 Based on estimations of the primordial mass required to form Uranus and Neptune, as well as bodies as large as Pluto "(see )", earlier models of the Kuiper belt had suggested that the number of large objects would increase by a factor of two beyond 50 AU, so this sudden drastic falloff, known as the "Kuiper cliff", was unexpected, and to date its cause is unknown.In 2003, Bernstein, Trilling, et al.found evidence that the rapid decline in objects of 100 km or more in radius beyond 50 AU is real, and not due to observational bias.