K2 News and Results

A statistically validated planet or brown dwarf orbiting the A-type star 2MASS J06101557+2436535.

A Substellar Companion to a Hot Star in K2's Campaign 0 Field
https://arxiv.org/abs/1907.06662

Four Small Planets Buried in K2 Systems: What Can We Learn for TESS?
https://arxiv.org/abs/1907.08244

Quote :

The Kepler, K2, and Transiting Exoplanet Survey Satellite (TESS) missions have provided a wealth of confirmed exoplanets, benefiting from a huge effort from the planet-hunting and follow-up community. With careful systematics mitigation, these missions provide precise photometric time series, which enable detection of transiting exoplanet signals. However, exoplanet hunting can be confounded by several factors, including instrumental noise, search biases, and host star variability. In this Letter, we discuss strategies to overcome these challenges using newly emerging techniques and tools. We demonstrate the power of new, fast open-source community tools (e.g., lightkurve, starry, celerite, exoplanet), and discuss four high signal-to-noise ratio (S/N) exoplanets that showcase specific challenges present in planet detection: K2-43c, K2-168c, K2-198c, and K2-198d. These planets have been undetected in several large K2 planet searches, despite having transit signals with S/N > 10. Two of the planets discussed here are new discoveries. In this work we confirm all four as true planets. Alongside these planet systems, we discuss three key challenges in finding small transiting exoplanets. The aim of this Letter is to help new researchers understand where planet detection efficiency gains can be made, and to encourage the continued use of K2 archive data. The considerations presented in this Letter are equally applicable to Kepler, K2, and TESS, and the tools discussed here are available for the community to apply to improve exoplanet discovery and fitting.

Detection of Hundreds of New Planet Candidates and Eclipsing Binaries in K2 Campaigns 0-8
https://arxiv.org/abs/1907.10806

This paper reports 374 new planet candidates.

It takes two planets in resonance to tango around K2-146
https://arxiv.org/abs/1907.11141

K2-146: Discovery of Planet c, Precise Masses from Transit Timing, and Observed Precession
https://arxiv.org/abs/1907.10620

Radial velocity confirmation of K2-100b: a young, highly irradiated, and low density transiting hot Neptune
https://arxiv.org/abs/1909.05252

Significant update for K2-138 system with the addition of another planet, K2-138 g.

Exoplanet characterisation in the longest known resonant chain: the K2-138 system seen by HARPS

Quote :

The detection of low-mass transiting exoplanets in multiple systems brings new constraints to planetary formation and evolution processes and challenges the current planet formation theories. Nevertheless, only a mere fraction of the small planets detected by Kepler and K2 have precise mass measurements, which are mandatory to constrain their composition. We aim to characterise the planets that orbit the relatively bright star K2-138. This system is dynamically particular as it presents the longest chain known to date of planets close to the 3:2 resonance. We obtained 215 HARPS spectra from which we derived the radial-velocity variations of K2-138. Via a joint Bayesian analysis of both the K2 photometry and HARPS radial-velocities (RVs), we constrained the parameters of the six planets in orbit. The masses of the four inner planets, from b to e, are 3.1, 6.3, 7.9, and 13.0 M⊕ with a precision of 34%, 20%, 18%, and 15%, respectively. The bulk densities are 4.9, 2.8, 3.2, and 1.8 g cm−3, ranging from Earth to Neptune-like values. For planets f and g, we report upper limits. Finally, we predict transit timing variations of the order two to six minutes from the masses derived. Given its peculiar dynamics, K2-138 is an ideal target for transit timing variation (TTV) measurements from space with the upcoming CHaracterizing ExOPlanet Satellite (CHEOPS) to study this highly-packed system and compare TTV and RV masses.

Zodiacal Exoplanets in Time (ZEIT) IX: a flat transmission spectrum and a highly eccentric orbit for the young Neptune K2-25b as revealed by Spitzer
https://arxiv.org/abs/1911.05744

Quote :

Transiting planets in nearby young clusters offer the opportunity to study the atmospheres and dynamics of planets during their formative years. To this end, we focused on K2-25b -- a close-in (P=3.48 days), Neptune-sized exoplanet orbiting a M4.5 dwarf in the 650Myr Hyades cluster. We combined photometric observations of K2-25 covering a total of 44 transits and spanning >2 years, drawn from a mix of space-based telescopes (Spitzer Space Telescope and K2) and ground-based facilities (Las Cumbres Observatory Network and MEarth). The transit photometry spanned 0.6 to 4.5μm, which enabled our study of K2-25b's transmission spectrum. We combined and fit each dataset at a common wavelength within an MCMC framework, yielding consistent planet parameters. The resulting transit depths ruled out a solar-composition atmosphere for K2-25b for the range of expected planetary masses and equilibrium temperature at a >4σ confidence level, and are consistent with a flat transmission spectrum. Mass constraints and transit observations at a finer grid of wavelengths (e.g., from HST) are needed to make more definitive statements about the presence of clouds or an atmosphere of high mean-molecular weight. Our precise measurements of K2-25b's transit duration also enabled new constraints on the eccentricity of K2-25's orbit. We find K2-25b's orbit to be eccentric (e>0.20) for all reasonable stellar densities and independent of the observation wavelength or instrument. The high eccentricity is suggestive of a complex dynamical history and motivates future searches for additional planets or stellar companions.

Constraining Orbital Periods from Nonconsecutive Observations: Period Estimates for Long-Period Planets in Six Systems Observed by K2 During Multiple Campaigns
https://arxiv.org/abs/1912.04287

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Most planetary discoveries with the K2 and TESS missions are restricted to short periods because of the limited duration of observation. However, the re-observation of sky area allows for the detection of longer period planets. We describe new transits detected in six candidate planetary systems which were observed by multiple K2 mission campaigns. One of these systems is a multiplanet system with four candidate planets; we present new period constraints for two planets in this system. In the other five systems, only one transit is observed in each campaign, and we derive period constraints from this new data. The period distributions are highly multimodal resulting from missed potential transits in the gap between observations. Each peak in the distribution corresponds to transits at an integer harmonic of the two observed transits. We further detail a generalized procedure to constrain the period for planets with multiple observations with intervening gaps. Because long period photometrically discovered planets are rare, these systems are interesting targets for follow-up observations and confirmation. Specifically, all six systems are bright enough (V = 10.4-12.7) to be amenable to radial velocity follow-up. This work serves as a template for period constraints in a host of similar yet-to-be-discovered planets in long baseline, temporally gapped observations conducted by the TESS mission.

K2 stellar parameter updates:

Hardegree-Ullman et al. "Scaling K2. I. Revised Parameters for 222,088 K2 Stars and a K2 Planet Radius Valley at 1.9 R_🜨"
https://arxiv.org/abs/2001.11511

This radius valley was previously identified in the Kepler planets and tentatively in the K2 sample.

Zink et al. "Scaling K2. II. Assembly of a Fully Automated C5 Planet Candidate Catalog Using EDI-Vetter"
https://arxiv.org/abs/2001.11515

72 planet candidates, including 7 multi-planet systems (5 double, 1 triple, 1 quadruple).

From A&A forthcoming, listed on EPE:

Hidalgo et al. "Three planets transiting the evolved star EPIC 249893012: a hot 8.8-M_⊕ super-Earth and two warm 14.7 and 10.2-M_⊕ sub-Neptunes"
https://doi.org/10.1051/0004-6361/201937080

Now on arXiv:
https://arxiv.org/abs/2002.01755

Lazarus wrote:

Zink et al. "Scaling K2. II. Assembly of a Fully Automated C5 Planet Candidate Catalog Using EDI-Vetter"
https://arxiv.org/abs/2001.11515

72 planet candidates, including 7 multi-planet systems (5 double, 1 triple, 1 quadruple).

For those keeping track, from this paper, there are thirteen planet candidates not yet previously reported as detected by K2.
Among these, EPIC 211399359.01 = TOI-515.01.
EPIC 211562654.04 would be in the gap between K2-183 b and K2-183 c.
EPIC 211711685.01 appears to be a viable candidate.
EPIC 211770867 has a detected even-odd transit depth difference, indicative of an eclipsing binary.
EPIC 211918985 has contamination from a nearby eclipsing binary of the same period.
EPIC 211942755.01 appears to be a viable candidate.
EPIC 211953244.01 appears to be a viable candidate.
EPIC 211958340 appears to be an eclipsing binary.
EPIC 212020330.01 appears to be a viable candidate.
EPIC 212066407 has an even-odd transit depth difference, indicative of an eclipsing binary.
EPIC 212119244.01 appears to be a viable candidate.
EPIC 212138198 has a significant centroid offset indicative of a background eclipsing binary.
EPIC 212172538.01 appears to be a viable candidate.

π Earth: a 3.14 days Earth-sized Planet from \textit{K2}'s Kitchen served warm by the SPECULOOS Team
https://arxiv.org/abs/2006.07308

Quote :

We report on the discovery of a transiting Earth-sized (0.95R⊕) planet around an M3.5 dwarf star at 57\,pc, EPIC~249631677. The planet has a period of ∼3.14 days, i.e. ∼π, with an instellation of 7.5\,S⊕. The detection was made using publicly available data from \textit{K2}'s Campaign 15. We observed three additional transits with SPECULOOS Southern and Northern Observatories, and a stellar spectrum from Keck/HIRES, which allowed us to validate the planetary nature of the signal. The confirmed planet is well suited for comparative terrestrial exoplanetology. While exoplanets transiting ultracool dwarfs present the best opportunity for atmospheric studies of terrestrial exoplanets with the \textit{James Webb Space Telescope}, those orbiting mid-M dwarfs within 100\,pc such as EPIC~249631677b will become increasingly accessible with the next generation of observatories (e.g., \textit{HabEx, LUVOIR, OST}).

K2-280b -- a low density warm sub-Saturn around a mildly evolved star
https://arxiv.org/abs/2007.07939

https://arxiv.org/abs/2007.01081

Characterization of the K2-38 planetary system. Unraveling one of the densest planets known to date

We characterized the transiting planetary system orbiting the G2V star K2-38 using the new-generation echelle spectrograph ESPRESSO. We carried out a photometric analysis of the available K2 photometric light curve of this star to measure the radius of its two known planets. Using 43 ESPRESSO high-precision radial velocity measurements taken over the course of 8 months along with the 14 previously published HIRES RV measurements, we modeled the orbits of the two planets through a MCMC analysis, significantly improving their mass measurements. Using ESPRESSO spectra, we derived the stellar parameters, Teff=5731±66, logg=4.38±0.11~dex, and [Fe/H]=0.26±0.05~dex, and thus the mass and radius of K2-38, M⋆=1.03 +0.04−0.02~M⊕ and R⋆=1.06 +0.09−0.06~R⊕. We determine new values for the planetary properties of both planets. We characterize K2-38b as a super-Earth with RP=1.54±0.14~R⊕ and Mp=7.3+1.1−1.0~M⊕, and K2-38c as a sub-Neptune with RP=2.29±0.26~R⊕ and Mp=8.3+1.3−1.3~M⊕. We derived a mean density of ρp=11.0+4.1−2.8~g cm−3 for K2-38b and ρp=3.8+1.8−1.1~g~cm−3 for K2-38c, confirming K2-38b as one of the densest planets known to date. The best description for the composition of K2-38b comes from an iron-rich Mercury-like model, while K2-38c is better described by a rocky model with a H2 envelope. The maximum collision stripping boundary shows how giant impacts could be the cause for the high density of K2-38b. The irradiation received by each planet places them on opposite sides of the radius valley. We find evidence of a long-period signal in the radial velocity time-series whose origin could be linked to a 0.25-3~MJ planet or stellar activity.

In collaboration with HARPS-N, ESPRESSo also measured the mass of the transiting planet K2-111 b and found a non-transiting companion. The article has just been submitted #Exo3

Several new K2 planet candidates, some of which are statistically validated.

Planetary candidates transiting cool dwarf stars from Campaigns 12 to 15 of K2
https://arxiv.org/abs/2007.12744

A couple of updates on the Hyades planet K2-25b:

Gaidos et al. "Zodiacal Exoplanets in Time. XI. The Orbit and Radiation Environment of the Young M Dwarf-Hosted Planet K2-25b"
https://arxiv.org/abs/2007.12701

Stefansson et al. "The Habitable-zone Planet Finder Reveals A High Mass and a Low Obliquity for the Young Neptune K2-25b"
https://arxiv.org/abs/2007.12766

More planetary candidates from K2 Campaign 5 by tran_k2
https://arxiv.org/abs/2008.10075

Quote :

CONTEXT. The exquisite precision of the space-based photometric surveys and the unavoidable presence of instrumental systematics and intrinsic stellar variability call for the development of sophisticated methods that separate these signal components from those caused by planetary transits. AIMS. Here we introduce tran_k2 a stand-alone Fortran code to search for planetary transits under the colored noise of stellar variability and instrumental effects. With this code we perform a survey for new candidates. METHODS. Stellar variability is represented by a Fourier series, and, if needed, by an autoregressive model to avoid excessive Gibbs overshoots at the edges. For the treatment of systematics, cotrending and external parameter decorrelation are employed by using cotrending stars with low stellar variability, the chip position and the background flux level at the target. The filtering is made within the framework of the standard weighted least squares, where the weights are determined iteratively, to allow robust fit and separate the transit signal from stellar variability and systematics. Once the periods of the transit components are determined from the filtered data by the box-fitting least squares method, we reconstruct the full signal and determine the transit parameters with a higher accuracy. This step greatly reduces the excessive attenuation of the transit depths and minimizes shape deformation. RESULTS. The code was tested on the field of Campaign 5 of the K2 mission. We detected 98% of the systems with all their candidate planets reported earlier by other authors, surveyed the whole field and discovered 15 new systems. Additional 3 planets were found in 3 multiplanetary systems and 2 more planets were found in a previously known single planet system.

TFAW survey. I. Wavelet-based denoising of K2 light curves. Discovery and validation of two new Earth-sized planets in K2 campaign 1
https://arxiv.org/abs/2009.09285

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The wavelet-based detrending and denoising method \texttt{TFAW} is applied for the first time to \texttt{EVEREST 2.0}-corrected light curves to further improve the photometric precision of almost all K2 observing campaigns (C1-C8, C12-C18). The performance of both methods is evaluated in terms of 6 hr combined differential photometric precision (CDPP), simulated transit detection efficiency, and planet characterization in different SNR regimes. On average, \texttt{TFAW} median 6hr CDPP is ∼30% better than the one achieved by \texttt{EVEREST 2.0} for all observing campaigns. Using the \texttt{transit least-squares} (\texttt{TLS}) algorithm, we show that the transit detection efficiency for simulated Earth-Sun-like systems is ∼8.5× higher for \texttt{TFAW}-corrected light curves than for \texttt{EVEREST 2.0} ones. Using the light curves of two confirmed exoplanets, K2-44 b (high-SNR) and K2-298 b (low-SNR), we show that \texttt{TFAW} yields better MCMC posterior distributions, transit parameters compatible with the cataloged ones but with smaller uncertainties and narrows the credibility intervals. We use the combination of \texttt{TFAW}'s improved photometric precision and \texttt{TLS} enhancement of the signal detection efficiency for weak signals to search for new transit candidates in K2 observing campaign 1. We report the discovery of two new K2-C1 Earth-sized planets statistically validated, using the \texttt{vespa} software: EPIC 201170410.02, with a radius of 1.047+0.276−0.257R⊕ planet orbiting an M-type star, and EPIC 201757695.02, with a radius of 0.908+0.059−0.064R⊕ planet orbiting a K-type star. EPIC 201757695.02 is the 9-th smallest planet ever discovered in K2-C1, and the 39-th smallest in all K2 campaigns.

A second, non-transiting planet has been found at K2-111.

K2-111: an old system with two planets in near-resonance
https://arxiv.org/abs/2010.01993

Ultra Short Period Planets in K2 III: Neighbors are Common with 12 New Multi-Planet Systems and 26 Newly Validated Planets in Campaigns 0-8, 10
https://arxiv.org/abs/2011.11698

Just a note: a mega-update at EPE has brought the exoplanet count up to 4678: 264 planets in just one day! Impressive.

Most of them were from from past K2 Mission's releases (2016-2018), plus some recent "Kepler residuals", a few former BD candidates and the latest two TESS planets. Maybe it was time for that.

And there is still some more left to update.

K2-290's planets are misaligned and coplanar.

A backward-spinning star with two coplanar planets
https://arxiv.org/abs/2102.07677

zorro83 wrote:

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

When can we expect the first confirmed exoplanets?

Today ! (5 years later)

https://academic.oup.com/mnras/article/505/4/5584/6315707

Kepler K2 Campaign 9 – I. Candidate short-duration events from the first space-based survey for planetary microlensing

ABSTRACT
We present the first short-duration candidate microlensing events from the Kepler K2 mission. From late April to early July 2016, Campaign 9 of K2 obtained high temporal cadence observations over a 3.7 deg2 region of the Galactic bulge. Its primary objectives were to look for evidence of a free-floating planet (FFP) population using microlensing, and demonstrate the feasibility of space-based planetary microlensing surveys. Though Kepler K2 is far from optimal for microlensing, the recently developed MCPM photometric pipeline enables us to identify and model microlensing events. We describe our blind event-selection pipeline in detail and use it to recover 22 short-duration events with effective time-scales teff < 10 d previously announced by the OGLE and KMTNet ground-based surveys. We also announce five new candidate events. One of these is a caustic-crossing binary event, modelled in a companion study. The other four have very short durations (teff < 0.1 d) typical of an Earth-mass FFP population. Whilst Kepler was not designed for crowded-field photometry, the K2C9 data set clearly demonstrates the feasibility of conducting blind space-based microlensing surveys towards the Galactic bulge.

A little old piece of news (2020) but worth of mention: the "vampire star" KSN:BS-C11a (or EPIC 203830112).

NASA's Kepler Witnesses Vampire Star System Undergoing Super-Outburst

Original paper is the following: Discovery of a new WZ Sagittae-type cataclysmic variable in the Kepler/K2 data

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We identify a new, bright transient in the K2/Kepler Campaign 11 field. Its light curve rises over 7 mag in a day and then declines 3 mag over a month before quickly fading another 2 mag. The transient was still detectable at the end of the campaign. The light curve is consistent with a WZ Sge-type dwarf nova outburst. Early superhumps with a period of 82 min are seen in the first 10 days and suggest that this is the orbital period of the binary, which is typical for the WZ Sge class. Strong superhump oscillations develop 10 days after peak brightness with periods ranging between 83 and 84 min. At 25 days after the peak brightness a bump in the light curve appears to signal a subtle rebrightening phase implying that this was an unusual type-A outburst. This is the only WZ Sge-type system observed by K2/Kepler during an outburst. The early rise of this outburst is well fitted with a broken power law. In first 10 h, the system brightened linearly and then transitioned to a steep rise with a power-law index of 4.8. Looking at archival K2/Kepler data and new TESS observations, a linear rise in the first several hours at the initiation of a superoutburst appears to be common in SU Ursa Majoris stars.

It reminds me the case of EF Eridani. Although WD mass is not reported, with q = 0.07 the companion is certainly substellar even with a massive 1 M_Sol white dwarf primary. The most likely value has got to be around 42-50 M_Jup assuming a canonical 0.6-0.7 M_Sol WD primary.