When I took a closer look at the planetoids, they had this weird texture glitch. Look at this screenshot:
Take a close look at the middle of the screenshot and there's the "thing".
Quick Edit: I found out that the area around the "thing" is translucent. Kind of see-through.
To replicate this glitch:
1. Place an Earth down in an empty simulation.
2. Launch a Moon at the Earth by clicking the number 4.
3. Take a look at the planetoids that are created in the collision.
4. You will find the strange texture glitch. This glitch also sometimes appears on generated asteroids, such as Mimas.