Heronius II was a class M planet in the Alpha or Beta Quadrant, the second planet orbiting binary star Heronius, and orbited by three moons. It was the site of a Federation colony in the mid-24th century.
Heronius II was notable for its constantly shifting magnetic fields. Every twenty-seven years, the planet underwent a magnetic phase shift with a duration of three days. The phase shift which occurred in early 2354 was observed by scientist Erin Hansen. The phase shift also made it possible for Seven of Nine, circa 2376, to travel back in time to try to dissuade her parents from their impending journey into the Delta Quadrant. (VOY short story: "A Ribbon for Rosie")
The story leaves it ambiguous as to whether or not Heronious II and The Tendara Colony are connected.