The Bleeding Heart
Galen was a planet that radiated good feelings from its core. This was caused by the effects of the Compassionate, a species who had been sealed inside a rift at Galen’s heart during the Last Great Time War.
No weapons were allowed on the planet. It was commonly used for peace talks between warring factions.
The Ninth Doctor arrived on the planet and found that the Compassionate were trying to free themselves from the rift by influencing the minds of the people on Galen. The Doctor sealed the Compassionate inside the rift for good, preventing their influence from escaping into the universe forever. (The Bleeding Heart)