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Pluto is a member of the Kuiper Belt; a region of space that starts at the orbit of Neptune (at 30 astronomical units) and then extends out to approximately 55 AU from the Sun. It’s similar to the asteroid belt between Mars and Jupiter, but its 20 times as wide and contains up to 200 times as much mass. There are currently more than 1,000 Kuiper Belt Objects known, and astronomers estimate that there are 70,000 larger than 100 km in diameter in the region.
Another large object in the Kuiper Belt is the newly discovered dwarf planet Eris, which orbits as far as 97 AU from the Sun. It’s a little larger than Pluto and even has a moon of its own.
Beyond Eris lies the heliosphere. This is the region where the influence of the Sun ends, and interstellar space begins. The solar wind from the Sun gets out to about 95 astronomical units before it interacts with the interstellar medium. The solar wind slows down and becomes more turbulent, forming a great oval structure known as the heliosheath.
Outside that is the theorized Oort cloud. This is a huge region that could contain trillions of icy objects, which are believed to be the source of all long-period comets. This region starts around 50 AU and could extend out as far as 2 light-years.