With the success of the New Horizon’s spacecraft visiting Pluto and the Kuiper Belt, it seemed appropriate to discuss hte major zones of the solar system. The solar system consists of the Sun and the objects bound to it through gravity. This includes the Sun, eight planets (since 2006 when Pluto was redesignated as a dwarf planet), their 158 currently known moons, and a large number of asteroids, meteoroids, planetoids, comets, and interplanetary dust. These may be conveniently divided into three distinct zones, each with its own characteristics. The inner zone closest to the Sun contains rocky planets, known as terrestrial planets because of their comparative relationship to Earth. The second part contains the four gas giant planets—Jupiter, Saturn, Uranus, and Neptune. The third zone includeds the Kuiper Belt, of which Pluto is the most famous (but not largest) body, and the Oort Cloud of distant long-period comets.
Each of these zones has its own challenges and opportunities. The terrestrial planets in this solar system occupy a habitable zone, defined as the region around a star that has conditions conducive to life as we understand it. Fundamentally, this means that the region has at least some planets with temperatures high enough to maintain liquid water. Of course, Earth is the only object in our solar system in this category but evidence supported the theory that Mars once had liquid water a well. Accordingly, the planets in this zone have long been viewed as possible abodes of life and were early targets for exploration because of this and because of their relative closeness to Earth. All have been visited, some of them many times, and the findings from the scientific investigations have both dashed hopes of finding life and of opening other possibilities for exploration.
Beyond the orbit of Mars but inside the orbit of Jupiter lies the main asteroid belt, consiting of thousands of rocky and metallic bodies. The total mass of this material is a small fraction contained in Earth’s Moon.
The second zone, the Jovian planets or gas giants, have enamored all astronomers from the time that Galileo turned the first telescope on them. Little wonder that as soon as the opportunity permitted, spacecraft began to visit these. The most stunning of these missions was the Voyager 1 and 2 probes to the outer solar system. Launched in 1977, these spacecraft visited their primary targets of Jupiter and Saturn and then went on to all the giant outer planets. The two Voyagers took well over 100,000 images of the outer planets, rings, and satellites, as well as millions of magnetic, chemical spectra, and radiation measurements. The Voyagers’ windshield tour of the Jovian planets in the second zone returned information to Earth that revolutionized the science of planetary astronomy.
The Kuiper Belt, comprising the third zone, is perhaps the most mysterious of those parts of the solar system since it is as yet unexplored by spacecraft. While there are more icy dwarf planets than in the other two zones, as yet no spacecraft has visited the region. This led the National Academy of Sciences to assign priority to missions to visit this third zone—especially the Pluto-Charon system—and the New Horizons mission was launched in 2006 to help fulfill this objective.
In the Kuiper Belt, icy dwarf planets represent a fascinating puzzle. Are they planetary embryos, whose growth was stunted? Are they relics of the origin of the solar system? Most scientists believe they are both, relics from the formation of the solar system more than 4 billion years ago. If they are the bodies out of which the larger planets accumulated, they promise new understandings of the origins and evolution of the solar system. With these efforts to explore the farthest zone of the solar system, humanity will complete the first stage of the reconnaissance of the Solar System. What is learned in the process will inform the direction of future explorations.