A state of emergency is a status assigned to planets in immediate peril.
In 2286, a state of emergency was issued on Earth, by the Federation President after the Whale Probe visited the planet and began vaporizing its oceans. In addition, a planetary distress signal was also sent out. (Star Trek IV: The Voyage Home)
In 2372, a state of emergency was again declared on Earth, after members of Red Squad infiltrated the Division of Planetary Operations on the orders of Admiral Leyton. At 0400 hours, local time, on the 23rd, they sabotaged the power distribution network by downloading a self-replicating disordering protocol into the control system, shutting down every power relay on the planet. The entire operation, from deployment to return, took less than three hours. (DS9: "Homefront", "Paradise Lost")