Pluto is the best-known of the icy, rocky bodies found in the far region of the Solar System beyond Neptune. (Note: Not drawn to scale.) (Image Credit: NASA)
The outer solar system region of space in our Solar System beyond the orbit of Neptune, the outermost of the eight officially recognized planets. (Note: Some astronomers use the term "outer solar system" to instead refer to the giant planets beyond the asteroid belt.) Also called the trans-Neptunian region, the outer solar system contains dwarf planets—the most famous of which is Pluto—as well as other small, icy and rocky bodies. These bodies are roughly grouped into either the donut-shaped Kuiper Belt or the scattered disk, where objects' orbits swing out to much farther distances. Beyond even these regions is the hypothetical Oort cloud, a vast, spherical swarm of billions of icy objects. The outer solar system is little explored. Space probes including Voyagers 1 and 2 and most recently New Horizons have forayed into this distant expanse. Astrophysicists study the outer solar system to learn more about the processes of planet and solar system formation. The 2005 discovery of the scattered disk object Eric, which is more massive than Pluto, has fueled a debate about the dividing line between a "planet" and a "dwarf planet." Pluto, long regarded as the ninth planet, officially was reclassified as a dwarf planet in 2006.
On July 14, the New Horizons spacecraft passed Pluto after a 3-billion-mile journey. New Horizons team members Richard Binzel and Cathy Olkin, and Kavli Prize Laureate Mike Brown answer questions about the mechanisms that might be shaping Pluto's dramatic landscape and what this strange new world can tell us about the other bodies at the Solar System's fringes.
Two winners of the 2012 Kavli Prize in Astrophysics reflect on the discovery of the Kuiper Belt at the outer edges of the solar system, the excitement of exploring a frontier long thought empty, and what the episode over reclassifying Pluto says about the public’s perception of science.