K2 News and Results

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Re: K2 News and Results

Post by Led_Zep on 24th October 2016, 9:44 pm

https://arxiv.org/abs/1610.07249

Spitzer Observations Confirm and Rescue the Habitable-Zone Super-Earth K2-18b for Future Characterization

The recent detections of two transit events attributed to the super-Earth candidate K2-18b have provided the unprecedented prospect of spectroscopically studying a habitable-zone planet outside the Solar System. Orbiting a nearby M2.5 dwarf and receiving virtually the same stellar insolation as Earth, K2-18b would be a prime candidate for the first detailed atmospheric characterization of a habitable-zone exoplanet using HST and JWST. Here, we report the detection of a third transit of K2-18b near the predicted transit time using the Spitzer Space Telescope. The Spitzer detection demonstrates the periodic nature of the two transit events discovered by K2, confirming that K2-18 is indeed orbited by a super-Earth in a 33-day orbit and ruling out the alternative scenario of two similarly-sized, long-period planets transiting only once within the 75-day K2 observation. We also find, however, that the transit event detected by Spitzer occurred 1.85 hours (7-sigma) before the predicted transit time. Our joint analysis of the Spitzer and K2 photometry reveals that this early occurrence of the transit is not caused by transit timing variations (TTVs), but the result of an inaccurate K2 ephemeris due to a previously undetected data anomaly in the K2 photometry likely caused by a cosmic ray hit. We refit the ephemeris and find that K2-18b would have been lost for future atmospheric characterizations with HST and JWST if we had not secured its ephemeris shortly after the discovery. We caution that immediate follow-up observations as presented here will also be critical in confirming and securing future planets discovered by TESS, in particular if only two transit events are covered by the relatively short 27-day TESS campaigns
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Re: K2 News and Results

Post by Edasich on 28th October 2016, 4:38 am

Transiting brown dwarf (or massive planet) in Ruprecht 147 open cluster:

EPIC 219388192 b - an inhabitant of the brown dwarf desert in the Ruprecht 147 open cluster

We report the discovery of EPIC 219388192 b, a transiting brown dwarf in a 5.3-day orbit around a member star of Ruprecht-147, the oldest nearby open cluster association, which was photometrically monitored by K2 during its Campaign 7. We combine the K2 time-series data with ground-based adaptive optics imaging and high resolution spectroscopy to rule out false positive scenarios and determine the main parameters of the system. EPIC 219388192 b has a radius of Rb=0.937±0.042~RJup and mass of Mb=36.50±0.09~MJup, yielding a mean density of 59.0±8.1~gcm−3. The host star is nearly a Solar twin with mass M⋆=0.99±0.05~M⊙, radius R⋆=1.01±0.04~R⊙, effective temperature Teff=5850±85~K and iron abundance [Fe/H]=0.03±0.08~dex. Its age, spectroscopic distance, and reddening are consistent with those of Ruprecht-147, corroborating its cluster membership. EPIC 219388192 b is the first brown dwarf with precise determinations of mass, radius and age, and serves as benchmark for evolutionary models in the sub-stellar regime.
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Re: K2 News and Results

Post by Sirius_Alpha on 14th November 2016, 12:29 am

K2-60b and epic 216468514b. A sub-jovian and a jovian planet from the k2 mission
https://arxiv.org/abs/1611.03704

We report the characterization and independant detection of K2-60b, as well as the detection and characterization of EPIC 216468514b, two transiting hot gaseous planets from the K2 space mission. We confirm the planetary nature of the two systems and determine their fundamental parameters combining the K2 time-series data with FIES@NOT and HARPS-N@TNG spectroscopic observations. K2-60b has a radius of 0.683 +/- 0.037 RJup and a mass of 0.426 +/- 0.037 MJup and orbits a G4V star with an orbital period of 3.00267 +/- 0.00006 days. EPIC 216468514b has a radius of 1.44 +/- 0.15RJup and a mass of 0.84 +/- 0.08 MJup and orbits an F9 IV star every 3.31392 +/- 0.00002 days. K2-60b is among the few planets at the edge of the so-called "desert" of short-period sub Jovian planets. EPIC 216468514b is a highly inflated Jovian planet orbiting an evolved star about to leave the main sequence.

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Re: K2 News and Results

Post by Sirius_Alpha on 15th November 2016, 6:14 pm

Mission Manager update.
A third module has failed. Kepler-K2 is in campaign 11. Among the targets are Enceladus and Titan.

http://www.nasa.gov/feature/ames/kepler/kepler-mission-manager-update-k2-marching-on-with-campaign-11

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Re: K2 News and Results

Post by Sirius_Alpha on 24th November 2016, 12:09 am

EPIC 220504338b: A dense hot-Jupiter transiting a solar analogue
https://arxiv.org/abs/1611.07614

We present the discovery of EPIC 220504338b, a dense hot-Jupiter discovered using photometry from Campaign 8 of the Kepler-2 (K2) mission and high-resolution spectroscopic follow up obtained with the FEROS spectrograph. The planet orbits a V=13.68 solar analogue in a P=5.81771+0.00004−0.00004 day orbit, has a radius of 0.91+0.10−0.07RJ and a mass of 1.28+0.11−0.12MJ. With a density of 2.08+0.66−0.57 gr/cm3, the planet is among the densest systems known having masses below 2 MJ and Teq>1000, and is just above the temperature limit at which inflation mechanisms are believed to start being important. Based on its mass and radius, we estimate that EPIC 220504338b should have a heavy element content on the order of ∼ 110 M⊕ or greater.

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Re: K2 News and Results

Post by zorro83 on 4th December 2016, 10:25 am

https://exofop.ipac.caltech.edu/k2/microlensing/event_list.php

When can we expect the first confirmed exoplanets?

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Re: K2 News and Results

Post by Led_Zep on 4th December 2016, 12:07 pm

Soon !

http://nexsci.caltech.edu/conferences/2017/microlensing/

21st International Microlensing Conference
February 1-3, 2017

« …We will highlight breaking results from K2's Campaign 9 (K2C9)… »
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Re: K2 News and Results

Post by Lazarus on 4th January 2017, 4:31 am

New papers on K2 low-mass stars:

Dressing et al. "Characterizing K2 Candidate Planetary Systems Orbiting Low-Mass Stars I: Classifying Low-mass Host Stars Observed During Campaigns 1-7"
https://arxiv.org/abs/1701.00586

Martinez et al. "Stellar & Planetary Parameters for K2's Late Type Dwarf Systems from C1 to C5"
https://arxiv.org/abs/1701.00588

The second one includes the systems K2-3 (EPIC 201367065) and K2-72 (EPIC 206209135) that were discussed upthread, stellar parameters are significantly revised, the stars are over twice as luminous as previously estimated, and ~50% more massive.
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Re: K2 News and Results

Post by Sirius_Alpha on 5th January 2017, 9:52 pm

The K2-ESPRINT Project VI: K2-105 b, a Hot-Neptune around a Metal-rich G-dwarf
https://arxiv.org/abs/1701.01294

We report on the confirmation that the candidate transits observed for the star EPIC 211525389 are due to a short-period Neptune-sized planet. The host star, located in K2 campaign field 5, is a metal-rich ([Fe/H] = 0.26±0.05) G-dwarf (T_eff = 5430±70 K and log g = 4.48±0.09), based on observations with the High Dispersion Spectrograph (HDS) on the Subaru 8.2m telescope. High-spatial resolution AO imaging with HiCIAO on the Subaru telescope excludes faint companions near the host star, and the false positive probability of this target is found to be <10−6 using the open source vespa code. A joint analysis of transit light curves from K2 and additional ground-based multi-color transit photometry with MuSCAT on the Okayama 1.88m telescope gives the orbital period of P = 8.266902±0.000070 days and consistent transit depths of Rp/R⋆∼0.035 or (Rp/R⋆)2∼0.0012. The transit depth corresponds to a planetary radius of Rp=3.59+0.44−0.39R⊕, indicating that EPIC 211525389 b is a short-period Neptune-sized planet. Radial velocities of the host star, obtained with the Subaru HDS, lead to a 3\sigma\ upper limit of 90 M⊕(0.00027M⊙) on the mass of EPIC 211525389 b, confirming its planetary nature. We expect this planet, newly named K2-105 b, to be the subject of future studies to characterize its mass, atmosphere, spin-orbit (mis)alignment, as well as investigate the possibility of additional planets in the system.

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Re: K2 News and Results

Post by Stalker on 2nd February 2017, 2:11 pm

EPIC201702477b: A Long Period Transiting Brown Dwarf from K2
We report the discovery of EPIC201702477b, a transiting brown dwarf in a long period (40.73691 +/- 0.00037 day) and eccentric (e=0.2281 +/- 0.0026) orbit. This system was initially reported as a planetary candidate based on two transit events seen in K2 Campaign 1 photometry and later validated as an exoplanet. We confirm the transit and refine the ephemeris with two subsequent ground-based detections of the transit using the LCOGT 1m telescope network. We rule out any transit timing variations above the level of 30s. Using high precision radial velocity measurements from HARPS and SOPHIE we identify the transiting companion as a brown dwarf with a mass, radius, and bulk density of 66.9 +/- 1.7 MJ, 0.757 +/- 0.065 RJ, and 191+/-51 g.cm−3 respectively. EPIC201702477b is the smallest radius brown dwarf yet discovered, with a mass just below the H-burning limit. It has the highest density of any planet, substellar mass object or main-sequence star discovered so far. We find evidence in the set of known transiting brown dwarfs for two populations of objects - high mass brown dwarfs and low mass brown dwarfs. The higher-mass population have radii in very close agreement to theoretical models, and show a lower-mass limit around 60 MJ. This may be the signature of mass-dependent ejection of systems during the formation process.

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Re: K2 News and Results

Post by Led_Zep on 2nd February 2017, 9:24 pm

https://arxiv.org/abs/1702.00013

Four Sub-Saturns with Dissimilar Densities : Windows into Planetary Cores and Envelopes

We present results from a Keck/HIRES radial velocity campaign to study four sub-Saturn-sized planets, K2-27b, K2-32b, K2-39b, and K2-108b, with the goal of understanding their masses, orbits, and heavy element enrichment. The planets have similar sizes (R P =4.5−5.5 R E ) , but have dissimilar masses (M P =16−60 M E ) , implying a diversity in their core and envelope masses. K2-32b is the least massive (M P =16.5±2.7 M E ) and orbits in close proximity to two sub-Neptunes near a 3:2:1 period commensurability. K2-27b and K2-39b are significantly more massive at M P =30.9±4.6 M E and M P =39.8±4.4 M E , respectively, and show no signs of additional planets. K2-108b is the most massive at M P =59.4±4.4 M E , implying a large reservoir of heavy elements of about ≈50 M E . Sub-Saturns as a population have a large diversity in planet mass at a given size. They exhibit remarkably little correlation between mass and size; sub-Saturns range from ≈6−60 M E , regardless of size. We find a strong correlation between planet mass and host star metallicity, suggesting that metal-rich disks form more massive planet cores. The most massive sub-Saturns tend to lack detected companions and have moderately eccentric orbits, perhaps as a result of a previous epoch of dynamical instability. Finally, we observe only a weak correlation between the planet envelope fraction and present-day equilibrium temperature, suggesting that photo-evaporation does not play a dominant role in determining the amount of gas sub-Saturns accrete from their protoplanetary disks.
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Re: K2 News and Results

Post by Sirius_Alpha on 2nd February 2017, 9:44 pm

EPIC 218916923 b: a low-mass warm Jupiter on a 29-day orbit transiting an active K0 V star
https://arxiv.org/abs/1702.00691

We announce the discovery of EPIC218916923 b, a transiting warm-Jupiter (Teq=555±11 K) on a 29-day orbit around an active (logR′HK = −4.49 ± 0.03) K0 V star in K2 Campaign 7. We derive the system's parameters by combining the K2 photometry with ground-based follow-up observations. With a mass of Mp=0.381±0.045MJup and radius of Rp=0.812±0.030RJup, EPIC218916923 b is one of the transiting warm Jupiters with the lowest mass known to date. The planetary mean density ρp=0.88±0.14gcm−3 can be explained with a core of ∼50M⊕. Given the brightness of the host star (V= 11.653 mag), the relatively short transit duration (∼5 hours), and the expected amplitude of the Rossiter-McLaughlin effect (∼25 ms), EPIC218916923 is an ideal target to measure the spin-orbit angle of a planetary system hosting a warm Jupiter.

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Re: K2 News and Results

Post by Sirius_Alpha on 22nd March 2017, 8:23 pm

Ten new validated planets.

Characterizing K2 Candidate Planetary Systems Orbiting Low-Mass Stars II: Planetary Systems Observed During Campaigns 1-7
https://arxiv.org/abs/1703.07416

We recently used near-infrared spectroscopy to improve the characterization of 76 low-mass stars around which K2 had detected 79 candidate transiting planets. Thirty of these worlds were new discoveries that have not previously been published. We calculate the false positive probabilities that the transit-like signals are actually caused by non-planetary astrophysical phenomena and reject five new transit-like events and three previously reported events as false positives. We also statistically validate 18 planets (eight of which were previously unpublished), confirm the earlier validation of 21 planets, and announce 17 newly discovered planet candidates. Revising the properties of the associated planet candidates based on the updated host star characteristics and refitting the transit photometry, we find that our sample contains 20 planets or planet candidates with radii smaller than 1.25 Earth radii, 20 super-Earths (1.25-2 Earth radii), 20 small Neptunes (2-4 Earth radii), three large Neptunes (4-6 Earth radii), and eight giant planets (> 6 Earth radii). Most of these planets are highly irradiated, but EPIC 206209135.04 (K2-72e, Rp = 1.29 (-0.13/+0.14) Earth radii), EPIC 211988320.01 (Rp = 2.86 (-0.15/+0.16) Earth radii), and EPIC 212690867.01 (Rp = 2.20 (-0.18/+0.19) Earth radii) orbit within optimistic habitable zone boundaries set by the "recent Venus" inner limit and the "early Mars" outer limit. In total, our planet sample includes eight moderately-irradiated 1.5-3 Earth radius planet candidates (Fp < 20 F_Earth) orbiting brighter stars (Ks < 11) that are well-suited for atmospheric investigations with Hubble, Spitzer, and/or the James Webb Space Telescope. Five validated planets orbit relatively bright stars (Kp < 12.5) and are expected to yield radial velocity semi-amplitudes of at least 2 m/s.

They are unable to confirm K2-14b (EPIC 201635569.01), finding a much larger radius for the star, bringing the FPP probability up to 4-7%, too high to claim validation. K2-45b (EPIC 201345483.01) and K2-83c (EPIC 210508766.02) also suffer from not being able to be validated in this study's program, but they note that it doesn't take into effect planetary multiplicity, which would boost the probability of any particular planet being real, so they argue in favor of a validated status for K2-83c.

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Re: K2 News and Results

Post by Sirius_Alpha on 31st March 2017, 9:14 pm

A Low-Mass Exoplanet Candidate Detected By K2 Transiting the Praesepe M Dwarf JS 183
https://arxiv.org/abs/1703.10250

(Also known as K2-95)

(I inadvertently posted this originally in the thread for the prime Kepler mission)


Last edited by Sirius_Alpha on 31st March 2017, 9:15 pm; edited 1 time in total

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Re: K2 News and Results

Post by Sirius_Alpha on 31st March 2017, 9:14 pm

Latest Exoplanet Results from NASA's Kepler/K2 Mission - Ian Crossfield (SETI Talks 2017)
https://www.youtube.com/watch?v=qPL9cFg8e38

RV monitoring of HD 3167 reveals a third, non-transiting planet at ~8.5 days.
The b planet (0.959 d) has a K = 3.58±0.26 m/s.
The c planet (29.85 d) has a K = 2.24±0.28 m/s.
The d planet (8.49 d) has a K = 2.39±0.24 m/s.

The b planet is consistent with a rocky composition. The c planet, unsurprising is more of a Neptune-like composition.

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Re: K2 News and Results

Post by Led_Zep on 6th April 2017, 2:33 pm

http://www.abc.net.au/news/2017-04-06/stargazing-live-four-planets-discovered-in-new-solar-system/8423142

Stargazing Live viewers find four-planet solar system via crowd-sourcing project
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Re: K2 News and Results

Post by Sirius_Alpha on 27th April 2017, 8:17 pm

EPIC 210894022b - A short period super-Earth transiting a metal poor, evolved old star
https://arxiv.org/abs/1704.08284

The star EPIC 210894022 has been identified from a light curve acquired through the K2 space mission as possibly orbited by a transiting planet. Our aim is to confirm the planetary nature of the object and derive its fundamental parameters. We combine the K2 photometry with reconnaissance spectroscopy and radial velocity (RV) measurements obtained using three separate telescope and spectrograph combinations. The spectroscopic synthesis package SME has been used to derive the stellar photospheric parameters that were used as input to various stellar evolutionary tracks in order to derive the parameters of the system. The planetary transit was also validated to occur on the assumed host star through adaptive imaging and statistical analysis. The star is found to be located in the background of the Hyades cluster at a distance at least 4 times further away from Earth than the cluster itself. The spectrum and the space velocities of EPIC 210894022 strongly suggest it to be a member of the thick disk population. We find that the star is a metal poor ([Fe/H]=-0.53+/-0.05 dex) and alpha-rich somewhat evolved solar-like object of spectral type G3 with Teff=5730+/-50 K, logg=4.15+/-0.1 (cgs), radius of 1.3+/-0.1 R_Sun, and mass of 0.88+/-0.02 M_Sun. The RV detection together with the imaging confirms with a high level of significance that the transit signature is caused by a super-Earth orbiting the star EPIC 210894022. We measure a mass of 8.6+/-3.9 M_Earth and a radius of 1.9+/-0.2 R_Earth. A second more massive object with a period longer than about 120 days is indicated by a long term linear acceleration. With an age of > 10 Gyrs this system is one of the oldest where planets is hitherto detected. Further studies of this planetary system is important since it contains information about the planetary formation process during a very early epoch of the history of our Galaxy.

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Re: K2 News and Results

Post by Sirius_Alpha on 10th May 2017, 6:03 pm

Sirius_Alpha wrote:EPIC 210894022b - A short period super-Earth transiting a metal poor, evolved old star
https://arxiv.org/abs/1704.08284
Now designated K2-111.

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Re: K2 News and Results

Post by Sirius_Alpha on 10th May 2017, 8:31 pm

K2-66b and K2-106b: Two extremely hot sub-Neptune-size planets with high densities
https://arxiv.org/abs/1705.03491

We report precise mass and density measurements of two extremely hot sub-Neptune-size planets from the K2 mission using radial velocities, K2 photometry, and adaptive optics imaging. K2-66 harbors a close-in sub-Neptune-sized (2.49+0.34−0.24R⊕) planet (K2-66b) with a mass of 21.3 ± 3.6 M⊕. Because the star is evolving up the sub-giant branch, K2-66b receives a high level of irradiation, roughly twice the main sequence value. K2-66b may reside within the so-called "photoevaporation desert", a domain of planet size and incident flux that is almost completely devoid of planets. Its mass and radius imply that K2-66b has, at most, a meager envelope fraction (< 5%) and perhaps no envelope at all, making it one of the largest planets without a significant envelope. K2-106 hosts an ultra-short-period planet (P = 13.7 hrs) that is one of the hottest sub-Neptune-size planets discovered to date. Its radius (1.82+0.20−0.14R⊕) and mass (9.0 ± 1.6 M⊕) are consistent with a rocky composition, as are all other small ultra-short-period planets with well-measured masses. K2-106 also hosts a larger, longer-period planet (Rp = 2.77+0.37−0.23R⊕, P = 13.3 days) with a mass less than 24.4 M⊕ at 99.7% confidence. K2-66b and K2-106b probe planetary physics in extreme radiation environments. Their high densities reflect the challenge of retaining a substantial gas envelope in such extreme environments.

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Re: K2 News and Results

Post by Sirius_Alpha on 11th May 2017, 9:22 pm

K2-106, a system containing a metal rich planet and a planet of lower density
https://arxiv.org/abs/1705.04163

Aims: Planets in the mass-range from 2 to 15 MEarth are very diverse. Some of them have very low, others very high densities. Method: We determined the masses, radii and the densities for the two transiting planets orbiting K2-106, which have been previously found. The inner one is an ultra-short period planet with an orbital period of 0.57 days. The outer planet has orbital period of 13.3 days. Results: The two planets have similar masses, though very different densities. For K2-106b we derive M_p=7.69+/-0.82 MEarth, Rp=1.52+/-0.16 REarth, and a high density of 12.0 -3.2+4.8 gcm-3. For K2-106c, we find 6.79+/-2.29 MEarth, R_p=2.59+/-0.27 REarth and a relatively low density of 2.4-1.1+1.6 gcm-3. Conclusions: Since the systems contains two planets of almost the same mass, but different distances from the host star, it is an excellent laboratory to study atmospheric escape. Comparing the mass and radius of the inner planet with composition models implies that it has an iron core containing at least 50% of its mass. Such a high metal content is surprising, particularly given that the star has solar abundance. We discuss various formation scenarios for this unusual planet.

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Re: K2 News and Results

Post by Sirius_Alpha on 18th May 2017, 3:36 pm

An update on the NASA Exoplanet Archive includes some name updates for K2 planets.

K2-97 = EPIC 211351816 (link)
K2-98 = EPIC 211391664 (link)
K2-100 = EPIC 211990866 (link)
K2-101 = EPIC 211913977 (link)
K2-102 = EPIC 211970147 (link)
K2-104 = EPIC 211969807 (link)

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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=1.019±0.070 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 1.095±0.018RJup the planet has a bulk density of 0.726±0.062ρJup. The host star has a [Fe/H] of 0.12±0.045, and from the K2 light curve we find a rotation period for the star of 16.3±0.1 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 8.0±1.9 M⊕ with a bulk density of 3.7±0.9 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

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