KMT-2017-BLG-1003L is a Sun-like G-type star (about 0.32 solar masses, 5,778 K), 7,090 light-years from Earth. KMT-2017-BLG-1003L b is a 5.2-Earth-mass planet, at 1.380 AU from its star, discovered in 2023 via microlensing. Microlensing detections occur when a foreground star's gravity briefly magnifies a background source, revealing planets that are otherwise too faint and distant to observe directly. KMT-2017-BLG-1003L b orbits at roughly the distance of Mars from the Sun. Its low mass makes KMT-2017-BLG-1003L b a Neptune- or super-Earth-class world rather than a gas giant.