Microlensing exoplanet discoveries

It seems like this is the Microlensing Planet Discoveries Month.

A companion on the planet/brown dwarf mass boundary on a wide orbit discovered by gravitational microlensing

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We present the discovery of a substellar companion to the primary host lens in the microlensing event MOA-2012-BLG-006. The companion-to-host mass ratio is 0.016, corresponding to a companion mass of ≈8 MJup(M∗/0.5M⊙). Thus, the companion is either a high-mass giant planet or a low-mass brown dwarf, depending on the mass of the primary M∗. The companion signal was separated from the peak of the primary event by a time that was as much as four times longer than the event timescale. We therefore infer a relatively large projected separation of the companion from its host of ≈10 a.u.(M∗/0.5M⊙)1/2 for a wide range (3-7 kpc) of host star distances from the Earth. We also challenge a previous claim of a planetary companion to the lens star in microlensing event OGLE-2002-BLG-045.

On the other hand, as a new planet arrives, another one seems to go away, OGLE-2002-BLG-045L b.

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It seems like this is the Microlensing Planet Discoveries Month.

Quoting myself, it's quite so!

MOA-2016-BLG-227Lb: A Massive Planet Characterized by Combining Lightcurve Analysis and Keck AO Imaging

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We report the discovery of a microlensing planet --- MOA-2016-BLG-227Lb --- with a massive planet/host mass ratio of q≃9×10−3. This event was fortunately observed by several telescopes as the event location was very close to the area of the sky surveyed by Campaign 9 of the K2 Mission. Consequently, the planetary deviation is well covered and allows a full characterization of the lensing system. High angular resolution images by the Keck telescope show excess flux other than the source flux at the target position, and this excess flux could originate from the lens star. We combined the excess flux and the observed angular Einstein radius in a Bayesian analysis which considers various possible origins of the measured excess flux as priors, in addition to a standard Galactic model. Our analysis indicates that it is unlikely that a large fraction of the excess flux comes from the lens. We compare the results of the Bayesian analysis using different priors for the probability of hosting planets with respect to host mass and find the planet is likely a gas-giant around an M/K dwarf likely located in the Galactic bulge. This is the first application of a Bayesian analysis considering several different contamination scenarios for a newly discovered event. Our approach for considering different contamination scenarios is crucial for all microlensing events which have evidence for excess flux irrespective of the quality of observation conditions, such as seeing, for example.

Edasich wrote:

On the other hand, as a new planet arrives, another one seems to go away, OGLE-2002-BLG-045L b.

Poleski et al didn't seem to think it was ever a secure detection anyway.

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However, the analyzed data were very limited. Even though the mass ratio was found to be in planetary regime, this event is not considered a secure planet detection.

Sirius_Alpha wrote:

An Earth-mass Planet in a 1-AU Orbit around a Brown Dwarf
https://arxiv.org/abs/1703.08548

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We combine Spitzer and ground-based KMTNet microlensing observations to identify and precisely measure an Earth-mass (1.32+0.41−0.28M⊕) planet OGLE-2016-BLG-1195Lb at 1.11+0.13−0.10 AU orbiting a 0.072+0.014−0.010M⊙ ultracool dwarf, likely a brown dwarf. This is the lowest-mass microlensing planet to date. At 4.20+0.29−0.34 kpc, it is the third consecutive case among the Spitzer "Galactic distribution" planets toward the Galactic bulge that lies in the Galactic disk as opposed to the bulge itself, hinting at a skewed distribution of planets. Together with previous microlensing discoveries, the seven Earth-size planets orbiting the ultracool dwarf TRAPPIST-1, and the detection of disks around young brown dwarfs, OGLE-2016-BLG-1195Lb suggests that such planets might be common around ultracool dwarfs. It therefore sheds light on the formation of both brown dwarfs and planetary systems at the limit of low-mass protoplanetary disks.

The Lowest Mass Ratio Planetary Microlens: OGLE 2016-BLG-1195Lb
https://arxiv.org/abs/1703.08639

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We report discovery of the lowest mass ratio exoplanet to be found by the microlensing method in the light curve of the event OGLE~2016--BLG--1195. This planet revealed itself as a small deviation from a microlensing single lens profile from an examination of the survey data soon after the planetary signal. The duration of the planetary signal is ∼2.5hours. The measured ratio of the planet mass to its host star is q=4.2±0.7×10−5. We further estimate that the lens system is likely to comprise a cold ∼3 Earth mass planet in a ∼2 AU wide orbit around a 0.2 Solar mass star at an overall distance of 7.1 kpc.

NASA press release :
https://www.nasa.gov/feature/jpl/iceball-planet-discovered-through-microlensing

'Iceball' Planet Discovered Through Microlensing

A couple more...

Mroz et al. "OGLE-2013-BLG-0132Lb and OGLE-2013-BLG-1721Lb: Two Saturn-mass Planets Discovered around M-dwarfs"
https://arxiv.org/abs/1705.01058

An update on OGLE-2015-BLG-0954

MOA Data Reveal a New Mass, Distance, and Relative Proper Motion for Planetary System OGLE-2015-BLG-0954L
https://arxiv.org/abs/1705.03937

Brown dwarf or giant planet detection around relatively massive star:

OGLE-2014-BLG-1112LB: A Microlensing Brown Dwarf Detected Through the Channel of a Gravitational Binary-Lens Event

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Due to the nature depending on only the gravitational field, microlensing, in principle, provides an important tool to detect faint and even dark brown dwarfs. However, the number of identified brown dwarfs is limited due to the difficulty of the lens mass measurement that is needed to check the substellar nature of the lensing object. In this work, we report a microlensing brown dwarf discovered from the analysis of the gravitational binary-lens event OGLE-2014-BLG-1112. We identify the brown-dwarf nature of the lens companion by measuring the lens mass from the detections of both microlens-parallax and finite-source effects. We find that the companion has a mass of (3.03±0.78)×10−2 M⊙ and it is orbiting a solar-type primary star with a mass of 1.07±0.28 M⊙. The estimated projected separation between the lens components is 9.63±1.33 au and the distance to the lens is 4.84±0.67 kpc. We discuss the usefulness of space-based microlensing observations in detecting brown dwarfs through the channel of binary-lens events.

Sirius_Alpha wrote:

Spitzer as Microlens Parallax Satellite: Mass Measurement for the OGLE-2014-BLG-0124L Planet and its Host Star
http://arxiv.org/abs/1410.4219

Update.

Combining Spitzer parallax and Keck II adaptive optics imaging to measure the mass of a solar-like star orbited by a cold gaseous planet discovered by microlensing
https://arxiv.org/abs/1709.00806

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To obtain accurate mass measurements for cold planets discovered by microlensing, it is usually necessary to combine light curve modeling with at least two lens mass-distance relations. Often, a constraint on the Einstein ring radius measurement is obtained from the caustic crossing time: This is supplemented by secondary constraints such as precise parallax measurements and/or measures of the lens luminosity using high angular resolution observations. We resolved the source+lens star from sub-arcsecond blends in H band using adaptive optics (AO) observations with NIRC2 mounted on Keck II telescope. We identify additional flux, coincident with the source to within 160 mas. We estimate the potential contributions to this blended light (chance-aligned star, additional companion to the lens or to the source) and find that 85 % of of the NIR flux is due to the lens star at H_L=16.63 +- 0.06 and K_L=16.46 +- 0.06. We combined the parallax constraint and the AO constraint to derive the physical parameters of the system. The lensing system is composed of a mid-late type G main sequence star of M_L=0.89 +- 0.05 Mo located at D_L = 3.6 +- 0.3 kpc in the Galactic disk. Taking the mass ratio and projected separation from the original study leads to a planet of M_p= 0.64 +- 0.044 M_Jupiter at 3.48 +- 0.22 AU. Excellent parallax measurement from simultaneous ground-space observations have been obtained on the microlensing event OGLE-2014-BLG-0124, but it is only when they are combined with ~ 30 min of Keck II AO observations that the physical parameters of the host star are well measured.

OGLE-2017-BLG-0173Lb: Low Mass-Ratio Planet in a "Hollywood" Microlensing Event
https://arxiv.org/abs/1709.08476

OGLE-2016-BLG-0613LABb: A Microlensing Planet in a Binary System
https://arxiv.org/abs/1710.00924

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We present the analysis of OGLE-2016-BLG-0613, for which the lensing light curve appears to be that of a typical binary-lens event with two caustic spikes but with a discontinuous feature on the trough between the spikes. We find that the discontinuous feature was produced by a planetary companion to the binary lens. We find 4 degenerate triple-lens solution classes, each composed of a pair of solutions according to the well-known wide/close planetary degeneracy. One of these solution classes is excluded due to its relatively poor fit. For the remaining three pairs of solutions, the most-likely primary mass is about M1∼0.7M⊙ while the planet is a super-Jupiter. In all cases the system lies in the Galactic disk, about half-way toward the Galactic bulge. However, in one of these three solution classes, the secondary of the binary system is a low-mass brown dwarf, with relative mass ratios (1 : 0.03 : 0.003), while in the two others the masses of the binary components are comparable. These two possibilities can be distinguished in about 2024 when the measured lens-source relative proper motion will permit separate resolution of the lens and source.

97-BLG-41 has finally its revenge.

Even the double planet solution with more massive inner companion doesn't sound bad, but I guess it would likely turn out a HD 202206's analogue.

OGLE-2016-BLG-1190Lb: First Spitzer Bulge Planet Lies Near the Planet/Brown-Dwarf Boundary
https://arxiv.org/abs/1710.09974

Not an exoplanet discovery, but this one does go to show that attempting to detect exomoons via microlensing is challenging.

Hwang et al. "OGLE-2015-BLG-1459L: The Challenges of Exo-Moon Microlensing"
https://arxiv.org/abs/1711.09651

Three models could explain the microlensing curve: 3 lens, 1 source (3L1S), 2L2S, 1L3S. In the 3L1S case, the system would consist of a brown dwarf with a Neptune-mass planet and a Mars-mass object (possibly a moon of the Neptune, or a planet on an independent orbit that happened to appear close to the Neptune in projection). However the colours derived by comparison with MOA data indicate that the 1L3S model is correct, with the source consisting of a bright primary and a dim binary.

The discussion (section 6) notes several of the problems that occur when trying to look for moons with microlensing.

A free-floating planet candidate from the OGLE and KMTNet surveys
https://arxiv.org/abs/1712.01042

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Current microlensing surveys are sensitive to free-floating planets down to Earth-mass objects. All published microlensing events attributed to unbound planets were identified based on their short timescale (below 2 d), but lacked an angular Einstein radius measurement (and hence lacked a significant constraint on the lens mass). Here we present the discovery of a Neptune-mass free-floating planet candidate in the ultrashort (tE=0.320±0.003 d) microlensing event OGLE-2016-BLG-1540. The event exhibited strong finite source effects, which allowed us to measure its angular Einstein radius of θE=9.2±0.5 uas. There remains, however, a degeneracy between the lens mass and distance. The combination of the source proper motion and source-lens relative proper motion measurements favors a Neptune-mass lens located in the Galactic disk. However, we cannot rule out that the lens is a Saturn-mass object belonging to the bulge population. We exclude stellar companions up to 15 au. Owing to the relatively large relative lens-source proper motion, any stellar companions should be detectable using the high-resolution imaging in the relatively near future.

A Saturn-like planet around an M dwarf at 3.7 kpc.

Calchi Novati et al. "Spitzer Microlensing Parallax for OGLE-2016-BLG-1067: a sub-Jupiter Orbiting an M-dwarf in the Disk"
https://arxiv.org/abs/1801.05806

OGLE-2017-BLG-1434Lb: Eighth q < 1 * 10^-4 Mass-Ratio Microlens Planet Confirms Turnover in Planet Mass-Ratio Function

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We report the discovery of a cold Super-Earth planet (m_p=4.4 +/- 0.5 M_Earth) orbiting a low-mass (M=0.23 +/- 0.03 M_Sun) M dwarf at projected separation a_perp = 1.18 +/- 0.10 AU, i.e., about 1.9 times the snow line. The system is quite nearby for a microlensing planet, D_Lens = 0.86 +/- 0.09 kpc. Indeed, it was the large lens-source relative parallax pi_rel=1.0 mas (combined with the low mass M) that gave rise to the large, and thus well-measured, "microlens parallax" that enabled these precise measurements. OGLE-2017-BLG-1434Lb is the eighth microlensing planet with planet-host mass ratio q < 1 * 10^-4.
We apply a new planet-detection sensitivity method, which is a variant of "V/V_max", to seven of these eight planets to derive the mass-ratio function in this regime. We find dN/d(ln q) ~ q^p, with p = 1.05 (+0.78,-0.68), which confirms the "turnover" in the mass function found by Suzuki et al. relative to the power law of opposite sign n = -0.93 +/- 0.13 at higher mass ratios q >~ 2 * 10^-4. We combine our result with that of Suzuki et al. to obtain p = 0.73 (+0.42,-0.34).

Microlensing detection in the infrared

Shvartzvald et al. "UKIRT-2017-BLG-001Lb: A giant planet detected through the dust"
https://arxiv.org/abs/1802.06795

~1.5 Jupiter mass planet orbiting a ~0.93 solar mass star at a (projected) distance of ~4.5 AU

Some more:

A low-mass binary (16 and 12 Jupiter masses):

Albrow et al. "OGLE-2016-BLG-1266: A Probable Brown-Dwarf/Planet Binary at the Deuterium Fusion Limit"
https://arxiv.org/abs/1802.09563

A sub-jovian planet and a new microlensing degeneracy to watch out for:

Skowron et al. "OGLE-2017-BLG-0373Lb: A Jovian Mass-Ratio Planet Exposes A New Accidental Microlensing Degeneracy"
https://arxiv.org/abs/1802.10067

Possibly another.

KMT-2016-BLG-0212: First KMTNet-Only Discovery of a Substellar Companion
https://arxiv.org/abs/1802.10246

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We present the analysis of KMT-2016-BLG-0212, a low flux-variation (Iflux−var∼20) microlensing event, which is well-covered by high-cadence data from the three Korea Microlensing Telescope Network (KMTNet) telescopes. The event shows a short anomaly that is incompletely covered due to the brief visibility intervals that characterize the early microlensing season when the anomaly occurred. We show that the data are consistent with two classes of solutions, characterized respectively by low-mass brown-dwarf (q=0.037) and sub-Neptune (q<10−4) companions, respectively. Future high-resolution imaging should easily distinguish between these solutions.

Spitzer Opens New Path to Break Classic Degeneracy for Jupiter-Mass Microlensing Planet OGLE-2017-BLG-1140Lb
https://arxiv.org/abs/1803.04437

OGLE-2017-BLG-1522: A giant planet around a brown dwarf located in the Galactic bulge
https://arxiv.org/abs/1803.05095

A Likely Detection of a Two-Planet System in a Low Magnification Microlensing Event
https://arxiv.org/abs/1803.05238

Large error bars on the lens mass for OGLE-2017-BLG-1522, could be a low-mass star. Nevertheless, it is interesting to see a gas giant in such a system.

OGLE-2017-BLG-0482Lb: A Microlensing Super-Earth Orbiting a Low-mass Host Star
https://arxiv.org/abs/1803.10830

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We report the discovery of a planetary system in which a super-earth orbits a late M-dwarf host. The planetary system was found from the analysis of the microlensing event OGLE-2017-BLG-0482, wherein the planet signal appears as a short-term anomaly to the smooth lensing light curve produced by the host. Despite its weak signal and short duration, the planetary signal was firmly detected from the dense and continuous coverage by three microlensing surveys. We find a planet/host mass ratio of q∼1.4×10−4. We measure the microlens parallax πE from the long-term deviation in the observed lensing light curve, but the angular Einstein radius θE cannot be measured because the source trajectory did not cross the planet-induced caustic. Using the measured event timescale and the microlens parallax, we find that the masses of the planet and the host are Mp=9.0+9.0−4.5 M⊕ and Mhost=0.20+0.20−0.10 M⊙, respectively, and the projected separation between them is a⊥=1.8+0.6−0.7 au. The estimated distance to the lens is DL=5.8+1.8−2.1 kpc. The discovery of the planetary system demonstrates that microlensing provides an important method to detect low-mass planets orbiting low-mass stars.

MOA-2015-BLG-337: A Planetary System with a Low-mass Brown Dwarf/Planetary Boundary Host, or a Brown Dwarf Binary
https://arxiv.org/abs/1804.00830

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We report the discovery and the analysis of the short timescale binary-lens microlensing event, MOA-2015-BLG-337. The lens system could be a planetary system with a very low mass host, around the brown dwarf/planetary mass boundary, or a brown dwarf binary. We found two competing models that explain the observed light curves with companion/host mass ratios of q~0.01 and ~0.17, respectively. From the measurement of finite source effects in the best-fit planetary model, we find a relatively small angular Einstein radius of theta_E ~ 0.03 mas which favors a low mass lens. We conduct a Bayesian analysis to obtain the probability distribution of the lens properties. The results for the planetary models strongly depend on the minimum mass, M_min, in the assumed mass function. In summary, there are two solutions of the lens system: (1) a brown dwarf/planetary mass boundary object orbited by a super-Neptune (the planetary model with M_min=0.001 M_sun) and (2) a brown dwarf binary (the binary model). If the planetary models is correct, this system can be one of a new class of planetary system, having a low host mass and also a planetary mass ratio (q <0.03) between the host and its companion. The discovery of the event is important for the study of planetary formation in very low mass objects. In addition, it is important to consider all viable solutions in these kinds of ambiguous events in order for the future comprehensive statistical analyses of planetary/binary microlensing events.

Hey we've got parallaxes to microlensing planetary systems from Gaia DR2.

Microlensing planets in the light of the second data release of Gaia
https://arxiv.org/abs/1804.10136

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Context. Extrasolar planets found by gravitational microlensing often require assumptions on the source star distance and relative proper motion. Only in a few cases has it been possible to confirm these findings with space-based observations or high-resolution follow-up. 25 planetary microlensing events can be positionally cross-matched with the second Gaia data release containing parallax and proper motion measurements Aims. In this work we subject all microlensing planets listed in the NASA's Exoplanet Archive to a consistency check by comparing them with Gaia data release 2 measurements. The resulting list is supposed to serve as a reference for the microlensing community and to test if standard assumptions that are made in microlensing studies hold. Methods. Gravitational microlensing can constrain the physical parameters lens mass and lens distance based on fit parameters, such as the event timescale, the microlensing parallax, and the source star crossing time. If some of these parameters are not available one needs to resort to indirect means of assessing the events, often involving a Galactic model. In this work, we seek to make an assessment of those parameters solely based on Gaia DR2. Results. We find that 19 of 25 planets are consistent within 2-sigma of their published lens and source distances. Changing the lens distance of the remaining 6 published planets could lead to revised parameter values that are accordingly compatible with the published ones. We expect the incompatible planet mass estimates to belong to a blend star.