K2 News and Results

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Re: K2 News and Results

Post by Daniel on 20th June 2017, 6:12 pm

Seeing double with K2: Testing re-inflation with two remarkably similar planets around red giant branch stars

https://arxiv.org/abs/1706.05865

Determining the mechanism that causes anomalously large radii of strongly irradiated exoplanets has remained a puzzle since before the radius of an exoplanet was first measured. Here, we report the discovery of a new inflated gas giant planet found with the NASA K2 Mission, EPIC2287.01, and a revised mass for the previously discovered inflated gas giant K2-97b. These planets orbit at moderate (~9 day) orbital distances around host stars which recently evolved into red giants. We constrain the irradiation history of these systems using a model constrained by parameters determined by asteroseismology and Keck/HIRES spectroscopy and radial velocity measurements. We find that both planets resided near the planet inflation irradiation threshold during their main sequence lifetimes. We also find that the current irradiation of these planets is typical for the population of planets with similar radii, but the main sequence irradiation of these planets would have been atypically small for the population of planets inflated to their size. Our precise constraints of the masses and radii of the stars and planets in this system allow us to constrain the planetary heating efficiencies of both systems to 0.03% +0.03%/-0.02%. These results are consistent with a planet re-inflation scenario, but suggest the efficiency of planet re-inflation is significantly lower than previously theorized. Finally, we discuss the similarity of both planetary systems (agreement within 10% of stellar masses and radii, and planet masses, radii, and orbital periods) and speculate that this may be due to selection bias in searching for planets around evolved stars.

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Re: K2 News and Results

Post by Sirius_Alpha on 21st June 2017, 11:53 pm

EPIC 228735255b - An eccentric 6.57 day transiting hot Jupiter in Virgo
https://arxiv.org/abs/1706.06865

We present the discovery of EPIC 228735255b, a P= 6.57 days Jupiter-mass (MP=1.0190.070 MJup) planet transiting a V=12.5 (G5-spectral type) star in an eccentric orbit (e=0.120+0.056−0.046) detected using a combination of K2 photometry and ground-based observations. With a radius of 1.0950.018RJup the planet has a bulk density of 0.7260.062ρJup. The host star has a [Fe/H] of 0.120.045, and from the K2 light curve we find a rotation period for the star of 16.30.1 days. This discovery is the 9th hot Jupiter from K2 and highlights K2's ability to detect transiting giant planets at periods slightly longer than traditional, ground-based surveys. This planet is slightly inflated, but much less than others with similar incident fluxes. These are of interest for investigating the inflation mechanism of hot Jupiters.

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Re: K2 News and Results

Post by Sirius_Alpha on 4th July 2017, 9:56 pm

Another paper on EPIC 228754001

A hot Saturn on an eccentric orbit around the giant star EPIC228754001
https://arxiv.org/abs/1707.00779

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Re: K2 News and Results

Post by Sirius_Alpha on 16th July 2017, 8:58 pm

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

The bright M dwarf K2-18 at 34 pc is known to host a transiting super-Earth-sized planet orbiting within the star's habitable zone; K2-18b. Given the superlative nature of this system for studying an exoplanetary atmosphere receiving similar levels of insolation as the Earth, we aim to characterize the planet's mass which is required to interpret atmospheric properties and infer the planet's bulk composition. We obtain precision radial velocity measurements with the HARPS spectrograph and couple those measurements with the K2 photometry to jointly model the observed radial velocity variation with planetary signals and a radial velocity jitter model based on Gaussian process regression. We measure the mass of K2-18b to be 8.01.9 M⊕ with a bulk density of 3.70.9 g/cm3 which may correspond to a predominantly rocky planet with a significant gaseous envelope or an ocean planet with a water mass fraction ≳50%. We also find strong evidence for a second, warm super-Earth K2-18c at ∼9 days with a semi-major axis 2.4 times smaller than the transiting K2-18b. After re-analyzing the available light curves of K2-18 we conclude that K2-18c is not detected in transit and therefore likely has an orbit that is non-coplanar with K2-18b. A suite of dynamical integrations with varying simulated orbital eccentricities of the two planets are used to further constrain each planet's eccentricity posterior from which we measure eb<0.43 and ec<0.47 at 99% confidence. The discovery of the inner planet K2-18c further emphasizes the prevalence of multi-planet systems around M dwarfs. The characterization of the density of K2-18b reveals that the planet likely has a thick gaseous envelope which along with its proximity to the Solar system makes the K2-18 planetary system an interesting target for the atmospheric study of an exoplanet receiving Earth-like insolation.

EPIC 228813918 b: an Earth-sized planet in a 4.3-hour orbit around an M-dwarf
https://arxiv.org/abs/1707.04549

We report the discovery from K2 of a transiting terrestrial planet in an ultra-short-period orbit around an M3-dwarf. EPIC 228813918 b completes an orbit in only 4.3 hours, the second-shortest orbital period of any known planet, just 4 minutes longer than that of KOI 1843.03, which also orbits an M-dwarf. Using a combination of archival images, AO imaging, RV measurements, and light curve modelling, we show that no plausible eclipsing binary scenario can explain the K2 light curve, and thus confirm the planetary nature of the system. The planet, whose radius we determine to be 0.89 +/- 0.09 Earth radii, and which must have a iron mass fraction greater than 0.45, orbits a star of mass 0.463 +/- 0.052 Msol and radius 0.442 +/- 0.044 Rsol.

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Re: K2 News and Results

Post by Sirius_Alpha on 25th July 2017, 8:35 pm

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

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.

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Re: K2 News and Results

Post by Sirius_Alpha on 31st July 2017, 9:08 pm

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

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Re: K2 News and Results

Post by Sirius_Alpha on 24th August 2017, 8:47 pm

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

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.

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Re: K2 News and Results

Post by Sirius_Alpha on 29th August 2017, 9:06 pm

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

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Re: K2 News and Results

Post by Edasich on 30th August 2017, 4:07 am

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.
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Re: K2 News and Results

Post by Lazarus on 30th August 2017, 5:07 am

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.
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Re: K2 News and Results

Post by Sirius_Alpha on 4th September 2017, 8:33 pm

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

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.

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Re: K2 News and Results

Post by Sirius_Alpha on 6th September 2017, 9:39 pm

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

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.

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Re: K2 News and Results

Post by Sirius_Alpha on 7th September 2017, 8:33 pm

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.

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