Planetary Systems around Low-mass Stars Unveiled by K2

Planetary Systems around Low-mass Stars Unveiled by K2 

Authors: 

Hirano et al 

Abstract: 

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 Rp on stellar insolation and metallicity [Fe/H]. We confirm that medium-sized planets (Rp=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.

EPIC 220504338b: A dense hot-Jupiter transiting a solar analogue

EPIC 220504338b: A dense hot-Jupiter transiting a solar analogue   

Authors:

Espinoza et al

Abstract:

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=5.81760+0.00003−0.00003/+0.00003 day orbit, has a radius of 0.93+0.10−0.07RJ/0.93−0.07+0.10RJ and a mass of 1.29+0.13−0.14MJ/1.29−0.14+0.13MJ. With a density of 1.97+0.60−0.53/1.97−0.53+0.60 gr/cm3, the planet is among the densest systems known having masses below 2 MJ and Teq greater than 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.

Two white dwarfs in ultrashort binaries with detached, eclipsing, likely substellar companions detected by K2

Two white dwarfs in ultrashort binaries with detached, eclipsing, likely substellar companions detected by K2

Authors:

Parsons et al

Abstract:
Using data from the extended Kepler mission in K2 Campaign 10 we identify two eclipsing binaries containing white dwarfs with cool companions that have extremely short orbital periods of only 71.2 min (SDSS J1205−0242, a.k.a. EPIC 201283111) and 72.5 min (SDSS J1231+0041, a.k.a. EPIC 248368963). Despite their short periods, both systems are detached with small, low-mass companions, in one case a brown dwarf, and the other case either a brown dwarf or a low-mass star. We present follow-up photometry and spectroscopy of both binaries, as well as phase-resolved spectroscopy of the brighter system, and use these data to place preliminary estimates on the physical and binary parameters. SDSS J1205−0242 is composed of a 0.39 ± 0.02M⊙ helium-core white dwarf which is totally eclipsed by a 0.049 ± 0.006M⊙ (51 ± 6MJ) brown dwarf companion, while SDSS J1231+0041 is composed of a 0.56 ± 0.07M⊙ white dwarf which is partially eclipsed by a companion of mass ≲ 0.095M⊙. In the case of SDSS J1205−0242 we look at the combined constraints from common-envelope evolution and brown dwarf models; the system is compatible with similar constraints from other post common-envelope binaries given the current parameter uncertainties, but has potential for future refinement.

EPIC 228735255b: an eccentric 6.57 day transiting hot Jupiter in Virgo

EPIC 228735255b - An eccentric 6.57 day transiting hot Jupiter in Virgo

Authors:

Giles et al

Abstract:

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.

K2-106 System: A Exoplanetary System With Two Very Different Worlds of the Same Mass

K2-106, a system containing a metal rich planet and a planet of lower density

Authors:

Guenther et al

Abstract:
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.

K2-66b and K2-106b: Two Extremely Hot Sub-Neptune-size Planets with High Densities

K2-66b and K2-106b: Two Extremely Hot Sub-Neptune-size Planets with High Densities

Authors:

Sinukoff et al

Abstract:

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 (${2.49}_{-0.24}^{+0.34}$ ${R}_{\oplus }$) planet (K2-66b) with a mass of $21.3\pm 3.6$ ${M}_{\oplus }$. Because the star is evolving up the subgiant 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 (less than 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 hr) that is one of the hottest sub-Neptune-size planets discovered to date. Its radius (${1.82}_{-0.14}^{+0.20}$ ${R}_{\oplus }$) and mass ($9.0\pm 1.6$ ${M}_{\oplus }$) 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 (${R}_{{\rm{p}}}$ = ${2.77}_{-0.23}^{+0.37}$ ${R}_{\oplus }$, P = 13.3 days) with a mass less than $24.4$ ${M}_{\oplus }$ 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.

Transit Shapes and Self Organising Maps as a Tool for Ranking Planetary Candidates: Application to Kepler and K2

Transit Shapes and Self Organising Maps as a Tool for Ranking Planetary Candidates: Application to Kepler and K2

Authors:

Armstrong et al

Abstract:

A crucial step in planet hunting surveys is to select the best candidates for follow up observations, given limited telescope resources. This is often performed by human ‘eyeballing’, a time consuming and statistically awkward process. Here we present a new, fast machine learning technique to separate true planet signals from astrophysical false positives. We use Self Organising Maps (SOMs) to study the transit shapes of Kepler and K2 known and candidate planets. We find that SOMs are capable of distinguishing known planets from known false positives with a success rate of 87.0%, using the transit shape alone. Furthermore, they do not require any candidates to be dispositioned prior to use, meaning that they can be used early in a mission's lifetime. A method for classifying candidates using a SOM is developed, and applied to previously unclassified members of the Kepler KOI list as well as candidates from the K2 mission. The method is extremely fast, taking minutes to run the entire KOI list on a typical laptop. We make Python code for performing classifications publicly available, using either new SOMs or those created in this work. The SOM technique represents a novel method for ranking planetary candidate lists, and can be used both alone or as part of a larger autovetting code.

K2-56b/BD+20594b is a High Density Subneptune/MegaEarth

DISCOVERY AND VALIDATION OF A HIGH-DENSITY SUB-NEPTUNE FROM THE K2 MISSION

Authors:

Espinoza et al

Abstract:

We report the discovery of K2-56b, a high-density sub-Neptune exoplanet, made using photometry from Campaign 4 of the two-wheeled Kepler (K2) mission, ground-based radial velocity (RV) follow-up from HARPS and high-resolution lucky and adaptive optics imaging obtained using AstraLux and MagAO, respectively. The host star is a bright (V = 11.04, K s = 9.37), slightly metal-poor ([Fe/H] = −0.15 ± 0.05 dex) solar analogue located at ${152.1}_{-7.4}^{+9.7}$ pc from Earth, for which we find a radius of ${R}_{* }={0.928}_{-0.040}^{+0.055}{R}_{\odot }$ and a mass of ${M}_{* }={0.961}_{-0.029}^{+0.032}{M}_{\odot }$. A joint analysis of the K2 photometry and HARPS RVs reveal that the planet is in a ≈42 day orbit around its host star, has a radius of ${2.23}_{-0.11}^{+0.14}{R}_{\oplus }$, and a mass of ${16.3}_{-6.1}^{+6.0}{M}_{\oplus }$. Although the data at hand put the planet in the region of the mass–radius diagram where we could expect planets with a pure rock (i.e., magnesium silicate) composition using two-layer models (i.e., between rock/iron and rock/ice compositions), we discuss more realistic three-layer composition models which can explain the high density of the discovered exoplanet. The fact that the planet lies in the boundary between "possibly rocky" and "non-rocky" exoplanets makes it an interesting planet for future RV follow-up.

previous preprint.

EPIC 211391664b: A 32-M⊕ Neptune-sized planet in a 10-day orbit around an F8 star

EPIC 211391664b: A 32-M⊕ Neptune-sized planet in a 10-day orbit transiting an F8 star

Authors:

Barragán et al

Abstract:

We report the discovery of EPIC 211391664b, a transiting Neptune-sized planet monitored by the K2 mission during its campaign 5. We combine the K2 time-series data with ground-based photometric and spectroscopic follow-up observations to confirm the planetary nature of the object and derive its mass, radius, and orbital parameters. EPIC 211391664 b is a warm Neptune-like planet in a 10-day orbit around a V=12.2~mag F-type star with M⋆=1.074±0.042M⊙, R⋆=1.311+0.083−0.048R⊙, and age of 5.2+1.2−1.0~Gyr. We derive a planetary mass and radius of Mp=32.2±8.1M⊕ and Rp=4.3+0.3−0.2R⊕. EPIC 211391664b joins the relatively small group of Neptune-sized planets whose mass and radius have been derived with a precision better than 3-σ. We estimate that the planet will be engulfed by EPIC 211391664 in ∼3~Gyr, due to the evolution of the host star towards the red giant branch.

Ultra Short Period Exoplanets in K2 Campaigns 0 to 5

ULTRA-SHORT-PERIOD PLANETS IN K2 SUPERPIG RESULTS FOR CAMPAIGNS 0–5

Authors:

Adams et al

Abstract:

We analyzed data from Campaigns 0–5 of the K2 mission and report 19 ultra-short-period candidate planets with orbital periods of less than one day (nine of which have not been previously reported). Planet candidates range in size from 0.7 to 16 Earth radii and in orbital period from 4.2 to 23.5 hr. One candidate (EPIC 203533312, Kp = 12.5) is among the shortest-period planet candidates discovered to date ($P=4.2$ hr), and, if confirmed as a planet, must have a density of at least $\rho =8.9\ {\rm{g}}\,{\mathrm{cm}}^{-3}$ to not be tidally disrupted. Five candidates have nominal radius values in the sub-Jovian desert (${R}_{P}=3\mbox{--}11\,{R}_{\oplus }$ and $P\leqslant 1.5$ days) where theoretical models do not favor their long-term stability; the only confirmed planet in this range is thought to be disintegrating (EPIC 201637175). In addition to the planet candidates, we report on four objects that may not be planetary, including one with intermittent transits (EPIC 211152484) and three initially promising candidates that are likely false positives based on characteristics of their light curves and on radial velocity follow-up. A list of 91 suspected eclipsing binaries identified at various stages in our vetting process is also provided. Based on an assessment of our survey's completeness, we estimate an occurrence rate for ultra-short-period planets among K2 target stars that is about half that estimated from the Kepler sample, raising questions as to whether K2 systems are intrinsically different from Kepler systems, possibly as a result of their different galactic location.

Exoplanet candidates in Praesepe

A PSF-based approach to Kepler/K2 data. II. Exoplanet candidates in Praesepe (M 44)

Authors:

Libralto et al

Abstract:

In this work we keep pushing K2 data to a high photometric precision, close to that of the Kepler main mission, using a PSF-based, neighbour-subtraction technique, which also overcome the dilution effects in crowded environments. We analyse the open cluster M 44 (NGC 2632), observed during the K2 Campaign 5, and extract light curves of stars imaged on module 14, where most of the cluster lies. We present two candidate exoplanets hosted by cluster members and five by field stars. As a by-product of our investigation, we find 1680 eclipsing binaries and variable stars, 1071 of which are new discoveries. Among them, we report the presence of a heartbeat binary star. Together with this work, we release to the community a catalogue with the variable stars and the candidate exoplanets found, as well as all our raw and detrended light curves.

Two Brown Dwarfs Detected by K2 Mission

Search for light curve modulations among Kepler candidates: Three very low-mass transiting companions

Authors:

Lillo-Box et al

Abstract:

Context.

Light curve modulations in the sample of Kepler planet candidates allows the disentangling of the nature of the transiting object by photometrically measuring its mass. This is possible by detecting the effects of the gravitational pull of the companion (ellipsoidal modulations) and in some cases, the photometric imprints of the Doppler effect when observing in a broad band (Doppler beaming).

Aims.

We aim to photometrically unveil the nature of some transiting objects showing clear light curve modulations in the phase-folded Kepler light curve.

Methods.

We selected a subsample among the large crop of Kepler objects of interest (KOIs) based on their chances to show detectable light curve modulations, i.e., close (a< 12 R⋆) and large (in terms of radius, according to their transit signal) candidates. We modeled their phase-folded light curves with consistent equations for the three effects, namely, reflection, ellipsoidal and beaming (known as REB modulations).

Results.

We provide detailed general equations for the fit of the REB modulations for the case of eccentric orbits. These equations are accurate to the photometric precisions achievable by current and forthcoming instruments and space missions. By using this mathematical apparatus, we find three close-in very low-mass companions (two of them in the brown dwarf mass domain) orbiting main-sequence stars (KOI-554, KOI-1074, and KOI-3728), and reject the planetary nature of the transiting objects (thus classifying them as false positives). In contrast, the detection of the REB modulations and transit/eclipse signal allows the measurement of their mass and radius that can provide important constraints for modeling their interiors since just a few cases of low-mass eclipsing binaries are known. Additionally, these new systems can help to constrain the similarities in the formation process of the more massive and close-in planets (hot Jupiters), brown dwarfs, and very low-mass companions.

Hunting for TRAPPIST-1-like Systems With the K2 Mission

Probing TRAPPIST-1-like systems with K2

Authors:

Demory et al

Abstract:

The search for small planets orbiting late M dwarfs holds the promise of detecting Earth-size planets for which their atmospheres could be characterised within the next decade. The recent discovery of TRAPPIST-1 entertains hope that these systems are common around hosts located at the bottom of the main sequence. In this Letter, we investigate the ability of the repurposed Kepler mission (K2) to probe planetary systems similar to TRAPPIST-1. We perform a consistent data analysis of 189 spectroscopically confirmed M5.5 to M9 late M dwarfs from campaigns 1-6 to search for planet candidates and inject transit signals with properties matching TRAPPIST-1b and c. We find no transiting planet candidates across our K2 sample. Our injection tests show that K2 is able to recover both TRAPPIST-1 planets for 10% of the sample only, mainly because of the inefficient throughput at red wavelengths resulting in Poisson-limited performance for these targets. Increasing injected planetary radii to match GJ1214b's size yields a recovery rate of 70%. The strength of K2 is its ability to probe a large number of cool hosts across the different campaigns, out of which the recovery rate of 10% may turn into bona-fide detections of TRAPPIST-1 like systems within the next two years.

EPIC201702477b: A Long Period Transiting Brown Dwarf from K2

EPIC201702477b: A Long Period Transiting Brown Dwarf from K2

Authors:

Bayliss et al

Abstract:

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.

K2 Campaigns 5 & 6 Have IDed 152 Exoplanet Candidates

Transiting exoplanet candidates from K2 Campaigns 5 and 6

Authors:

Pope et al

Abstract:

We introduce a new transit search and vetting pipeline for observations from the K2 mission, and present the candidate transiting planets identified by this pipeline out of the targets in Campaigns 5 and 6. Our pipeline uses the Gaussian Process-based K2SC code to correct for the K2 pointing systematics and simultaneously model stellar variability. The systematics-corrected, variability-detrended light curves are searched for transits with the Box Least Squares method, and a period-dependent detection threshold is used to generate a preliminary candidate list. Two or three individuals vet each candidate manually to produce the final candidate list, using a set of automatically-generated transit fits and assorted diagnostic tests to inform the vetting. We detect 147 single-planet system candidates and 5 multi-planet systems, independently recovering the previously-published hot~Jupiters EPIC 212110888b, WASP-55b (EPIC 212300977b) and Qatar-2b (EPIC 212756297b). We also report the outcome of reconnaissance spectroscopy carried out for all candidates with Kepler magnitude Kp less than 13, identifying 12 targets as likely false positives. We compare our results to those of other K2 transit search pipelines, noting that ours performs particularly well for variable and/or active stars, but that the results are very similar overall. All the light curves and code used in the transit search and vetting process are publicly available, as are the follow-up spectra.

Tabby's Star has a Sister

New research conducted by a team of astronomers, led by Simone Scaringi of the Max Planck Institute for Extraterrestrial Physics in Germany, examines peculiar dimming of a newly found young stellar object designated EPIC 204278916. The study tries to explain the nature of these dipping events observed in the object's light curve. The results were published Aug. 25 in a paper available on arXiv.org.

EPIC 204278916, is a young, pre-main-sequence star, about 5 million years old, of spectral type M1, located in the Upper Scorpius sub-group of the Scorpius-Centaurus OB association. It is the size of our sun in diameter, but has only 0.5 solar masses. This young stellar object was discovered by NASA Kepler spacecraft's prolonged mission known as K2, during its Campaign 2 between Aug. 23 and Nov. 13, 2014. Moreover, follow-up observations made with the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile revealed that EPIC 204278916 also has a resolved tilted disk.

In a recent paper, Scaringi and his colleagues analyze the data provided by K2 and ALMA regarding EPIC 204278916, available in the Ecliptic Plane Input Catalog (EPIC) and the Mikulski Archive for Space Telescope (MAST) archive, with the aim to examine the object's light curve and its irregular dimmings in detail.

"We examine the K2 light curve in detail and hypothesize that the irregular dimmings are caused by either a warped inner-disk edge or transiting cometary-like objects in either circular or eccentric orbits," the researchers wrote in the paper.

According to the data provided by K2, EPIC 204278916 exhibited irregular dimmings of up to 65 percent for about 25 consecutive days out of 79 days of observations. The researchers also noted that when it comes to the remaining days of observation, this variability is highly periodic and could be attributed to stellar rotation.

One of the two most plausible explanations offered by the astronomers to explain the irregular dips in the object's light curve is that they are caused by non-axisymmetric structures in the inner disk edge occulting EPIC 204278916. Due to the fact that these dimmings are at a level of up to 65 percent, the occulting material must have a large scale height comparable to the size of the object.

The researchers also noted that the dips in young stellar objects like EPIC 204278916 might be caused by transiting circumstellar objects. They emphasized that if transiting cometary-like bodies are responsible for the observed dips, the events are most likely occurring close to periastron passage.

However, more observations are definitely needed to fully understand the mysterious behavior of EPIC 204278916. Moreover, further continuous photometric and spectroscopic monitoring of this system for subsequent dipping events will help determine whether this behavior is periodic or not.

link.

paper link.

Predictions for the Detection and Characterization of a Population of Free-Floating Planets with K2 Campaign 9

Predictions for the Detection and Characterization of a Population of Free-Floating Planets with K2 Campaign 9

Authors:

Penny et al

Abstract:

K2 Campaign 9 (K2C9) offers the first chance to measure parallaxes and masses of members of the large population of free-floating planets (FFPs) that has previously been inferred from measurements of the rate of short-timescale microlensing events. Using detailed simulations of the nominal campaign (ignoring the loss of events due to Kepler's emergency mode) and ground-based microlensing surveys, we predict the number of events that can be detected if there is a population of 1-Jupiter-mass FFPs matching current observational constraints. Using a Fisher matrix analysis we also estimate the number of detections for which it will be possible to measure the microlensing parallax, angular Einstein radius and FFP mass. We predict that between 1.4 and 7.9 events will be detected in the K2 data, depending on the noise floor that can be reached, but with the optimistic scenario being more likely. For nearly all of these it will be possible to either measure the parallax or constrain it to be probabilistically consistent with only planetary-mass lenses. We expect that for between 0.42 and 0.98 events it will be possible to gain a complete solution and measure the FFP mass. For the emergency mode truncated campaign, these numbers are reduced by 20~percent. We argue that when combined with prompt high-resolution imaging of a larger sample of short-timescale events, K2C9 will conclusively determine if the putative FFP population is indeed both planetary and free-floating.

197 Candidates and 104 Validated Planets in K2's First Five Fields

197 Candidates and 104 Validated Planets in K2's First Five Fields

Authors:

Crossfield et al

Abstract:

We present 197 planet candidates discovered using data from the first year of the NASA K2 mission (Campaigns 0-4), along with the results of an intensive program of photometric analyses, stellar spectroscopy, high-resolution imaging, and statistical validation. We distill these candidates into sets of 104 validated planets (57 in multi-planet systems), 30 false positives, and 63 remaining candidates. Our validated systems span a range of properties, with median values of R_P = 2.3 R_E, P=8.6 d, Tef = 5300 K, and Kp=12.7 mag. Stellar spectroscopy provides precise stellar and planetary parameters for most of these systems. We show that K2 has increased by 30% the number of small planets known to orbit moderately bright stars (1-4 R_E, Kp=9-13 mag). Of particular interest are 37 planets smaller than 2 R_E, 15 orbiting stars brighter than Kp=11.5, five receiving Earth-like irradiation levels, and several multi-planet systems -- including four planets orbiting the M dwarf K2-72 near mean-motion resonances. By quantifying the likelihood that each candidate is a planet we demonstrate that our candidate sample has an overall false positive rate of 15-30%, with rates substantially lower for small candidates (< 2 R_E) and larger for candidates with radii > 8 R_E and/or with P < 3 d. Extrapolation of the current planetary yield suggests that K2 will discover between 500-1000 planets in its planned four-year mission -- assuming sufficient follow-up resources are available. Efficient observing and analysis, together with an organized and coherent follow-up strategy, is essential to maximize the efficacy of planet-validation efforts for K2, TESS, and future large-scale surveys.

Doppler Monitoring of five K2 Transiting Planetary Systems

Doppler Monitoring of five K2 Transiting Planetary Systems

Authors:

Dai et al

Abstract:

In an effort to measure the masses of planets discovered by the NASA {\it K2} mission, we have conducted precise Doppler observations of five stars with transiting planets. We present the results of a joint analysis of these new data and previously published Doppler data. The first star, an M dwarf known as K2-3 or EPIC~201367065, has three transiting planets ("b", with radius 2.1 R⊕; "c", 1.7 R⊕; and "d", 1.5 R⊕). Our analysis leads to the mass constraints: Mb=8.1+2.0−1.9 M⊕ and Mc less than 4.2 M⊕~(95\%~conf.). The mass of planet d is poorly constrained because its orbital period is close to the stellar rotation period, making it difficult to disentangle the planetary signal from spurious Doppler shifts due to stellar activity. The second star, a G dwarf known as K2-19 or EPIC~201505350, has two planets ("b", 7.7 R⊕; and "c", 4.9 R⊕) in a 3:2 mean-motion resonance, as well as a shorter-period planet ("d", 1.1 R⊕). We find Mb= 28.5+5.4−5.0 M⊕, Mc= 25.6+7.1−7.1 M⊕ and Md less than 14.0 M⊕~(95\%~conf.). The third star, a G dwarf known as K2-24 or EPIC~203771098, hosts two transiting planets ("b", 5.7 R⊕; and "c", 7.8 R⊕) with orbital periods in a nearly 2:1 ratio. We find Mb= 19.8+4.5−4.4 M⊕ and Mc = 26.0+5.8−6.1 M⊕.

K2 Mission Finds an Additional 172 Exoplanet Candidates

New planetary and EB candidates from Campaigns 1-6 of the K2 mission

Authors:

Barros et al

Abstract:

With only two functional reaction wheels, Kepler cannot maintain stable pointing at its original target field and entered a new mode of observation called K2. Our method is based on many years of experience in planet hunting for the CoRoT mission. Due to the unstable pointing, K2 light curves present systematics that are correlated with the target position in the CCD. Therefore, our pipeline also includes a decorrelation of this systematic noise. Our pipeline is optimised for bright stars for which spectroscopic follow-up is possible. We achieve a maximum precision on 6 hours of 6 ppm. The decorrelated light curves are searched for transits with an adapted version of the CoRoT alarm pipeline. We present 172 planetary candidates and 327 eclipsing binary candidates from campaigns 1, 2, 3, 4, 5 and 6 of K2. Both the planetary candidates and eclipsing binary candidates lists are made public to promote follow-up studies. The light curves will also be available to the community.