Planet Two was the second planet in the Taurean system, located in a sector unfamiliar (but not un-trafficked) to Starfleet, which had been previously chosen as a new homeworld by Theela's species, a warp-capable humanoid species who settled on the planet when their original homeworld began to die.
The planet's climate included rain and lightning. Water "accumulations" also existed.
The novelization of "The Lorelei Signal" depicted this planet as having rainstorms that were "typically sudden."
The entrance to a temple
These people built for themselves an advanced civilization, including a temple and numerous other architectural wonders. Unfortunately, these new inhabitants did not know at the time that the planet drained humanoid energy. The women's bodies were able to develop a glandular secretion that enabled them to survive and to manipulate certain areas of the males' brains to influence their emotions, ultimately draining the men. This caused the men to weaken and eventually die. The females realized that, in order to survive, they had to revitalize every 27 years, as they became eternal prisoners of the planet. They were neither capable of aging, dying, or bearing children. They revitalized themselves by means of summoning male crew members of starships from as far as twenty light years away.
In 2269, the USS Enterprise was investigating several disappearances within range of Taurean system when they fell victim to their summons. After reaching an agreement with the inhabitants, the crew of the Enterprise arranged for Starfleet to send a ship crewed by women. These women would transport them to a suitable planet where they could live out their lives as mortal women. (TAS: "The Lorelei Signal")
This planet was identified in Scott's log as "Planet Two in the Taurean System", and was so capitalized in the episode's script. While generally an atypical means of identifying a planet by name, the name is likewise unique to only this planet.