KMT-2025-BLG-1616Lb: First Microlensing Bound Planet From DREAMS

TL;DR

This study demonstrates DREAMS' capability to identify and characterize a bound microlensing planet, KMT-2025-BLG-1616Lb, through high-cadence, precise photometry that breaks a central–resonant degeneracy. By combining KMTNet, PRIME, DREAMS data, and CMD-based source characterization with a Bayesian Galactic-prior analysis, the authors infer a bulge M-dwarf host ($\sim 0.26$–$0.3\,M_\odot$) with a Neptune-to-Saturn-mass planet ($\sim 42\,M_\oplus$) at a projected separation $a_{\perp} \sim 1.6\ \mathrm{AU}$, at $D_L \sim 7.5\ \mathrm{kpc}$. The analysis highlights DREAMS' role in resolving degeneracies and constraining source colors for faint events, while also outlining DREAMS' detection limits for Mars- and Moon-mass free-floating planets. These results advance our understanding of planetary demographics in the Galactic bulge and demonstrate DREAMS' potential to probe low-mass FFP populations in future surveys.

Abstract

We present observations and analysis of the bound planetary microlensing event KMT-2025-BLG-1616. The planetary signal was captured by the Korea Microlensing Telescope Network (KMTNet) and the DECam Rogue Earths and Mars Survey (DREAMS). DREAMS's minute-cadence observations break the central/resonant degeneracy in the binary-lens models. The color of the faint source star ($I=22$) is measured from the DREAMS's $r - z$ color. The planetary system has a planet-host mass ratio of $q \sim 5 \times 10^{-4}$. A Bayesian analysis yields a host-star mass of $\sim 0.3\,M_\odot$, a planetary mass of $\sim 40\,M_{\oplus}$, a projected planet-host separation of $\sim 1.6~\mathrm{au}$, and a lens distance of $\sim 7.5~\mathrm{kpc}$. Based on the photometric precision achieved by DREAMS for this event, we simulate free-floating planet (FFP) detections and find that DREAMS is sensitive to Mars-mass FFPs in the Galactic bulge and Moon-mass FFPs in the Galactic disk.

Figures (7)

• Figure 1: Light curve of the microlensing event, KMT-2025-BLG-1616, with the 2L1S models (solid black and orange lines) and the underlying 1L1S model (dashed grey line). Different data sets are plotted in different colors. The upper panels shows the 8-day data around the peak. The lower panels present a close-up of the planetary anomaly and the residuals relative to the 2L1S models.
• Figure 2: The $\chi^2$ surface in the $(\log s, \log q)$ plane obtained from the 2L1S grid search. The upper panel shows the coarse grid, while the lower panel zooms in on the dense grid around the global minimum. Dark red, red, yellow, green, blue, and dark blue denote grid points within $\Delta\chi^2<1n$, $<2n$, $<3n$, $<4n$, $<5n$, and $<6n$, respectively, where $n = 80$. Grid points with $\Delta\chi^2 > 480$ are left blank. The labels "Close Central", "Wide Central", "Close Resonant", and "Wide Resonant" in the lower panel mark the four local minima.
• Figure 3: Caustic geometries of the four 2L1S solutions. In each panel, the red lines show the caustic, the blue dot marks the location of the host star, the black line represents the source-lens relative trajectory, and the arrow indicates the direction of source motion. The right panels provide close-up views of the caustic-crossing regions, with the radii of the magenta circles indicating the source size.
• Figure 4: Color-magnitude diagram for KMT-2025-BLG-1616. Black dots show OGLE-III field stars OGLEIII within a $1.5'$ from the event position. The red asterisk and blue dot mark the centroid of the red clump and the source star, respectively. Green points show the HST CMD from HSTCMD, and the HST red clump centroid, $(V-I,I)_{\rm cl,HST}=(1.62,15.15)$MB07192, has been shifted to match the OGLE-III red clump.
• Figure 5: Posterior probability distributions from our Bayesian analysis are shown for the host mass $M_1$, the planetary mass $M_2$, the lens distance $D_{\rm L}$, the projected planet-host separation $a_\bot$, and the heliocentric lens-source relative proper motion $\mu_{\rm hel,rel}$. In each panel, the solid black curve indicates the median, and the two dashed black curves mark the 15.9% and 84.1% credible limits. Contributions from bulge and disk lens populations are shown in red and blue, respectively.