The Withers' Planet is made entirely out of obsidian, with a few glowstone scattered around, since it is hard to see the planet in the darkness of space.
As the name suggests, it is home to a large colony of hostile withers, which makes the planet really dangerous, as the Withers will kill everyone that sets foot on the planet. That is exactly what happened to ExplodingTNT. Due to the planet being almost entirely covered by Obsidian, it doesn't suffer any damage from the Withers.
- The Withers Planet might be the Withers' homeworld, explaining their alien-like shape. However, since they are closely related to the Nether, this fact is unlikely.