An accretion disk around a black hole consists of particles in approximately circular orbits about a central mass. The solar system is made up of planets $($made of particles$)$ in orbit about a central mass. The planets in the solar system have been orbiting stably for billion of years, whereas gas in a black hole accretion disk spirals toward the black hole. Explain why there is this difference in behavior.

Please explain reasoning and draw diagrams if possible.