Aldea is a legendary planet concealed by a cloaking shield, whose sterile people kidnapped children from the Enterprise.
Aldea was long thought to be a myth, a planet hidden from view in the Epsilon Mynos system. In When the Bough Breaks it revealed itself to the Enterprise. Its people had an advanced civilisation run by an ancient computer called the Custodian, which they relied on but no longer understood.
The Aldeans could no longer have children, and they took several children from the Enterprise, including Wesley Crusher, to raise as their own. Dr. Crusher found that the Aldeans were being harmed by radiation that reached the surface because their shield had damaged the ozone layer. Data disabled the Custodian, and the children were returned.