Hobbies And Interests
Composition
As evidenced in our own solar system, planets exhibit variation in their composition. The terrestrial planets, such as Mercury, Venus, Earth, and Mars, possesses a greater percentage of rocky material in their composition than other planets do. The Earth is composed of a thick, solid crust of silicate rocks such as basalt and granite, a solid mantle of ferro-magnesium-rich silicate rocks, and a solid outer and liquid inner iron-nickel core. The outer planets, however, gas giants such as Jupiter, Uranus, and Neptune, are primarily pressurized gases such as hydrogen, and only possess a solid core. Jupiter has a liquid hydrogen outer mantel and a metallic hydrogen inner mantel.
Mass and Diameter
The mass and diameter of planets varies considerably, and this is clearly reflected in the solar system. The Earth has a mass of 1.32 times 10^26 pounds and a diameter of 7,926 miles. Mercury, the smallest solar system planet now that Pluto has been declassified, possesses a mass five percent that of Earth and a diameter of 3,030 miles. Jupiter, the solar system's largest planet, has a mass 317 times that of the Earth and a diameter of 88,846 miles. Recent advances in detecting extra solar planets, orbiting around other stars in the Milky Way, have revealed that there are planets many times bigger than Jupiter, and advances in technology should soon tell us how common smaller, rocky planets such as the Earth are.
Orbital Period
The distance and shape of a planet's orbit around a star determines its orbital period, the time it takes to make one complete rotation of its star. The Earth takes 365 days to orbit the Sun, while Mercury, the closest planet to the Sun, takes 88 days. Pluto, whose planetary status is controversial, takes 90,465 days, or over 247 Earth years. Recently discovered exoplanets, planets which orbit in other star systems, have been observed to possess both orbits very close to their stars, with searingly hot surface temperatures, and orbits even further out into space than Pluto.
Satellites
Satellites or moons are celestial bodies which orbit planets. The moon, also called Luna, is the Earth's only natural satellite. Planets may have many satellites -- such as Jupiter which possesses 50 official moons, and Saturn which possesses 53 -- or none at all, such as in the case of Mercury and Venus. Just like planets, moons vary in their masses and compositions, and just as planets exert gravitational effects on them, keeping them within their orbit, so moons can also effect gravitational effects on their planets, such as cycle of tides on Earth.