The Solar System in a Nutshell the Solar System in a Nutshell

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A lot is known about the Solar System. We diameter, spinning around a massive center, know how big it is, about how old it is, a lot with smaller whirligigs circling the center at about the distribution of matter in it, and a various distances. good bit about how the pieces move around. This article is a short review of the Just about all the matter in the Solar System knowledge of the Solar System that has been gravitated to the center and became the Sun. accumulated over the centuries by thousands The Sun accounts for about 99.8% of the of curious humans. mass of the system. The 0.2% that was not drawn into the Sun formed eight planets, Something over 4.5 billion years ago a several dwarf planets, scores of moons, modest accumulation of dust and gases in and a collection of lesser objects like our part of the Milky Way galaxy began to asteroids, comets, and unincorporated draw together as a result of gravity. As the debris out on the fringes of the Solar System. vast cloud, known as the solar nebula, condensed, it began to spin and ﬂ atten into a The Sun rules. Because of its mass, it exerts disk, like a huge phonograph record. Most of a mighty gravitational force on all the other the matter was drawn into the center of the members of the system, and it is responsible vortex, but not all of it. In several places for maintaining the orbits of the planets. away from the center, smaller accumulations Also, during the Sun’s formative phase its of dust and gas began to spin. This was the gravity powerfully condensed and situation as the Solar System was in its early compressed the hydrogen and other gases. stages of formation—a kind of cosmic Things got hotter and hotter until a carousel at least 10 billion kilometers in thermonuclear reaction began. Hydrogen

NNewGrid_Planet_Science_45-104.in84ewGrid_Planet_Science_45-104.in84 8484 55/12/08/12/08 9:18:139:18:13 AMAM began to fuse into helium at a furious rate, terrestrial planets formed without releasing huge quantities of energy. The Sun atmospheres and without volatile materials. “turned on,” and a star was born. Planetary scientists think that it was some While the Sun was condensing, so were the time after the terrestrial planets formed smaller accumulations of matter orbiting that they acquired atmospheres as a result around it. These nine (or more) spinning of gases and water escaping from the locations became the planets. interiors of the planets (out-gassing through volcanism) and bombardment by The planets were undoubtedly formed from water-rich comets. The evolution and fates the same raw materials as the Sun, but they of these volatile materials varied from are quite different in properties. The four planet to planet, resulting in the unique planets closest to the Sun are known as the family of inner planets we see today. terrestrial planets. They are often considered together because of their Gas Giants similarities. Today they are all composed Planets 5–8 are the four gas giants. They primarily of rock and metal and have solid are truly huge compared to the little surfaces and gaseous atmospheres of various terrestrials closer to the Sun. And they are densities and compositions. The four considerably farther from the Sun; light terrestrial planets are all relatively close to reaches Jupiter, the ﬁ fth planet out, 43 the Sun; light reaches Mars, the fourth planet minutes after leaving the Sun, and Neptune, out, just 12.6 minutes after it leaves the Sun. the eighth planet out, receives light 4.2 hours after it leaves the Sun. It is believed that the four terrestrial planets formed in a long and violent process At these distances from the Sun, the more of condensation, gravitational attraction, and volatile materials did condense and become collisions. Because the action took place so part of the planetary mass, including ice. close to the Sun, the lighter, more volatile Because the gas giants acquired so much materials, like water, nitrogen, carbon matter, their gravity was powerful enough dioxide, hydrogen, and helium, did not to hold the volatile materials, especially the condense, but were driven away from the gases hydrogen and helium. In fact, the developing planets. The rocky and metallic gravitational compression of gases in the matter collected into larger and larger objects. giants was so intense that they began to Eventually they sorted themselves out into heat up, starting a process like the one that the four planets closest to the Sun. led to the formation of the Sun. However, because they are way too small to develop These planets are small. The combination of the internal pressures needed to start the heat (generated by collisions and the Sun) thermonuclear star process, they may have and the weak gravitational attraction of the simply glowed red for a few years before small planets made it impossible for them to cooling off. attract and hold the lighter, more volatile materials, such as gases and water. The And what about the little dwarf planet Pluto, whose origin is still a subject of ongoing

NNewGrid_Planet_Science_45-104.in85ewGrid_Planet_Science_45-104.in85 8585 55/12/08/12/08 9:18:159:18:15 AMAM how careful you would have to be in the Pluto has a moon, Charon, which orbits future if you signed on to work at the Pluto once every 6 Earth days. And Pluto Neptune science station for a year? Be sure to rotates once every 6 Earth days. That ask if that is an Earth year or a Neptune year. means the day and the month are the same length on Pluto. But a year on Pluto takes Pluto 248 Earth years. Beyond Neptune is the dwarf planet Pluto. There is no general agreement at this time as Comets to where Pluto came from. It has been The only signiﬁ cant other players in the suggested that there might have been a host Solar System are the long-range wanderers, of similar objects on the perimeter of the Solar the comets. There are lots of these “dirty System, and Pluto is one of only a few that snowballs” that infrequently pass through survived the early throes of Solar System the inner Solar System on exaggerated formation. elliptical orbits. Most of the time they are way out in the far reaches of the Solar Pluto follows an elliptical orbit. Most of the System, but they can create quite a lot of time it is a lot farther from the Sun than excitement when they drop in for a visit. Neptune, but at times it is actually closer to The characteristic tail of a comet is a stream the Sun. Pluto is cold—so cold in fact that of tiny particles that ﬂ ows off the comet when it is at its greatest distance from the body as it comes close to the Sun. When Sun, its thin atmosphere freezes and falls to these minute particles are illuminated by Pluto’s rocky surface as a kind of snow. the Sun, we can see one of nature’s outstanding celestial light shows.

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