Aldea was hidden from view for millennia by its powerful planetary defense system, devised by the Progenitors and run by the Custodian. The system included an enormous planet-wide cloaking device that hid it from marauders and other threats. In addition to the cloaking device, Aldea was defended by an immensely powerful shield that could only be penetrated by Aldean technology and a repulsor beam powerful enough to push starships several hundred light years distant.
For thousands of years the existence of Aldea was considered to be only a rumor, similar to the cultural myths of Atlantis or Neinman. However, by the late 24th century the planet-wide cloaking device was responsible for cumulative damage to the planet's ozone layer, resulting in widespread radiation poisoning from the Aldean sun's ultraviolet rays, causing the species to become sterile. To try to rectify their dire situation, they attempted to kidnap various Enterprise-D children, including Wesley Crusher, to repopulate their planet. After the failed attempt, the Aldeans agreed to accept help from the Enterprise-D to dismantle their shield and cloaking device, which reversed the radiation poisoning in most of the population. (TNG: "When The Bough Breaks")