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Planets of the Solar System
Overview As the solar system emerged from its parent nebula, a young star was surrounded by a disk of dust and gas that was starting to clump in various places. As these clumps grew larger, they collapsed under their mutual gravity and to accrete (accumulate) more matter. Clumps closer to the sun were deprived of a lot of gas because the young star's radiation forced most of the lighter materials outward into what would become the outer solar system. While the inner planets became mostly rocky, the outer planets were able to accrete much more gas and to evolve voluminous atmospheres. Eventually, the sun's wind blew most of the remaining material away, and the new planets could grow no larger. Mercury, Venus, Earth, and Mars became the rocky, terrestrial planets of the inner solar system, while Jupiter, Saturn, Uranus, and Neptune became the gas giants of the outer solar system. Pluto, the "planet" beyond Neptune, is not gaseous like the other planets of the outer solar system, nor is it rocky like those of the inner; it is actually most like a giant comet. Left-over building blocks were herded into the region between Mars and Jupiter to become the asteroid belt. Other material, farther from the Sun so able to retain more volatile components, formed the comets of the Kuiper Belt and Oort Cloud. Even though there was no longer much planet-building material left, the solar system looked different than it does today. Rogue asteroids roamed the solar system, as evidenced by the crater patterns on the Earth's Moon. The interplanetary asteroids frequently struck planets and moons until approximately 3.8 billion years ago. After that period, most of the rogue asteroids had either been ejected from the solar system, captured as moons, struck planets or moons, or been shepherded into relatively stable orbits, mostly between Mars and Jupiter in the asteroid belt. Since then, the solar system has been relatively calm, though each object in it has had a unique evolution that is explored in the pages of this section. What Is a Planet? Historically, the solar system was composed of Mercury, Venus, Earth, Mars, Jupiter, Saturn, the Sun, and a smattering of moons. Then comets were realized and asteroids were discovered, and Uranus and Neptune were found. The solar system was still pretty cut-and-dry: There was a star, around which there were eight planets. Between the fourth and fifth was a belt of asteroids that were tiny bodies significantly smaller than any planet. Around most planets were moons. Pluto's discovery in 1930 and its subsequent classification as a planet was not questioned because another planet was expected to be there. But lately, many more objects have been discovered around, within, and outside of Pluto's orbit. For this reason, and for revised theories of how the solar system formed and what objects it should contain far from the Sun, many astronomers throughout the 1990s and early 2000s no longer considered Pluto to be a planet, but rather was the "King Comet" of the Kuiper Belt. Then, it was announced in Summer 2005 by a group led by Michael Brown that an object they had discovered in 2003 (2003 UB313, now named "Eris") was in fact larger than Pluto. Consequently, after two years of review by a sub-committee, the August 2006 meeting of International Astronomical Union (IAU) passed a resolution that defines a lower limit for what a planet is. The definition is liked by very few, it has many issues with it, and it will probably be changed within the next few years. The new definition, first of all, is really only applicable to our Solar System. It also only sets a lower limit, not an upper limit, which means that there is still an issue with how large a planet can get before it is no longer considered to be a planet*. Originally, the definition listed four criteria for an object to be classified as a planet:
However, this proposed definition led to a huge backlash from mainly the astronomers who study solar system dynamics. It also increased the number of planets from 9 to 12 by adding the largest asteroid, Ceres, keeping Pluto, adding its moon Charon, and adding Eris. Also, weird things could happen over time, such as Earth's moon would eventually become a planet because, since the moon is slowly moving away from Earth, the center of mass of the Earth-Moon system would eventually be outside Earth's surface, so the moon would be considered to be orbiting the Sun, not Earth. Thus, another criterion was added:
This removes Pluto because it crosses Neptune's orbit - a planet over 10,000 times more massive than it. This would also mean that Earth's moon would never become a planet because Earth dominates the orbit. But, there are still problems with this definition, mainly with the first and third point. It is not defined how spherical an object must be to be considered a "sphere" in this sense. Ceres is now considered a dwarf planet, but its axes only differ by about 30 km (about 3%). Jupiter, however, is very squashed, where its axes differ by a factor of about 6.5%, and the bulge at its equator is clearly visible in a telescope or even high-powered binoculars. A problem with the third aspect of this definition is what does it mean to "dominate?" All planets have asteroids that cross their path, so no planet orbits alone. Jupiter, which must be a planet by any sense of the word, has over 600 Trojan asteroids that precede and follow it in its orbit, so it has not cleared its orbit. And Pluto crosses Neptune's orbit, so should Neptune not be a planet because it has this giant ball of icy rock that crosses its path. These are all questions that have led many to denounce the Resolution. But besides astronomers, there was much public outcry, with polls showing that up to 75% of the population believed that Pluto should still be a planet. The public likes an underdog, and Pluto had always (at least for 75 years, anyway) been that. Childrens' letters to NASA begged them not to change Pluto's status, and many believed that Pluto should be "grandfathered" in as a planet because it had been considered one since its discovery. And, every encyclopedia and textbook was made out-of-date because of the new classification, making only eight planets in the solar system. But others see this as a learning opportunity, and it is a chance to educate people on what it means to actually have a "definition" (after all, Pluto hasn't changed in any way, shape, nor form just because the IAU says it is no longer a planet), and about the dynamic nature of astronomy. *Extra-solar planets that have been discovered since 1995 beg for an upper-mass cut-off as well. This has been, relatively unofficially, put at about 13 times the mass of Jupiter. Anything larger is generally considered to be a brown dwarf - a "failed star" that wasn't able to accrete enough mass to start nuclear fusion in its core. Data for the Planets
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