arXiv subjects

Matthew T. Penny

Publications and source records attributed to Matthew T. Penny.

At least 19 recordsLinked to original sources

arXiv2026

Prospects for Reconstructing the Free-floating Planet Mass Function at the Population Level with the Nancy Grace Roman Space Telescope

Free-floating planets comprise one of the most enigmatic populations of exoplanets in the Galaxy. Though ground-based observations point to a large abundance of these worlds, little is known about their origins and demographics. In the coming years, the Nancy Grace Roman Space Telescope's Galactic Bulge Time Domain Survey is expected to detect several hundred free-floating planets, providing the first opportunity to characterize these worlds at the population level. We present a first study of Roman's prospects for reconstructing the mass distribution of free-floating planets through population-level statistical inference. We find that depending on the true underlying mass distribution of free-floating planets in the Galaxy, Roman will be able to improve upon existing estimates of the abundance by orders of magnitudes in the largely unexplored mass range below that of Earth. When applied to Roman's observations, the methodology we present herein will be capable of discriminating between different hypothesized mass distributions at high statistical significance, opening a new window into the origins of these rogue worlds.

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arXiv2026

An Updated SynthPop Model for Microlensing Simulations I: Model Description & Evaluation

The optimization and interpretation of microlensing surveys depends on having an accurate model of the Milky Way. However, existing population synthesis Galactic modeling tools often perform poorly in replicating the stellar contents of the inner Galactic bulge region and reproducing microlensing survey results. We present an updated Galactic model implementation within the \synthpop framework that has been tuned for simulating the upcoming {\it Nancy Grace Roman Space Telescope}'s Galactic Bulge Time Domain Survey (RGBTDS). We evaluate the model against stellar catalogs and kinematics from optical and infrared surveys toward the Galactic bulge, finding good agreement in much of the bulge, including the RGBTDS' contiguous lower bulge fields. However, within Galactic latitudes of $b\lesssim0.5^\circ$ of the Galactic plane, some inconsistencies arise which may impact projections for the RGBTDS' Galactic center field. The model over-predicts optical microlensing event rate per star measurements by a $\sim20$\%, but detailed comparisons to near-infrared measurements are hampered by their lack of detection efficiencies. {\it Roman}'s GBTDS and Galactic Plane Survey will be instrumental in resolving the remaining model inconsistencies and improving our understanding of the structure of the central few degrees of our Galaxy.

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arXiv2026

An HST Wide Field Survey of the Galactic Bulge: Overview, Strategy, and First Results

We present an HST imaging survey of a 1.1 sq. degree sky area toward the Milky Way Galactic Bulge. This field significantly overlaps with the upcoming Nancy Grace Roman Galactic Bulge Time Domain Survey (GBTDS). High angular resolution imaging of this area with HST before the start of the Roman Galactic Exoplanet Survey (RGES) will greatly strengthen Roman's ability to characterize detected exoplanet systems, as well as provide a rich and wide-field archive for use as a legacy dataset toward the Galactic Bulge for the broader community. We conduct coordinated-parallel imaging with both wide-field cameras on HST, Wide-field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS), utilizing the F606W and F814W passbands. Approximately 70% of the survey was conducted during HST Cycle 32, with the remaining 30% conducted during Cycle 33. In this paper, the first in a series, we give a general overview of the program and the observing strategy, and present early results. This campaign secures HST's lasting impact on the high-precision study of stellar populations, dynamics, exoplanet systems, interstellar extinction, metallicities, cluster associations, and more toward the center of our Galaxy.

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arXiv2025

Predictions of the Nancy Grace Roman Space Telescope Galactic Exoplanet Survey. III. Detectability of Giant Exomoons of Wide Separation Giant Planets

The Nancy Grace Roman Space Telescope (Roman) will conduct a Galactic Exoplanet Survey (RGES) to discover bound and free-floating exoplanets using gravitational microlensing. Roman should be sensitive to lenses with mass down to ~ 0.02 $M_{\oplus}$, or roughly the mass of Ganymede. Thus the detection of moons with masses similar to the giant moons in our Solar System is possible with Roman. Measuring the demographics of exomoons will provide constraints on both moon and planet formation. We conduct simulations of Roman microlensing events to determine the effects of exomoons on microlensing light curves, and whether these effects are detectable with Roman. We focus on giant planets from 30 $M_{\oplus}$ to 10 $M_{Jup}$ on orbits from 0.3 to 30 AU, and assume that each planet is orbited by a moon with moon-planet mass ratio from $10^{-4}$ to $10^{-2}$ and separations from 0.1 to 0.5 planet Hill radii. We find that Roman is sensitive to exomoons, although the number of expected detections is only of order one over the duration of the survey, unless exomoons are more common or massive than we assumed. We argue that changes in the survey strategy, in particular focusing on a few fields with higher cadence, may allow for the detection of more exomoons with Roman. Regardless, the ability to detect exomoons reinforces the need to develop robust methods for modeling triple lens microlensing events to fully utilize the capabilities of Roman.

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arXiv2025

The Multiband Imaging Survey for High-Alpha PlanetS (MISHAPS) I: Preliminary Constraints on the Occurrence Rate of Hot Jupiters in 47 Tucanae

The first generation of transiting planet searches in globular clusters yielded no detections, and in hindsight, only placed occurrence rate limits slightly higher than the measured occurrence rate in the higher-metallicity Galactic thick disk. To improve these limits, we present the first results of a new wide field search for transiting hot Jupiters in the globular cluster 47~Tucanae. We have observed 47~Tuc as part of the Multiband Imaging Survey for High-Alpha Planets (MISHAPS). Using 24 partial and full nights of observations taken with the Dark Energy Camera on the 4-m Blanco telescope at CTIO, we perform a search on 19,930 stars in the outer regions of the cluster. Though we find no clear planet detections, by combining our result with the upper limit enabled by Gilliland et al.'s 2000 Hubble search for planets around an independent sample of 34,091 stars in the inner cluster, we place the strongest limit to date on hot Jupiters with periods of $0.8 \leq P \leq 8.3$ days and $0.5~R_{\rm Jup} \leq R_{\rm P} \leq 2.0~R_{\rm Jup}$ of $f_{\rm HJ} < 0.11\%$, a factor of ${\sim}$4 below the occurrence rate in the \textit{Kepler} field. Our search found 35 transiting planet candidates, though we are ultimately able to rule out each without follow-up observations. We also found 4 eclipsing binaries, including 3 previously-uncataloged detached eclipsing binary stars.

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arXiv2025

MOA-2020-BLG-135Lb: A New Neptune-class Planet for the Extended MOA-II Exoplanet Microlens Statistical Analysis

We report the light-curve analysis for the event MOA-2020-BLG-135, which leads to the discovery of a new Neptune-class planet, MOA-2020-BLG-135Lb. With a derived mass ratio of $q=1.52_{-0.31}^{+0.39} \times 10^{-4}$ and separation $s\approx1$, the planet lies exactly at the break and likely peak of the exoplanet mass-ratio function derived by the MOA collaboration (Suzuki et al. 2016). We estimate the properties of the lens system based on a Galactic model and considering two different Bayesian priors: one assuming that all stars have an equal planet-hosting probability and the other that planets are more likely to orbit more massive stars. With a uniform host mass prior, we predict that the lens system is likely to be a planet of mass $m_\mathrm{planet}=11.3_{-6.9}^{+19.2} M_\oplus$ and a host star of mass $M_\mathrm{host}=0.23_{-0.14}^{+0.39} M_\odot$, located at a distance $D_L=7.9_{-1.0}^{+1.0}\;\mathrm{kpc}$. With a prior that holds that planet occurrence scales in proportion to the host star mass, the estimated lens system properties are $m_\mathrm{planet}=25_{-15}^{+22} M_\oplus$, $M_\mathrm{host}=0.53_{-0.32}^{+0.42} M_\odot$, and $D_L=8.3_{-1.0}^{+0.9}\; \mathrm{kpc}$. This planet qualifies for inclusion in the extended MOA-II exoplanet microlens sample.

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arXiv2023

KMT-2021-BLG-1547Lb: Giant microlensing planet detected through a signal deformed by source binarity

We investigate the previous microlensing data collected by the KMTNet survey in search of anomalous events for which no precise interpretations of the anomalies have been suggested. From this investigation, we find that the anomaly in the lensing light curve of the event KMT-2021-BLG-1547 is approximately described by a binary-lens (2L1S) model with a lens possessing a giant planet, but the model leaves unexplained residuals. We investigate the origin of the residuals by testing more sophisticated models that include either an extra lens component (3L1S model) or an extra source star (2L2S model) to the 2L1S configuration of the lens system. From these analyses, we find that the residuals from the 2L1S model originate from the existence of a faint companion to the source. The 2L2S solution substantially reduces the residuals and improves the model fit by $Δχ^2=67.1$ with respect to the 2L1S solution. The 3L1S solution also improves the fit, but its fit is worse than that of the 2L2S solution by $Δχ^2=24.7$. According to the 2L2S solution, the lens of the event is a planetary system with planet and host masses $(M_{\rm p}/M_{\rm J}, M_{\rm h}/M_\odot)=\left( 1.47^{+0.64}_{-0.77}, 0.72^{+0.32}_{-0.38}\right)$ lying at a distance $\D_{\rm L} =5.07^{+0.98}_{-1.50}$~kpc, and the source is a binary composed of a subgiant primary of a late G or an early K spectral type and a main-sequence companion of a K spectral type. The event demonstrates the need of sophisticated modeling for unexplained anomalies for the construction of a complete microlensing planet sample.

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arXiv2023

Transiting Exoplanet Yields for the Roman Galactic Bulge Time Domain Survey Predicted from Pixel-Level Simulations

The Nancy Grace Roman Space Telescope (Roman) is NASA's next astrophysics flagship mission, expected to launch in late 2026. As one of Roman's core community science surveys, the Galactic Bulge Time Domain Survey (GBTDS) will collect photometric and astrometric data for over 100 million stars in the Galactic bulge to search for microlensing planets. To assess the potential with which Roman can detect exoplanets via transit, we developed and conducted pixel-level simulations of transiting planets in the GBTDS. From these simulations, we predict that Roman will find between $\sim$60,000 and $\sim$200,000 transiting planets, over an order of magnitude more planets than are currently known. While the majority of these planets will be giants ($R_p>4R_\oplus$) on close-in orbits ($a<0.3$ au), the yield also includes between $\sim$7,000 and $\sim$12,000 small planets ($R_p<4 R_\oplus$). The yield for small planets depends sensitively on the observing cadence and season duration, with variations on the order of $\sim$10-20% for modest changes in either parameter, but is generally insensitive to the trade between surveyed area and cadence given constant slew/settle times. These predictions depend sensitively on the Milky Way's metallicity distribution function, highlighting an opportunity to significantly advance our understanding of exoplanet demographics, particularly across stellar populations and Galactic environments.

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arXiv2023

Systematic KMTNet Planetary Anomaly Search. X. Complete Sample of 2017 Prime-Field Planets

We complete the analysis of planetary candidates found by the KMT AnomalyFinder for the 2017 prime fields that cover $\sim 13\,{\rm deg}^2$. We report 3 unambiguous planets: OGLE-2017-BLG-0640, OGLE-2017-BLG-1275, and OGLE-2017-BLG-1237. The first two of these were not previously identified, while the last was not previously published due to technical complications induced by a nearby variable. We further report that a fourth anomalous event, the previously recognized OGLE-2017-BLG-1777, is very likely to be planetary, although its light curve requires unusually complex modeling because the lens and source both have orbiting companions. One of the 3 unambiguous planets, OGLE-2017-BLG-1275 is the first AnomalyFinder discovery that has a {\it Spitzer} microlens parallax measurement, $π_E \sim 0.045\pm0.015$, implying that this planetary system almost certainly lies in the Galactic bulge. In the order listed, the four planetary events have planet-host mass ratios $q$, and normalized projected separations $s$, of $(\log q,s)$ = $(-2.31,0.61)$, $(-2.06,0.63/1.09)$, $(-2.10,1.04)$, and $(-2.86,0.72)$. Combined with previously published events, the 2017 AnomalyFinder prime fields contain 11 unambiguous planets with well-measured $q$ and one very likely candidate, of which 3 are AnomalyFinder discoveries. In addition to these 12, there are three other unambiguous planets with large uncertainties in $q$.

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arXiv2023

MOA-2020-BLG-208Lb: Cool Sub-Saturn Planet Within Predicted Desert

We analyze the MOA-2020-BLG-208 gravitational microlensing event and present the discovery and characterization of a new planet, MOA-2020-BLG-208Lb, with an estimated sub-Saturn mass. With a mass ratio $q = 3.17^{+0.28}_{-0.26} \times 10^{-4}$ and a separation $s = 1.3807^{+0.0018}_{-0.0018}$, the planet lies near the peak of the mass-ratio function derived by the MOA collaboration (Suzuki et al. 2016), near the edge of expected sample sensitivity. For these estimates we provide results using two mass law priors: one assuming that all stars have an equal planet-hosting probability, and the other assuming that planets are more likely to orbit around more massive stars. In the first scenario, we estimate that the lens system is likely to be a planet of mass $m_\mathrm{planet} = 46^{+42}_{-24} \; M_\oplus$ and a host star of mass $M_\mathrm{host} = 0.43^{+0.39}_{-0.23} \; M_\odot$, located at a distance $D_L = 7.49^{+0.99}_{-1.13} \; \mathrm{kpc}$. For the second scenario, we estimate $m_\mathrm{planet} = 69^{+37}_{-34} \; M_\oplus$, $M_\mathrm{host} = 0.66^{+0.35}_{-0.32} \; M_\odot$, and $D_L = 7.81^{+0.93}_{-0.93} \; \mathrm{kpc}$. As a cool sub-Saturn-mass planet, this planet adds to a growing collection of evidence for revised planetary formation models and qualifies for inclusion in the extended MOA-II exoplanet microlensing sample.

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arXiv2023

KMT-2022-BLG-0440Lb: A New $q < 10^{-4}$ Microlensing Planet with the Central-Resonant Caustic Degeneracy Broken

We present the observations and analysis of a high-magnification microlensing planetary event, KMT-2022-BLG-0440, for which the weak and short-lived planetary signal was covered by both the KMTNet survey and follow-up observations. The binary-lens models with a central caustic provide the best fits, with a planet/host mass ratio, $q = 0.75$--$1.00 \times 10^{-4}$ at $1σ$. The binary-lens models with a resonant caustic and a brown-dwarf mass ratio are both excluded by $Δχ^2 > 70$. The binary-source model can fit the anomaly well but is rejected by the ``color argument'' on the second source. From Bayesian analyses, it is estimated that the host star is likely a K or M dwarf located in the Galactic disk, the planet probably has a Neptune-mass, and the projected planet-host separation is $1.9^{+0.6}_{-0.7}$ or $4.6^{+1.4}_{-1.7}$ au, subject to the close/wide degeneracy. This is the third $q < 10^{-4}$ planet from a high-magnification planetary signal ($A \gtrsim 65$). Together with another such planet, KMT-2021-BLG-0171Lb, the ongoing follow-up program for the KMTNet high-magnification events has demonstrated its ability in detecting high-magnification planetary signals for $q < 10^{-4}$ planets, which are challenging for the current microlensing surveys.

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arXiv2023

KMT-2021-BLG-2010Lb, KMT-2022-BLG-0371Lb, and KMT-2022-BLG-1013Lb: Three microlensing planets detected via partially covered signals

We inspect 4 microlensing events KMT-2021-BLG-1968, KMT-2021-BLG-2010, KMT-2022-BLG-0371, and KMT-2022-BLG-1013, for which the light curves exhibit partially covered short-term central anomalies. We conduct detailed analyses of the events with the aim of revealing the nature of the anomalies. We test various models that can give rise to the anomalies of the individual events including the binary-lens (2L1S) and binary-source (1L2S) interpretations. Under the 2L1S interpretation, we thoroughly inspect the parameter space to check the existence of degenerate solutions, and if they exist, we test the feasibility of resolving the degeneracy. We find that the anomalies in KMT-2021-BLG-2010 and KMT-2022-BLG-1013 are uniquely defined by planetary-lens interpretations with the planet-to-host mass ratios of $q\sim 2.8\times 10^{-3}$ and $\sim 1.6\times 10^{-3}$, respectively. For KMT-2022-BLG-0371, a planetary solution with a mass ratio $q\sim 4\times 10^{-4}$ is strongly favored over the other three degenerate 2L1S solutions with different mass ratios based on the $χ^2$ and relative proper motion arguments, and a 1L2S solution is clearly ruled out. For KMT-2021-BLG-1968, on the other hand, we find that the anomaly can be explained either by a planetary or a binary-source interpretation, making it difficult to firmly identify the nature of the anomaly. From the Bayesian analyses of the identified planetary events, we estimate that the masses of the planet and host are $(M_{\rm p}/M_{\rm J}, M_{\rm h}/M_\odot) = (1.07^{+1.15}_{-0.68}, 0.37^{+0.40}_{-0.23})$, $(0.26^{+0.13}_{-0.11}, 0.63^{+0.32}_{-0.28})$, and $(0.31^{+0.46}_{-0.16}, 0.18^{+0.28}_{-0.10})$ for KMT-2021-BLG-2010L, KMT-2022-BLG-0371L, and KMT-2022-BLG-1013L, respectively.

astro-ph.EP↗

arXiv2023

Systematic KMTNet Planetary Anomaly Search. VIII. Complete Sample of 2019 Subprime Field Planets

We complete the publication of all microlensing planets (and ``possible planets'') identified by the uniform approach of the KMT AnomalyFinder system in the 21 KMT subprime fields during the 2019 observing season, namely KMT-2019-BLG-0298, KMT-2019-BLG-1216, KMT-2019-BLG-2783, OGLE-2019-BLG-0249, and OGLE-2019-BLG-0679 (planets), as well as OGLE-2019-BLG-0344, and KMT-2019-BLG-0304 (possible planets). The five planets have mean log mass-ratio measurements of $(-2.6,-3.6,-2.5,-2.2,-2.3)$, median mass estimates of $(1.81,0.094,1.16,7.12,3.34)\, M_{\rm Jup}$, and median distance estimates of $(6.7,2.7,5.9,6.4,5.6)\, {\rm kpc}$, respectively. The main scientific interest of these planets is that they complete the AnomalyFinder sample for 2019, which has a total of 25 planets that are likely to enter the statistical sample. We find statistical consistency with the previously published 33 planets from the 2018 AnomalyFinder analysis according to an ensemble of five tests. Of the 58 planets from 2018-2019, 23 were newly discovered by AnomalyFinder. Within statistical precision, half of all the planets have caustic crossings while half do not (as predicted by Zhu et al. 2014), an equal number of detected planets result from major-image and minor-image light-curve perturbations, and an equal number come from KMT prime fields versus subprime fields.

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arXiv2022

OGLE-2018-BLG-0799Lb: a $q \sim 2.7 \times 10^{-3}$ Planet with Spitzer Parallax

We report the discovery and analysis of a planet in the microlensing event OGLE-2018-BLG-0799. The planetary signal was observed by several ground-based telescopes, and the planet-host mass ratio is $q = (2.65 \pm 0.16) \times 10^{-3}$. The ground-based observations yield a constraint on the angular Einstein radius $θ_{\rm E}$, and the microlensing parallax vector $\vecπ_{\rm E}$, is strongly constrained by the Spitzer data. However, the 2019 Spitzer baseline data reveal systematics in the Spitzer photometry, so there is ambiguity in the magnitude of the parallax. In our preferred interpretation, a full Bayesian analysis using a Galactic model indicates that the planetary system is composed of an $M_{\rm planet} = 0.26_{-0.11}^{+0.22}~M_{J}$ planet orbiting an $M_{\rm host} = 0.093_{-0.038}^{+0.082}~M_{\odot}$, at a distance of $D_{\rm L} = 3.71_{-1.70}^{+3.24}$ kpc. An alternate interpretation of the data shifts the localization of the minima along the arc-shaped microlens parallax constraints. This, in turn, yields a more massive host with median mass of $0.13 {M_{\odot}}$ at a distance of 6.3 kpc. This analysis demonstrates the robustness of the osculating circles formalism, but shows that further investigation is needed to assess how systematics affect the specific localization of the microlens parallax vector and, consequently, the inferred physical parameters.

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arXiv2022

KMT-2021-BLG-0171Lb and KMT-2021-BLG-1689Lb: Two Microlensing Planets in the KMTNet High-cadence Fields with Followup Observations

Follow-up observations of high-magnification gravitational microlensing events can fully exploit their intrinsic sensitivity to detect extrasolar planets, especially those with small mass ratios. To make followup more uniform and efficient, we develop a system, HighMagFinder, based on the real-time data from the Korean Microlensing Telescope Network (KMTNet) to automatically alert possible ongoing high-magnification events. We started a new phase of follow-up observations with the help of HighMagFinder in 2021. Here we report the discovery of two planets in high-magnification microlensing events, KMT-2021-BLG-0171 and KMT-2021-BLG-1689, which were identified by the HighMagFinder. We find that both events suffer the ``central-resonant'' caustic degeneracy. The planet-host mass-ratio is $q\sim4.7\times10^{-5}$ or $q\sim 2.2\times10^{-5}$ for KMT-2021-BLG-0171, and $q\sim2.5\times10^{-4}$ or $q\sim 1.8\times10^{-4}$ for KMT-2021-BLG-1689. Together with two events reported by Ryu et al. (2022), four cases that suffer such degeneracy have been discovered in the 2021 season alone, indicating that the degenerate solutions may have been missed in some previous studies. We also propose a new factor for weighting the probability of each solution from the phase-space. The resonant interpretations for the two events are disfavored under this consideration. This factor can be included in future statistical studies to weight degenerate solutions.

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arXiv2021

Systematic KMTNet Planetary Anomaly Search, Paper II: Six New $q<2\times 10^{-4}$ Mass-ratio Planets

We apply the automated AnomalyFinder algorithm of Paper I (Zang et al. 2021b) to 2018-2019 light curves from the $\simeq 13\,{\rm deg}^2$ covered by the six KMTNet prime fields, with cadences $Γ\geq 2\,{\rm hr}^{-1}$. We find a total of 11 planets with mass ratios $q<2\times 10^{-4}$, including six newly discovered planets, one planet that was reported in Paper I, and recovery of four previously discovered planets. One of the new planets, OGLE-2018-BLG-0977Lb, is in a planetary-caustic event, while the other five (OGLE-2018-BLG-0506Lb, OGLE-2018-BLG-0516Lb, OGLE-2019-BLG-1492Lb, KMT-2019-BLG-0253, and KMT-2019-BLG-0953) are revealed by a "dip" in the light curve as the source crosses the host-planet axis on the opposite side of the planet. These subtle signals were missed in previous by-eye searches. The planet-host separations (scaled to the Einstein radius), $s$, and planet-host mass ratios, $q$, are, respectively, $(s,q\times 10^5) = (0.88, 4.1)$, $(0.96\pm 0.10, 8.3)$, $(0.94\pm 0.07, 13)$, $(0.97\pm 0.07, 18)$, $(0.97\pm0.04,4.1)$, and $(0.74,18)$, where the "$\pm$" indicates a discrete degeneracy. The 11 planets are spread out over the range $-5<\log q < -3.7$. Together with the two planets previously reported with $q\sim 10^{-5}$ from the 2018-2019 non-prime KMT fields, this result suggests that planets toward the bottom of this mass-ratio range may be more common than previously believed.

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arXiv2021

A Multi-Parameter Degeneracy in Microlensing Events with Extreme Finite Source Effects

For microlenses with sufficiently low mass, the angular radius of the source star can be much larger than the angular Einstein ring radius of the lens. For such extreme finite source effect (EFSE) events, finite source effects dominate throughout the duration of the event. Here, we demonstrate and explore a continuous degeneracy between multiple parameters of such EFSE events. The first component in the degeneracy arises from the fact that the directly-observable peak change of the flux depends on both the ratio of the angular source radius to the angular Einstein ring radius and the fraction of the baseline flux that is attributable to the lensed source star. The second component arises because the directly-observable duration of the event depends on both the impact parameter of the event and the relative lens-source proper motion. These two pairwise degeneracies become coupled when the detailed morphology of the light curve is considered, especially when including a limb-darkening profile of the source star. We derive these degeneracies mathematically through analytic approximations and investigate them further numerically with no approximations. We explore the likely physical situations in which these mathematical degeneracies may be realized and potentially broken. As more and more low-mass lensing events (with ever decreasing Einstein ring radii) are detected with improving precision and increasing cadence from microlensing surveys, one can expect that more of these EFSE events will be discovered. In particular, the detection of EFSE microlensing events could increase dramatically with the Roman Space Telescope Galactic Bulge Time Domain Survey.

astro-ph.HE↗

arXiv2021

An Earth-mass Planet in a Time of Covid-19: KMT-2020-BLG-0414Lb

We report the discovery of KMT-2020-BLG-0414Lb, with a planet-to-host mass ratio $q_2 = 0.9$--$1.2 \times 10^{-5} = 3$--$4~q_{\oplus}$ at $1σ$, which is the lowest mass-ratio microlensing planet to date. Together with two other recent discoveries ($4 \lesssim q/q_\oplus \lesssim 6$), it fills out the previous empty sector at the bottom of the triangular $(\log s, \log q)$ diagram, where $s$ is the planet-host separation in units of the angular Einstein radius $θ_{\rm E}$. Hence, these discoveries call into question the existence, or at least the strength, of the break in the mass-ratio function that was previously suggested to account for the paucity of very low-$q$ planets. Due to the extreme magnification of the event, $A_{\rm max}\sim 1450$ for the underlying single-lens event, its light curve revealed a second companion with $q_3 \sim 0.05$ and $|\log s_3| \sim 1$, i.e., a factor $\sim 10$ closer to or farther from the host in projection. The measurements of the microlens parallax $π_{\rm E}$ and the angular Einstein radius $θ_{\rm E}$ allow estimates of the host, planet, and second companion masses, $(M_1, M_2, M_3) \sim (0.3M_{\odot}, 1.0M_{\oplus}, 17M_{J})$, the planet and second companion projected separations, $(a_{\perp,2}, a_{\perp,3}) \sim (1.5, 0.15~{\rm or}~15)$~au, and system distance $D_{\rm L} \sim 1$ kpc. The lens could account for most or all of the blended light ($I \sim 19.3$) and so can be studied immediately with high-resolution photometric and spectroscopic observations that can further clarify the nature of the system. The planet was found as part of a new program of high-cadence follow-up observations of high-magnification events. The detection of this planet, despite the considerable difficulties imposed by Covid-19 (two KMT sites and OGLE were shut down), illustrates the potential utility of this program.

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