KMT-2016-BLG-1337L: A Saturn-mass planet orbiting within a binary system of low-mass stars

TL;DR

This study reports a microlensing discovery of a planetary companion in a low-mass stellar binary toward the Galactic bulge. A triple-lens single-source model provides the best fit, revealing two viable solutions: a Saturn-mass planet with $M_3\sim0.3\,M_J$ at $a_{\perp,3}\sim4$ au and a more massive planet with $M_3\sim7\,M_J$ at $a_{\perp,3}\sim1.5$ au, both orbiting a binary M-dwarf host ($M_1\sim0.54\,M_\odot$, $M_2\sim0.40\,M_\odot$) at $D_L\sim7$ kpc. The event underscores microlensing’s sensitivity to planets in dynamically complex environments and contributes to the demographic census of planets in multiple-star systems, thereby informing formation scenarios in binaries. Remaining degeneracies stem from caustic geometry and gaps in caustic-entrance coverage, highlighting the need for dense, multi-site observations in future events.

Abstract

We report the discovery and characterization of a planetary companion in the microlensing event KMT-2016-BLG-1337, which was produced by a binary system of low-mass stars. The light curve of the event exhibits a short-term anomaly superposed on the profile of a binary-lens single-source (2L1S) model. To investigate the nature of this anomaly, we performed detailed modeling under both the binary-lens binary-source (2L2S) and triple-lens single-source (3L1S) interpretations. The 3L1S model provides a substantially better fit to the data, strongly favoring the presence of a planetary companion in the lens system. Two viable $3L1S$ solutions describe the event nearly equally well. In one solution, the planet has a mass of $M_3 \sim 0.3~M_{\mathrm{J}}$ and lies at a projected separation of $a_{\perp,3} \sim 4~{\rm au}$ from the heavier member of the host binary. In the alternative solution, the planet has a mass of $M_3 \sim 7~M_{\mathrm{J}}$ and a projected separation of $a_{\perp,3} \sim 1.5~{\rm au}$. The host binary consists of early M-type dwarfs with masses of $M_1 \sim 0.54~M_\odot$ and $M_2 \sim 0.40~M_\odot$, separated in projection by $a_{\perp,2} \sim 3.5~{\rm au}$. The system is located at a distance of $D_{\rm L} \sim 7~{\rm kpc}$ toward the Galactic bulge. This event demonstrates the sensitivity of microlensing to planets in dynamically complex stellar environments, including systems beyond the reach of other detection techniques. It thereby contributes to a more comprehensive understanding of planet formation in multiple-star systems.

Figures (8)

• Figure 1: Light curve of the microlensing event KMT-2016-BLG-1337. The lower panel presents the full light curve of the event, with labeled arrows marking the times of the major anomaly features. The upper panels provide enlarged views of the regions around $t_1$, $t_3$, and $t_4$. The dotted and solid curves plotted over the data points represent the model fits for the binary-lens single-source (2L1S) and triple-lens single-source (3L1S: solution A) models, respectively. The colors of the data points correspond to the telescopes used for the observations, as indicated in the legend.
• Figure 2: Difference images taken during the planet-induced anomaly around $t_1$. The label in each panel indicates the time at which the image was obtained.
• Figure 3: Lens-system configurations of the 2L1S and 2L2S models. In each panel, the two blue dots mark the positions of the binary lens components $M_1$ and $M_2$, where the bigger dot indicates the heavier component. The red closed cuspy curve represents the caustic, and the arrowed line indicates the source trajectory. In the case of the 2L2S model, two source trajectories are shown, one corresponding to the primary source $S_1$ and the other to the secondary source $S_2$. The origin of the coordinate system is set at the barycenter of the lens, and spatial scales are given in units of the angular Einstein radius associated with the combined mass of the lens.
• Figure 4: Comparison of the two 3L1S models (solutions A and B) and the 2L2S model in the regions encompassing the three major anomalies around $t_1$, $t_3$, and $t_4$.
• Figure 5: Cumulative distribution of $\chi^2$ difference between the 2L2S and 3L1S model. The light curve in the upper panel is presented to show the region of $\chi^2$ difference.