems, not warm Jupiters.

Three statistically validated K2 transiting warm Jupiter exoplanets confirmed as low-mass stars
https://arxiv.org/abs/1708.08455

K2 detections do not seem that robust after all. I wonder whether also the hundreds of Kepler planets recently announced are going to suffer similar disprovals.

Jupiter-size planets are particularly prone to this, due to the overlap in the radius distribution with small stars. Section 4.1 goes into the reasons for why these objects were misidentified.

Three small transiting planets around the M dwarf host star LP 358-499
https://arxiv.org/abs/1709.01025

Quote :

We report on the detection of three transiting small planets around the low-mass star LP 358-499, using photometric data from the Kepler-K2 mission. The three detected planets have orbital periods of ca. 3, 4.9, and 11 days and transit depths of ca. 700, 1000, and 2000 ppm, respectively. We determine the spectral type of the host star to be M1V from multi-band photometry. Using the transit parameters and the stellar properties, we estimate that the innermost planet may be rocky. All planets are closer to the host star than the inner edge of the habitable zone in that system.

Three Small Super-Earths Transiting the nearby star GJ 9827
https://arxiv.org/abs/1709.01527

Quote :

We report on the discovery of three transiting planets around GJ~9827. The planets have radii of 1.75+0.11−0.12, 1.36+0.09−0.09, and 2.10+0.15−0.15~R⊕, 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.

A better paper, with confidence intervals.

A System of Three Super Earths Transiting the Late K-Dwarf GJ 9827 at Thirty Parsecs
https://arxiv.org/abs/1709.01957

Edit: It seems I confused the first paper for GJ 9827 with the paper for LP 358-499. Both GJ 9827 papers report confidence intervals.

Planets in the Hyades!

Zodiacal Exoplanets in Time (ZEIT) VI: a three-planet system in the Hyades cluster including an Earth-sized planet
https://arxiv.org/abs/1709.10328

Quote :

Planets in young clusters are powerful probes of the evolution of planetary systems. Here we report the discovery of three planets transiting EPIC 247589423, a late K dwarf in the Hyades (~800 Myr) cluster, and robust detection limits for additional planets in the system. The planets were identified from their K2 light curves, as part of our survey of young clusters and star forming regions. The smallest planet has a radius comparable to Earth (0.99+/-0.05 Earth radii), making it one of the few Earth-sized planets with a known, young age. The two larger planets are likely a mini-Neptune and a super-Earth, with radii of 2.91+/-0.11 and 1.45+/-0.10 Earth radii, respectively. The predicted radial velocity signals from these planets are between 0.4 and 2 m/s, achievable with modern precision RV spectrographs. Because the target star is bright (V=11.2) and has relatively low-amplitude stellar variability for a young star (2-6 mmag), EPIC 247589423 hosts the best planets known in a young open cluster for precise radial velocity follow-up, enabling a robust test of earlier claims that young planets are less dense than their older counterparts.

K2-nnnA~b: A Binary System in the Hyades Cluster Hosting a Neptune-Sized Planet
https://arxiv.org/abs/1709.10398

Quote :

We report the discovery of a Neptune-size planet (R_p = 3.0 R_Earth) in the Hyades Cluster. The host star is in a binary system, comprising a K5V star and M7/8V star with a projected separation of 40 AU. The planet orbits the primary star with an orbital period of 17.3 days and a transit duration of 3 hours. The host star is bright (V=11.2, J=9.1) and so may be a good target for precise radial velocity measurements. K2-nnnA~b is the first Neptune-sized planet to be found orbiting in a binary system within an open cluster. The Hyades is the nearest star cluster to the Sun, has an age of 625-750 Myr, and forms one of the fundamental rungs in the distance ladder; understanding the planet population in such a well-studied cluster can help us understand and set constraints on the formation and evolution of planetary systems.

I think this makes EPIC 247589423 the second multiplanet system in an open cluster, after Pr0211. Unless I've missed one?

I think you're right. I can't think of any others. From my notes, there's,...

Hyades
Eps Tau, HD 285507, K2-25

M4
PSR 1620-26

NGC 2423
NGC-2423-3

NGC 2632/M44/Beehive/Praesepe
Pr0201, Pr0211, K-95, K-100, K-101, K-102, K-103, K-104

NGC 2682/M67
NGC 2682 SAND364, NGC 2682 SAND978, NGC 2682 YBP401, NGC 2682 YBP1194, NGC 2682 YBP1514

NGC 4349
NGC 4349-127

NGC 6811
Kepler-56, Kepler-67

Upper Sco AB
K2-33

A new ultra-short-period planet around EPIC 228732031, likely with a water-rich envelope.

Dai et al. "The discovery and mass measurement of a new ultra-short-period planet: EPIC 228732031b"
https://arxiv.org/abs/1710.00076

https://arxiv.org/abs/1710.03239

Planetary Systems around Low-mass Stars Unveiled by K2

We present the detection and follow-up observations of planetary candidates around low-mass stars observed by the {\it K2} mission. Based on light-curve analysis, adaptive-optics imaging, and optical spectroscopy at low and high resolution (including radial velocity measurements), we validate 16 planets around 12 low-mass stars observed during {\it K2} campaigns 5--10. Among the 16 planets, 12 are newly validated, with orbital periods ranging from 0.96--33 days. For one of the planets (EPIC 220621087.01) we present ground-based transit photometry, allowing us to refine the ephemerides. We also identify EPIC 220187552 as a false positive, based on the multiple stars seen in a high-resolution image and double lines in a high-resolution spectrum. Combining our {\it K2} M-dwarf planets together with the validated or confirmed planets found previously, we investigate the dependence of planet radius R p on stellar insolation and metallicity [Fe/H]. We confirm that medium-sized planets (R p =2−5 R ⊕ ) seem to have experienced shrinkage --- plausibly due to photoevaporation --- and we find evidence that the shrinkage occurs at lower insolation for the coolest M dwarfs. Planets larger than ≈3 R ⊕ are only found around the most metal-rich M dwarfs, and for the coolest M dwarfs (≲3500 K) there appears to be a correlation between planet size and metallicity.

Potential 4th planet around star LP 358 499 https://academic.oup.com/mnrasl/article/doi/10.1093/mnrasl/slx171/4554394/Three-small-transiting-planets-around-the-M-dwarf
Interesting will be comparing masses and composition of c and e planets when RV data will be available as they straddle a boundary between rocky and gaseous planets.

tommi59 wrote:

Potential 4th planet around star LP 358 499 https://academic.oup.com/mnrasl/article/doi/10.1093/mnrasl/slx171/4554394/Three-small-transiting-planets-around-the-M-dwarf
Interesting will be comparing masses and composition of c and e planets when RV data will be available as they straddle a boundary between rocky and gaseous planets.

arXiv paper has been updated to match this, also it now has uncertainties on the values: https://arxiv.org/abs/1709.01025v3

I'm fairly confident this is a separate planetary system from the ones we've seen reported so far.
Update: Nope! Thanks Lazarus.

Three Small Planets Transiting a Hyades Star
https://arxiv.org/abs/1710.07203

Quote :

We present the discovery of three small planets transiting LP 358-348, a late K dwarf in the Hyades. The planets have orbital periods of 7.9757±0.0011, 17.30681+0.00034−0.00036, and 25.5715+0.0038−0.0040 days, and radii of 1.05±0.16, 3.14±0.36, and 1.55+0.24−0.21 R⊕, 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) and moderately inactive 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.

It's the same star (LP 358-348 = EPIC 247589423) as the three planet system in this post.

The same star but different stellar radius. First paper temperature 4499 K and radius 0.67 second paper 4359 K and 073 .First values seems to be more correct

Some K2 designations have been assigned.
K2-133 = LP 358-499
K2-134 = WASP-151

Fast one!
EPIC 246393474 b: A 5-M⊕ super-Earth transiting a K7 V star every 6.7 hours
https://arxiv.org/abs/1711.02097

Quote :

We report on the discovery of EPIC 246393474 b, an ultra-short-period super-Earth on a 6.7-hour orbit transiting an active K7 V star based on data from K2 campaign 12. We confirmed the planet's existence and measured its mass with a series of follow-up observations: seeing-limited MuSCAT imaging, NESSI high-resolution speckle observations, and FIES and HARPS high-precision radial-velocity monitoring. EPIC 246393474 b has a mass of 5.31±0.46 M⊕ and radius of 1.54+0.10−0.09 R⊕, yielding a mean density of 8.00+1.83−1.45 gcm−3 and suggesting a rocky-iron composition. Models indicate that iron cannot exceed ∼70 % of the total mass. With an orbital period of only 6.7 hours, EPIC 246393474 b is the shortest-period planet known to date with a precisely determined mass.

Planet Candidates from K2 Campaigns 5-8 and Follow-Up Optical Spectroscopy
https://arxiv.org/abs/1711.06377

Quote :

We present 151 planet candidates orbiting 141 stars from K2 campaigns 5-8 (C5-C8), identified through a systematic search of K2 photometry. In addition, we identify 16 targets as likely eclipsing binaries, based on their light curve morphology. We obtained follow-up optical spectra of 105/141 candidate host stars and 8/16 eclipsing binaries to improve stellar properties and to identify spectroscopic binaries. Importantly, spectroscopy enables measurements of host star radii with ≈10% precision, compared to ≈40% precision when only broadband photometry is available. The improved stellar radii enable improved planet radii. Our curated catalog of planet candidates provides a starting point for future efforts to confirm and characterize K2 discoveries.

http://www.ifa.hawaii.edu/info/press-releases/PuffyPlanets/

Newly Discovered Twin Planets Could Solve Puffy Planet Mystery

"Seeing double with K2: Testing re-inflation with two remarkably similar planets orbiting red giant stars" has been published in November 27th edition of The Astronomical Journal as and is available online at https://arxiv.org/abs/1706.05865

K2 limits on planet occurrence around white dwarf stars.

van Sluijs & Van Eylen "The occurrence of planets and other substellar bodies around white dwarfs using K2"
https://arxiv.org/abs/1711.09691

Sirius_Alpha wrote:

Characterization of the K2-18 multi-planetary system with HARPS: A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit
https://arxiv.org/abs/1707.04292

http://nouvelles.umontreal.ca/en/article/2017/12/05/two-super-earths-around-star-k2-18/

Two Super-Earths around star K2-18