This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
Essof IV was a planetoid and the fourth body in its star system. It had wildly fluctuating temperatures and a carbon monoxide atmosphere laced with perchlorate dust, which was rapidly lethal to oxygen-breathing lifeforms; an unprotected Human would suffocate painfully in two minutes. The planet was notably rich in deuterium.
One of the testing sites for Section 31's Daedalus Project was a research facility on Essof IV. In 2257, USS Discovery and NCIA-93 arrived at the planet so as to harness its deuterium for a plasma reactor that could power the phase discriminators necessary to trap the Red Angel. Commander Michael Burnham and Lieutenant Spock had also devised a plan to compel the Angel's appearance by exposing Burnham to Essof IV's toxic atmosphere. Their plan succeeded in trapping the Angel within the facility. (DIS: "The Red Angel")
Once the Red Angel suit was trapped, its anchor to the 32nd century began to cause gravimetric instability on Essof IV. Eventually, the suit and its pilot, Gabrielle Burnham, were pulled back into the future. Immediately afterward, Captain Christopher Pike ordered the Section 31 facility destroyed by photon torpedoes in an attempt to neutralize Control. (DIS: "Perpetual Infinity")
In the 32nd century, Gabrielle Burnham landed again on Essof IV, which by that time had been colonized. The colonists brought her to the Qowat Milat, who healed her injuries and eventually inducted her into their order. (DIS: "Unification III")