When discovered in 1930, it was considered to be the ninth planet; this changed in 2006 with the adoption of a formal definition of planet.
Ocenjujo, da je v Kuiperjevem pasu več kot 100.000 teles s premerom večjim od 50 km, skupna masa Kuiperjevega pasu pa je le desetina ali celo samo stotina Zemljine.