"A perfect place for suntanning!"
The Solar room is used to turn Puddle slimes into Rainbow slimes. Also you can use it to give yourself a tan. Keeping slimes in there will evaporate them due to scorching heat. However, the Solar Room cannot make a Tarr evaporate. If you try this, you'll get the achievement "Why isn't it working?" Like it's cousin, the Golden Tarr will not evaporate if placed in a Solar Room. If you try this, you'll get the achievement "Golden legends never die." Anthro Slimes avoid Solar Rooms and will quickly run out if placed inside.