key: cord-0308236-r0l3slam
authors: Han, Cheongho; Gould, Andrew; Albrow, Michael D.; Chung, Sun-Ju; Hwang, Kyu-Ha; Jung, Youn Kil; Kim, Doeon; Lee, Chung-Uk; Mao, Shude; Ryu, Yoon-Hyun; Shin, In-Gu; Shvartzvald, Yossi; Yee, Jennifer C.; Zang, Weicheng; Cha, Sang-Mok; Kim, Dong-Jin; Kim, Hyoun-Woo; Kim, Seung-Lee; Lee, Dong-Joo; Lee, Yongseok; Park, Byeong-Gon; Pogge, Richard W.; Kim, Chun-Hwey
title: KMT-2018-BLG-1988Lb: microlensing super-Earth orbiting a low-mass disk dwarf
date: 2021-11-17
journal: nan
DOI: 10.1051/0004-6361/202142077
sha: c35821cabda4863bcda793aeb68d3a97a88cb018
doc_id: 308236
cord_uid: r0l3slam

We reexamine high-magnification microlensing events in the previous data collected by the KMTNet survey with the aim of finding planetary signals that were not noticed before. In this work, we report the planetary system KMT-2018-BLG-1988L that was found from this investigation. The planetary signal appears as a deviation with $lesssim 0.2$~mag from a single-lens light curve and lasted for about 6 hours. The deviation exhibits a pattern of a dip surrounded by weak bumps on both sides of the dip. The analysis of the lensing light curve indicates that the signal is produced by a low mass-ratio ($qsim 4times 10^{-5}$) planetary companion located near the Einstein ring of the host star. The mass of the planet, $M_{rm planet}=6.8^{+4.7}_{-3.5}~M_oplus$ and $5.6^{+3.8}_{-2.8}~M_oplus$ for the two possible solutions, estimated from the Bayesian analysis indicates that the planet is in the regime of a super-Earth. The host of the planet is a disk star with a mass of $M_{rm host} = 0.47^{+0.33}_{-0.25}~M_odot$ and a distance of $D_{rm L}= 4.2^{+1.8}_{-.14}$~kpc. KMT-2018-BLG-1988Lb is the seventeenth microlensing planet with a mass below the upper limit of a super-Earth. The fact that 14 out of 17 microlensing planets with masses $lesssim 10~M_oplus$ were detected during the last 5 years since the full operation of the KMTNet survey indicates that the KMTNet database is an important reservoir of very low-mass planets.

Microlensing searches for planets are being conducted by inspecting light curves of more than 3000 lensing events, which are annually detected by massive lensing surveys monitoring stars lying in the Galactic bulge field. Planetary signals in lensing light curves appear as short-lasting perturbations to the lensing light curves produced by the planet hosts. Some planetary signals may escape detection for two major reasons. The first is the short duration of a signal relative to the cadence of observations. The duration of a planetary signal becomes shorter as the mass ratio q between the planet and host decreases, and thus signals of lower-mass planets are more likely to be missed. The second cause arises due to the weakness of planetary signals. A majority of published planetary signals are produced by the crossings of source stars over planet-induced caustics, and the signals produced via this caustic-crossing channel tend to be strong. However, planetary signals produced via a non-caustic-crossing channel tend to be weak, and may escape detection (Zhu et al. 2014) . Planetary signals can also be weak if lensing events are affected by severe finite-source effects or involve faint source stars.

Complete detections of planets including short and weak signals are important for the accurate estimation of the planet frequency and a solid demographic census of microlensing planetary systems. The basis for the statistical assessment of these planet properties is the detection efficiency, which is assessed as the ratio of the number of events with detected planets to the total number of lensing events, for example, Gould et al. (2010) . If a planetary signal is missed despite its strength being above the detection threshold, the efficiency would be underestimated, A&A proofs: manuscript no. ms and this would subsequently lead to the erroneous estimation of planet properties.

From a series of projects conducted during the Covid-19 time, microlensing data collected in previous years have been reinvestigated for the purpose of finding missing planets. The reinvestigation has been carried out via two approaches. The first approach is visually inspecting lensing light curves with weak or short anomaly features. From the visual reinvestigation of faint-source lensing events in the 2016-2017 season data, Han et al. (2020) reported four microlensing planets (KMT-2016 -BLG-2364Lb, KMT-2016 -BLG-2397Lb, OGLE-2017 -BLG-0604Lb, and OGLE-2017 , for which the planetary signals had not been noticed due to the weakness of the signals caused by the faintness of source stars. From the reexamination of the 2018-2019 season data, Han et al. (2021a,b) reported the detections of four planets (KMT-2018 -BLG-1025Lb, KMT-2018 -BLG-1976Lb, KMT-2018 -BLG-1996Lb, and OGLE-2019 , for which the planetary signals were missed due to their weakness caused by the non-caustic-crossing nature. The second approach was applying an automated algorithm to the previous data to search for buried signals of planets. Application of this algorithm to the 2018-2019 prime-field data of the Korea Microlensing Telescope Network (KMTNet: Kim et al. 2016 ) survey by Zang et al. (2021b) and Hwang et al. (2021) led to the discoveries of six planets (OGLE-2018-BLG-0977Lb, OGLE-2018-BLG-0506Lb, OGLE-2018-BLG-0516Lb, OGLE-2019-BLG-1492Lb, and KMT-2019-BLG-0253, OGLE-2019-BLG-1053) with weak and short signals. The automated searches for short and weak planetary signatures is planned to be extended to the data of the entire KMTNet fields.

In this work, we report the discovery of a super-Earth planet detected from the systematic reinvestigation of highmagnification lensing events conducted with the aim of finding missing planets in the 2018 season data obtained by the KMT-Net survey. The planet was not noticed before because its signal was not only weak but also short due to its non-caustic-crossing origin and the very small planet/host mass ratio.

For the presentation of the planet discovery, we organize the paper according to the following structure. In Sect. 2, we describe the observation of the lensing event and the data used in the analysis. In Sect. 3, we depict the procedure of the lensing light curve analysis and present the lens system configuration found from the analysis. We also mention the degeneracy encountered in the interpretation of the lensing event. In Sect. 4, we investigate the possibility of measuring higher-order lensing observables. In Sect. 5, we define the source type by measuring the color and magnitude and estimate the angular Einstein radius. In Sect. 6, we describe the Bayesian analysis conducted to estimate the physical lens parameters. We summarize the result and conclude in Sect. 7.

The planet was detected from the analysis of the lensing event KMT-2018 -BLG-1988 . The source star of the event is located toward the Galactic bulge field with equatorial coordinates (R.A., decl.) J2000 = (17:41:07.81, −35:35:40.92), which corresponds to the galactic coordinates (l, b) = (−6 • .168, −2 • .686).

The lensing event was detected from the post-season inspection of the 2018 KMTNet data using the EventFinder algorithm (Kim et al. 2018) . The source was located in the KMTNet field BLG37, which was covered with a 2.5 hour cadence. This cadence was substantially lower than that of the KMTNet prime fields, for which the cadence was 15 minutes. The field corresponds to the BLG610 field of the Optical Gravitational Lensing Experiment (OGLE: Udalski et al. 2015) , but no OGLE microlensing observation was conducted for this field in the 2018 season.

Observations of the event by the KMTNet survey were carried out using the three telescopes that are located at the Siding Spring Observatory (KMTA) in Australia, the Cerro Tololo Inter-American Observatory (KMTC) in Chile, and the South African Astronomical Observatory (KMTS) in South Africa. The telescopes are identical with a 1.6 m aperture, and each telescope is mounted with a camera yielding 2 × 2 deg 2 field of view. Images were taken mainly in the I band, and a fraction of V-band images were obtained for the source color measurement.

Reduction of the images and photometry of the source were done using the KMTNet pipeline developed by Albrow et al. (2009) . The pipeline is a customized version of the pySIS code established based on the difference imaging method (Tomaney & Crotts 1996; Alard & Lupton 1998) . The error bars of the photometry data assessed by the automatized pipeline were readjusted using the method described in Yee et al. (2012) in order for them to be consistent with the scatter of data and to make the χ 2 per degree of freedom for each data set unity. For a subset of KMTC I-and V-band data, additional photometry was done utilizing the pyDIA software (Albrow 2017) to construct the color-magnitude diagram (CMD) of stars around the source and to measure the reddening-and extinction-corrected (dereddened) source color and brightness. We describe the detailed procedure of the source color measurement in Sect. 5. Figure 1 shows the lensing light curve of KMT-2018-BLG-1988 constructed with the combined data from the three KMT-Net telescopes. At first glance, it appears to show a smooth and symmetric form of an event produced by a single-mass lens magnifying a single source (1L1S). Drawn over the data points is the 1L1S model curve with the lensing parameters (u 0 , t E ) ∼ (0.014, 50 days), where u 0 denotes the lens-source separation (scaled to the angular Einstein radius θ E ) at the time of the closest lens-source approach t 0 , and t E is the event time scale. The full lensing parameters of the 1L1S model is presented in Table 1 . The event was highly magnified with a peak magnification of A peak ∼ 1/u 0 ∼ 70. A close look at the region around the peak, shown in the inset of Figure 1 and the top panel of Figure 2 , reveals that there exists a short-term anomaly. The anomaly exhibits deviations with 0.2 mag from the 1L1S model and lasted for about 6 hours. The anomaly was found from the close inspection of the event conducted in the project of reexamining high-magnification events in the previous KMTNet data. In this project, high-magnification events are selected as targets for close examination due to their high sensitivities to planets (Griest & Safizadeh 1998) .

The signature of the anomaly comes mainly from the two KMTC points acquired at the epochs of HJD ′ ≡ HJD − 2450000 = 8210.719 and 8210.776, which exhibit deviations from the 1L1S model of ∆I = −0.182 mag (compared to the photometric uncertainty of σ I = 0.026 mag) and −0.132 mag (σ I = 0.026 mag), respectively. The KMTS point at HJD ′ = 8210.617 (with ∆I = +0.032 mag) and the KMTC point at HJD ′ = 8210.863 (with ∆I = +0.057 mag) also show deviations, although the signals are weaker than the previous two points. We designate the four points taken at HJD ′ = 8210.617, 8210.719, 8210.776, and 8210.863 as t 1 , t 2 , t 3 , and t 4 , respectively. The residuals from the 1L1S model, presented in the bottom panel of Figure 2 , show that the points at t 2 and t 3 manifest negative deviations, while the data points at t 1 and t 4 exhibit slight positive deviations, and thus the anomaly appears to be a dip surrounded by weak bumps on both sides of the dip.

We checked whether the anomaly were attributed to some kinds of correlated noise, for example, fake signals due to correlations with seeing. For this check, we examined the image quality of these data points and compared the quality with that of adjacent data points. The upper three panels of Figure 3 show the difference images obtained by subtracting a template image taken before the lensing magnification from the object images of the three anomalous KMTC points (at t 2 , t 2 , and t 3 ), and they are compared to the other three images taken about one day after the anomaly (at HJD ′ =8211.718, 8211.776, and 8211.863). In the panels, we insert the seeing (FWHM) values of the individual images. We find that the quality of the images taken during the anomaly is similar to that of the images obtained after the anomaly, indicating that there is no noticeable correlations with seeing. There exists a saturated bright star on the lower left side of the source, but we find that it does not affect the photometry of the source. Therefore, we conclude that the anomaly is real.

The fact that the anomaly lasted for a short period of time and it occurred near the peak of a high-magnification event suggests the possibility that the anomaly was produced by a planetary companion. The fact that the anomaly does not exhibit a sharp spike indicates that the source did not cross a planetinduced caustic. The anomaly pattern characterized by a dip surrounded by shallow bumps suggests that the anomaly was produced by the source star's passage through the planet-host axis on the opposite side of the planet. . The possibility of a binary-source interpretation (Gaudi 1998; Gaudi & Han 2004) for the origin of the anomaly is excluded because a source companion produces only positive deviations, while the observed anomaly exhibits a negative deviation.

Considering the possible planet origin of the anomaly, we conduct a binary-lens (2L1S) modeling for the observed light curve. A 2L1S lensing light curve is described by 7 parameters: t 0 , u 0 , t E , s, q, α, and ρ. The first three (t 0 , u 0 , t E ) are 1L1S parameters describing the lens-source approach, and the next three pa- 

Notes. HJD ′ = HJD − 2450000.

rameters (s, q, α) characterize the binary lens, denoting the projected separation (scaled to θ E ) and mass ratio between the lens components (M 1 and M 2 ), and the source trajectory angle defined as the angle between the direction of the source motion and M 1 -M 2 axis, respectively. The last parameter (normalized source radius), defined as the ratio of the angular source radius θ * to the Einstein radius, that is, ρ = θ * /θ E , is included to account for finite-source effects during caustic crossings, although the anomaly is unlikely to be produced by a caustic crossing.

In the 2L1S modeling, we divide the lensing parameters into two groups, in which (s, q) in the first group are searched for using a grid approach, while the other parameters in the second group are searched for via a downhill approach. We use the Markov Chain Monte Carlo (MCMC) algorithm for the downhill approach. Because a central anomaly can be produced not only by a planetary companion lying near the Einstein ring but also by a wide or a close binary companion with a similar mass to that of the primary lens (Han & Gaudi 2008) , we set the ranges of the grid parameters s and q wide enough, −1.0 ≤ log s < 1.0 and −5.5 ≤ log q < 1.0, to check the binary origin of the anomaly. From this first-round modeling, we identify local solutions in the ∆χ 2 map on the s-q plane. In the second-round modeling, we refine the identified local solutions by releasing all parameters as free parameters.

In Table 1 , we list the lensing parameters of the solutions found from the 2L1S modeling conducted under the assumption of a rectilinear relative lens-source motion (standard model). We identify two local solutions with s close ∼ 0.97 (close solution) and s wide ∼ 1.01 (wide solution), indicating that the companion to the lens is located very close to the Einstein ring regardless of the solution. The estimated lens mass ratio is of the order of 10 −5 , indicating that the companion to the lens is a very lowmass planet. The model curves of the solutions are drawn over the data points and the residuals from the solutions are presented in Figure 2 . Both the close and wide 2L1S models explain all the anomalous points, improving the fit with ∆χ 2 ∼ 80 relative to the 1L1S model.

In Figure 4 , we present the lens system configurations, which show the source trajectory (the curve with an arrow) with respect to the caustic (red concave closed figure), of the close (upper panel) and wide (lower panel) solutions. We note that the presented configurations are for the models considering higherorder effects causing non-rectilinear lens-source motion to be discussed below, but the configurations of the standard models are very similar to the presented ones. As predicted from the pattern and location, the anomaly was generated by the passage of the source through the negative anomaly region formed around the planet-host axis on the opposite side of the planet. We mark the positions of the source at the four epochs of the anomaly: t 1 , t 2 , t 3 , and t 4 . The two KMTC data points at t 2 and t 3 correspond to the epochs of the source passage through the negative perturbation region, and the other two points at t 1 and t 4 correspond to the epochs when the source passed the positive perturbation regions extending from the back-end cusps of the caustic.

The degeneracy between the close and wide solutions is very severe with ∆χ 2 = 0.1. The type of the degeneracy is similar to that identified by Yee et al. (2021) , who found that there existed a continuous transition between the two well-known types of degeneracy in planetary lensing events: "close-wide" (Griest & Safizadeh 1998; Dominik 1999; An 2005) and "innerouter" (Gaudi & Gould 1997) Han et al.: KMT-2018 : microlensing super-Earth orbiting a low-mass disk dwarf Fig. 5 . Scatter plots on the u 0 -ρ (left panel) and π E,E -π E,N (right panel) planes. The color coding is set to represent points with < 1σ (red), < 2σ (yellow), < 3σ (green), < 4σ (cyan), and < 5σ (blue). panion induces two types of caustics, in which one is located close to the planet host (central caustic) and the other lies away from the host (planetary caustic). The close-wide degeneracy arises due to the similarity between the two central caustics of the solutions with s < 1.0 and s > 1.0. The inner-outer degeneracy arises due to the similarity between the two light curves resulting from the source trajectories passing the inner (with respect to the primary) and outer region of the planetary caustic. From the geometry of the source trajectory with respect to the central caustic, on one hand, the light curve of KMT-2018-BLG1988 is subject to the close-wide degeneracy because the source approached the central caustic. From the geometry of the source trajectory relative to the planetary caustic, on the other hand, the light curve is also subject to the inner-outer degeneracy because the source passed the outer (right) and inner (left) region of the planetary caustic, as shown in the insets of the individual panels of Figure 4 . As a result, the degeneracy is caused by the combination of the two types of degeneracy.

The physical parameters of the mass M and distance D L to the lens can be constrained by measuring the lensing observables including the event time scale t E , angular Einstein radius θ E , and microlens-parallax π E . The first two observables are related to M and D L by

where µ represents the relative lens-source proper motion, κ = 4G/(c 2 AU), π rel = AU(D −1 L − D −1 S ) is the relative lens-source parallax, and D S denotes the distance to the source star. The additional measurement of π E would enable one to uniquely determine M and D L by

where π S = AU/D S represents the annual parallax of the source star (Gould 2000) . The event time scale is measured from the light curve modeling and presented in Table 1 . The Einstein radius is estimated by θ E = θ * /ρ, where ρ is measured from the analysis of the deviations in the lensing light curve caused by finite-source effects, and θ * is deduced from the source color and brightness. We will discuss in detail about the procedure of θ * determination in the following section. The microlens parallax is measured from the deviation of the light curve caused by the non-rectilinear lens-source motion induced by the orbital motion of Earth around the Sun (Gould 1992) . For the measurement π E , we conduct an additional modeling considering the microlensparallax effect. In this modeling, we also consider the orbital motion of the lens (Dominik 1998) , which is known to produce similar deviations to those induced by the parallax effect (Batista et al. 2011) . The inclusion of the higher-order effects in modeling requires one to include two extra pairs of lensing parameters: (π E,E , π E,N ) and (ds/dt, dα/dt). The first pair denote the north and east components of the microlens parallax vector π E = (π rel /θ E )(µ/µ), respectively, and the other pair represent the annual change rates of the binary separation and source trajectory angle, respectively. The lensing parameters obtained from the modeling considering the higher-order effects (higher-order 2L1S model) are listed in Table 1 . We find that the consideration of the higherorder effects improves the fit by ∆χ 2 ∼ 11 with respect to the standard models. It is found that finite-source effects cannot be firmly detected because the source did not cross the caustic. Nevertheless, the upper limit of ρ can be placed. The left panel of Figure 5 shows the distribution of points in the MCMC chain (scatter plot) on the u 0 -ρ parameter plane. We set a conservative upper limit as ρ max = 0.003. The right panel of Figure 5 shows the scatter plot on the π E,E -π E,N plane. It shows that the uncertainties of the parallax parameters are big, especially the north component π E,N . Similarly, the uncertainties of the orbital parameters are considerable.

For the estimation of the angular Einstein radius, we specify the source type by measuring its dereddened color and brightness, (V − I, I) 0 . In order to measure (V − I, I) 0 from the uncalibrated values, (V − I, I), in the instrumental color-magnitude diagram (CMD), we use the Yoo et al. (2004) method, in which the centroid of red giant clump (RGC) is used for calibration. Figure 6 shows the source location (empty circle with error bars) with respect to the RGC centroid (red filled dot) in the instrumental CMD of stars around the source constructed using the pyDIA photometry of the KMTC I-and V-band data. Also marked is the location of the blend (green filled dot). The 

indicating that the spectral type of the source is K2V.

Article number, page 5 of 8 A&A proofs: manuscript no. ms The angular source radius is estimated by first converting the measured V − I color into V − K color using the color-color relation of Bessell & Brett (1988) , and then deduce θ * from the (V − K)-θ * relation of Kervella et al. (2004) . From this procedure, it is estimated that the source has an angular radius of θ * = 0.650 ± 0.061 µas.

(4)

Combined with the measured event time scale and the upper limit of the normalized source radius, the lower limits of the Einstein radius and the relative lens-source proper motion are set as

and µ min = θ E,min t E = 1.62 mas yr −1 ,

respectively. We check whether a significant fraction of the blended light comes from the lens. The flux from a lens contributes to the blended flux, and thus if the lens is bright enough, the lens can be additionally constrained by analyzing the blended flux, for example, the planetary events OGLE-2018-BLG-1269 (Jung et al. 2020a ) and OGLE-2018-BLG-0740 (Han et al. 2019) . For this check, we measure the astrometric offset between the baseline object and the source. The position of the source is measured from the difference images obtained during lensing magnifications. The measured offsets in the east and north directions are ∆θ(E, N) = (37.2 ± 5.6, 370.0 ± 4.8) mas. The offset is well beyond the measurement error, indicating that the blended flux is likely to come from an adjacent star unrelated to lensing. Considering that the blend lies on the main-sequence branch of disk stars, it is unlikely that the blend is a binary companion to the source which lies in the bulge. This is also supported by the fact that the lens brightness expected from the mass and distance, to be estimated in the next section, is much fainter than the brightness of the blend.

We estimate the physical parameters of the planetary system based on the measured lensing observables. Among the lensing observables of (t E , θ E , π E ), the event time scale is relatively well constrained, but the uncertainty of the microlens parallax is substantial, and only the upper limit of the Einstein radius is constrained, making it difficult to uniquely determine M and D L using the relation in Equation (2). We, therefore, estimate the physical lens parameters by conducting a Bayesian analysis using the constraints from the observables together with a prior Galactic model. In the Bayesian analysis, we first conduct a Monte Carlo simulation to produce a large number (2 × 10 6 ) of artificial lensing events, for which the locations of the lens and source, lenssource transverse speeds, v ⊥ , and lens masses are derived from the Galactic model. The Galactic model is constructed by adopting the Han & Gould (2003) physical distribution, Han & Gould (1995) dynamical distribution, and Zhang et al. (2020) mass function of Galactic objects. The further description about the Galactic model is found in Jung et al. (2021) . For the individual simulated events, we compute lensing observables by t E = D L θ E /v ⊥ , θ E = (κMπ rel ) 1/2 , and π E = π rel /θ E , and then construct

MichaelDAlbrow/pyDIA: Initial Release on

Acknowledgements. Work by C.H. was supported by the grants of National Research Foundation of Korea (2020R1A4A2002885 and 2019R1A2C2085965). This research has made use of the KMTNet system operated by the Korea Astronomy and Space Science Institute (KASI) and the data were obtained at three host sites of CTIO in Chile, SAAO in South Africa, and SSO in Australia.

Cheongho Han et al.: KMT-2018 : microlensing super-Earth orbiting a low-mass disk dwarf Figure 7 shows the posterior distributions of the planet host mass and distance to the planetary system. In Table 2 , we summarize the masses of the host, M host , and planet, M planet , distance, and projected host-planet separation, a ⊥ , estimated based on the lensing parameters of the close and wide solutions. For each parameter, the median of the Bayesian distribution is taken as a represent value, and the lower and upper limits are set as the 16% and 84% of the distribution, respectively. The estimated mass, M planet = 6.8 +4.7 −3.5 M ⊕ for the close solution and 5.6 +3.8 −2.8 M ⊕ for the wide solution, indicates that the planet is in the regime of a super-Earth, which is defined as a planet with a mass higher than the mass of Earth, but substantially lower than those of the Solar System's ice giants, that is, Uranus and Neptune (Valencia et al. 2007 ). The mass of the planet host, M host = 0.47 +0.33 −0.25 M ⊙ , indicates that the host is a late K or an early M dwarf. The estimated distance to the lens, D L = 4.2 +1.8 −.14 kpc, suggests that the lens is likely to lie in the disk. According to the Bayesian estimation of the disk (blue curve in Figure 7 ) and bulge (red curve) contributions, the probability of a disk lens is ∼ 82%. We note that the uncertainties of the estimated physical lens parameters are considerable due to the combination of the statistical nature of the Bayesian analysis and the weak constraints of lensing observables. Furthermore, the estimated parameters can vary depending on adopted models (Yang et al. 2021 ).The planetary system KMT-2018-BLG-1988L demonstrates the importance of high-cadence microlensing surveys in finding low-mass planets. By setting the upper mass limit of a super-Earth as ∼ 10 M ⊕ , there exist 17 microlensing planetary systems with planet masses less than this upper limit, including the system reported in this work. In Table 3 , we list these planets discovered so far from the lensing surveys that have been conducted for almost three decades. In the table, we list the masses of planets and hosts together with the references and brief comments for notable systems. We note that the list is arranged according to the chronological order of the lensing event discovery not by the times of the planet discovery. Among these discovered planets, 14 planets have been found since 2016, when the KMT-Net survey conducted its full operation, indicating that the KMT-Net database is an important reservoir of low-mass microlensing planets.

We reported the super-Earth planet KMT-2018-BLG-1988L detected from the reinvestigation of high-magnification events in the previous microlensing data collected by the KMTNet survey. The planetary signal appeared as a deviation with 0.2 mag from a 1L1S curve and lasted for about 6 hours. The analysis of the lensing light curve indicated that the signal was produced by a low mass-ratio planetary companion located at around the Einstein ring of the host star. The mass of the planet, M planet = 6.8 +4.7 −3.5 M ⊕ and 5.6 +3.8 −2.8 M ⊕ for the two possible solutions, estimated from the Bayesian analysis indicated that the planet was in the regime of a super-Earth. The planet belonged to a disk star with a mass of M host = 0.47 +0.33 −0.25 M ⊙ located at a distance of D L = 4.2 +1.8 −.14 kpc. KMT-2018-BLG-1988L is the seventeenth microlensing planet with a mass below the upper limit of a super-Earth. The fact that 14 out of 17 planets with masses 10 M ⊕ were detected during the last 5 years since the full operation of the KMTNet survey indicates that the KMTNet database is an important reservoir of very low-mass planets.