The asteroid had least two continents, an ocean, and was orbited by a double-banded, green-colored ring system. Buried at its core was a Giant Brain cosmozoan which had been there for at least 107,600 years.
Its class M atmosphere supported a humanoid population that had advanced to the point of building cities. Their minds responded to telepathic emanations from the Giant Brain. As it was afflicted by Akwood's syndrome, the populace suffered from random bouts of planet-wide insanity.
In 2267, the starship USS Enterprise discovered the asteroid and beamed down a landing party. When geologist Freyer tasted the soil, it connected him to the emanations of the Giant Brain. He eventually was able to lead James T. Kirk and Leonard McCoy deep inside the planet and within the Brain to help inject an antidote into the cosmozoan. (TOS comic: "The Brain-Damaged Planet")
The asteroid's surface acted as a protective shell around the Giant Brain entity, similar to the shell containing the life energy mass within the hollow planet Tristas, in TOS comic: "The Final Truth", and both planets supported humanoid civilizations. Likewise, Argus IV was discovered to be a shell surrounding a nascent cosmozoan which hatched in 2274, in TOS comic: "It's a Living". How the asteroid shell came to be around the Giant Brain entity was unclear. It was also unknown whether the humanoid population evolved on its surface or had been transported there.
While this planetary object was called an asteroid in "The Brain-Damaged Planet", by definition it would more accurately be considered a dwarf planet or planetoid as it was spherical with an atmosphere, oceans, continents, and a ring system.