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Sun
The Sun is a star that provides energy through the fusion of hydrogen into helium. It is the most massive object in the solar system, containing 99.8 percent of the total mass of the solar system. The Sun has a magnetic field that extends past Pluto, causing radio interference and contributing to magnetic storms on the planets. The outer layers of the Sun exhibit differential rotation, meaning that the poles and the equator rotate at different rates. The differential rotation causes sunspots and solar flares on the Sun.
Kepler's Laws of Planetary Motion
Kepler, after careful planetary observations, developed three laws of planetary motion. The first law states that each planet's orbit is an ellipse, with the Sun at one focus of the ellipse. An ellipse is oval and the foci are points within the oval. The farther the foci are from one another, the more elliptical the orbit. Kepler's second law states that each planet sweeps out equal areas of the ellipse in equal times. In other words, when a planet is in perihelion, it moves a farther in its orbit than it does at aphelion. The third law of planetary motion states that the ratio of squares of a revolutionary period for two planets is equal to the ratio of the cubes of their semimajor axis. This law implies that the amount of time it takes for a planet to orbit the Sun increases quickly with the planet's radius from the Sun.
Inner Planets
The inner planets are also known as the terrestrial planets, as they are made of mostly rock. Mercury, Venus, Earth and Mars are all terrestrial planets. Mercury is heavily cratered and has a highly eccentric orbit. It lacks a significant atmosphere because the heat from its proximity to the Sun has boiled off any gases that made up an atmosphere. Venus is about the same size as Earth. Its atmosphere is composed of carbon dioxide, causing a runaway greenhouse effect on the planet. The temperature on Venus is hot enough to melt lead. Earth is the only planet with liquid water, which is essential to life as we know it. The heat capacity of the oceans keeps the temperature relatively stable, and therefore makes the oceans inhabitable. Mars has an extremely elliptical orbit. Alluvial patterns and channels indicate that fluid once flowed on Mars' surface. The planet's Olympus Mons is the largest volcano in the solar system.
Outer Planets
The outer planets are also known as the Jovian, or gas giant, planets. Jupiter, Saturn, Uranus and Neptune are the gas giants. Jupiter is the largest planet in the solar system, and it radiates more energy than it receives from the Sun. The Great Red Spot is a giant storm that has been observed for more than 300 years. Saturn is known for its rings, which are 250,000 km in diameter, but less than a kilometer thick. Saturn has a density less than water. Uranus rotates on its side and has a magnetic field tilted 60 degrees in respect to its axis of rotation. Neptune was found in the 1920s due to a wobble in Uranus's orbit. It, too, has an internal heat source.
Pluto
Pluto's status as a planet has been highly controversial. In 2006, it was downgraded to a dwarf planet. While it fits the definition of planet in its orbit and shape, Pluto has not cleared its area of other objects, technically making it a dwarf planet. So technically, there are only eight real planets in our solar system, not nine as was previously thought.