(covers information from several alternate timelines)
This page contains information regarding Star Trek: Picard, and thus may contain spoilers.
A planetary defense system was a network of space-based and/or ground-based defenses intended to protect a planet from attack.
In 2258 of the alternate reality, Earth's border protection grids had several access codes which were known to Starfleet captains. Nero captured Christopher Pike and tortured him to learn the codes. (Star Trek)
In 2369, when Starfleet assumed operational control of Deep Space 9, the station became a planetary defense system for Bajor as one of its main duties was to protect the planet from hostile attack. (DS9: "Emissary")
By 2385, Mars was protected by a deflector shield and a defense net that included orbiting satellites. These systems were deactivated or compromised by rogue synths during the Attack on Mars. (PIC: "Remembrance", "Maps and Legends")
In 2267, the USS Enterprise encountered Apollo's hand near the planet Pollux IV. This enormously-powerful force field was capable of stopping the ship from approaching the planet, preventing it from firing its weapons, and also preventing escape. (TOS: "Who Mourns for Adonais?")
A moon in the Gamma Quadrant, home of the Ennis and the Nol-Ennis, had a sophisticated array of orbiting satellites which inhibited transporter functions, and also actively sought out and attacked orbiting vessels. (DS9: "Battle Lines")
Chancellor Gowron moved the Klingon military headquarters from Qo'noS to Ty'Gokor in 2373. Ty'Gokor was protected by a powerful defensive shield and a tachyon detection grid, along with a large number of warships and space stations. (DS9: "Apocalypse Rising")
In late 2374, during the Dominion War, the Cardassians developed unmanned orbital weapon platforms to protect planets. The platforms were deployed in the Chin'toka system and around Cardassia Prime. (DS9: "Tears of the Prophets", "What You Leave Behind")
As of 2399, Vashti possessed a standard Romulan planetary defense system comprised of orbital killer drones, described as "primitive but effective". It had been bought secondhand and installed with the help of the Fenris Rangers. (PIC: "Absolute Candor")