Which exoplanet was the first discovered via the microlensing method?

OGLE-2003-BLG-235L b was the first exoplanet discovered through gravitational microlensing. It was identified in the 2003 event also designated MOA-2003-BLG-53L b.

Microlensing works when a foreground star passes almost directly in front of a more distant background star. The foreground star's gravity bends and magnifies the background star's light. A planet orbiting the foreground star creates a brief additional distortion in that brightening pattern, revealing its presence without directly seeing the planet.

OGLE and MOA independently observed the 2003 event toward the constellation Sagittarius. Analysis of the light curve led to the planetary detection in 2004, making this system a milestone for exoplanet science. The planet is a gas giant orbiting an orange K-type dwarf roughly 19,000 light-years away.

The designation can be confusing because the same system has two survey names: OGLE-2003-BLG-235L and MOA-2003-BLG-53L. This was not the first exoplanet discovered overall—that distinction belongs to the pulsar planets and, for a main-sequence star, 51 Pegasi b—but it was the first found using microlensing.