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We report discovering an exoplanet from following up a microlensing event alerted by Gaia. The event Gaia22dkv is toward a disk source rather than the traditional bulge microlensing fields. Our primary analysis yields a Jovian planet with ${M}_{{\rm{p}}}={0.59}_{-0.05}^{+0.15}\,{M}_{{\rm{J}}}$ at a projected orbital separation ${r}_{\perp }={1.4}_{-0.3}^{+0.8}$ au, and the host is a ∼1.1 M _⊙ turnoff star at ∼1.3 kpc. At $r^{\prime} \approx 14$ , the host is far brighter than any previously discovered microlensing planet host, opening up the opportunity to test the microlensing model with radial velocity (RV) observations. RV data can be used to measure the planet’s orbital period and eccentricity, and they also enable searching for inner planets of the microlensing cold Jupiter, as expected from the “inner–outer correlation” inferred from Kepler and RV discoveries. Furthermore, we show that Gaia astrometric microlensing will not only allow precise measurements of its angular Einstein radius θ _E but also directly measure the microlens parallax vector and unambiguously break a geometric light-curve degeneracy, leading to the definitive characterization of the lens system.

During the last 25 yr, hundreds of binary stars and planets have been discovered toward the Galactic bulge by microlensing surveys. Thanks to a new generation of large-sky surveys, it is now possible to regularly detect microlensing events across the entire sky. The OMEGA Key Projet at the Las Cumbres Observatory carries out automated follow-up observations of microlensing events alerted by these surveys with the aim of identifying and characterizing exoplanets as well as stellar remnants. In this study, we present the analysis of the binary lens event Gaia20bof. By automatically requesting additional observations, the OMEGA Key Project obtained dense time coverage of an anomaly near the peak of the event, allowing characterization of the lensing system. The observed anomaly in the lightcurve is due to a binary lens. However, several models can explain the observations. Spectroscopic observations indicate that the source is located at ≤2.0 kpc, in agreement with the parallax measurements from Gaia. While the models are currently degenerate, future observations, especially the Gaia astrometric time series as well as high-resolution imaging, will provide extra constraints to distinguish between them.

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 $q={3.17}_{-0.26}^{+0.28}\times {10}^{-4}$ , the planet lies near the peak of the mass-ratio function derived by the MOA collaboration and 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 ${m}_{\mathrm{planet}}={46}_{-24}^{+42}\,{M}_{\oplus }$ and a host star of mass ${M}_{\mathrm{host}}={0.43}_{-0.23}^{+0.39}\,{M}_{\odot }$ , located at a distance ${D}_{L}={7.49}_{-1.13}^{+0.99}\,\mathrm{kpc}$ . For the second scenario, we estimate ${m}_{\mathrm{planet}}={69}_{-34}^{+37}\,{M}_{\oplus }$ , ${M}_{\mathrm{host}}={0.66}_{-0.32}^{+0.35}\,{M}_{\odot }$ , and ${D}_{L}={7.81}_{-0.93}^{+0.93}\,\mathrm{kpc}$ . The planet has a projected separation as a fraction of the Einstein ring radius $s={1.3807}_{-0.0018}^{+0.0018}$ . As a cool sub-Saturn-mass planet, this planet adds to a growing collection of evidence for revised planetary formation models.