Okay, well, Saturn's rings form a wide and complex system, consisting mostly of particles and pieces of ice, and are highly visible. They may have formed from one or more moons that broke up due to a collision, or are left over from early debris that never coalesced into a moon... And, The rings of Uranus are thin and hard to see, consisting mostly of chunks of carbon and hydrocarbons with very little reflectivity. They may also have formed from the breakup of a small moon due to a collision. They may be kept thin by the presence of shepherd moons.
Based on the information in the question, a light year is the distance traveled light in one year. Then if the light from the star Centauri takes 4.2 years to reach the earth, then its distance from earth is 4.2 light years.
