Beyond the orbit of Neptune lies the Kuiper belt. This thick disk of icy rocks is more or less aligned with orbit of planets in the solar system. It is similar to asteroid belt spanning between orbits of Mars and Jupiter, but considerably larger. It is believed that Neptune and Saturn have got some moons from Kuiper belt. Around 100000 icy and rocky objects including Pluto and some other dwarf planets are contained in Kuiper belt. First spacecraft to explore Kuiper belt was New Horizon.
Oort cloud – lies beyond Kuiper belt and is gigantic sphere of icy rocks. It is believed that Oort cloud surrounds whole of solar system and is 2 light years away from earth. Edge of the Oort cloud is regarded as edge of our solar system. Not much is known about Oort cloud except the fact that majority of comets in our solar system come from it. Objects in the Oort cloud are very dark and difficult to observe as Oort cloud is very far from the sun. No spacecraft has till now reached Oort cloud and Voyager 1 & 2 will take 300 years to reach inner edge of Oort cloud. 30000 years will be needed by Voyager missions to go past Oort cloud.
The distance of the Oort cloud from us can be visualized by comparing the distance range of Pluto from the sun with the assumed distance range of the Oort cloud from the sun. The orbit of Pluto is elliptical and takes it 30 to 50 AU away from the sun. In comparison inner edge of the Oort cloud is believed to be between 2000 and 5000 AU away while the outer edge is between 10000 and 100000 AU away from the sun. The inner edge of the Kuiper belt is just 30 AU away from the sun. A ray of sunlight takes 8 minutes to reach the earth, 4.5 hours to reach Neptune’s orbit, and 7.5 hours to reach the outer edge of the Kuiper belt, and will need 10 to 28 days to reach the inner edge of the Oort cloud.
Kuiper belt icy objects are remnants from the process of formation of the solar system and in that way quite similar to objects residing in the Oort cloud. These objects present in the Kuiper belt and Oort cloud are called planetesimals.
It is believed that icy planetesimals present in the Oort Cloud were not this far away from the sun initially, the gravity of planets shoved some planetesimals into the Oort Cloud while some were banished from the solar system forever.
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