Y dwarfs are a type of very cool and dim star, often referred to as "failed stars" due to their low mass and temperature. They are part of the spectral class Y and are the coldest known type of star, with temperatures ranging from 200 to 500 degrees Celsius. Y dwarfs are often found in the outer regions of planetary systems and are difficult to detect due to their faintness. Studying Y dwarfs can provide valuable insights into the formation and evolution of stars and their associated planetary systems.