The Kuiper belt is a concentration of asteroid-like objects in orbit around the Sun, beyond Neptune. Since the belt was discovered in 1992, the number of known Kuiper belt objects (KBOs) has increased to over a thousand. And computed projections indicate that there are likely to be more than 70,000 of them. But, running contrary to the calculations, observations show that the number of KBOs dramatically falls off at around 47.8 AU (about 7170 million km) from the Sun. This is known as the Kuiper cliff. There is no agreed explanation for the sudden falloff.
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