Our solar system consists of the sun, nine planets (and their moons), an asteroid belt, and many comets and meteors. The sun is the center of our solar system; the planets, their moons, the asteroids, comets, and other rocks and gas all orbit the sun. The nine planets that orbit the sun are (in order from the sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. A belt of asteroids (minor planets made of rock and metal) lies between Mars and Jupiter. These objects all orbit the sun in roughly circular orbits that lie in the same plane, the ecliptic (Pluto is an exception; it has an elliptical orbit tilted over 17° from the ecliptic). Easy ways to remember the order of the planets are the mnemonics: "My Very Excellent Mother Just Sent Us Nine Pizzas" and "My Very Easy Method Just Simplifies Us Naming Planets" The first letter of each of these words represents a planet - in the correct order. The largest planet is Jupiter. It is followed by Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury, and finally, tiny Pluto. Jupiter is so big that all the other planets could fit inside it. The Inner Planets vs. the Outer Planets The inner planets (those planets that orbit close to the sun) are quite different from the outer planets (those planets that orbit far from the sun). The inner planets are: Mercury, Venus, Earth, and Mars. They are relatively small, composed mostly of rock, and have few or no moons. The outer planets include: Jupiter, Saturn, Uranus, Neptune, and Pluto. They are mostly huge, mostly gaseous, ringed, and have many moons (again, the exception is Pluto, which is small, rocky, and has one large moon plus two tiny ones). Temperatures on the Planets Generally, the farther from the Sun, the cooler the planet. Differences occur when the greenhouse effect warms a planet (like Venus) surrounded by a thick atmosphere. The outer, gaseous planets are much less dense than the inner, rocky planets. The Earth is the densest planet. Saturn is the least dense planet; it would float on water. The Mass of the Planets Jupiter is by far the most massive planet; Saturn trails it. Uranus, Neptune, Earth, Venus, Mars, and Pluto are orders of magnitude less massive. Gravitational Forces on the Planets The planet with the strongest gravitational attraction at its surface is Jupiter. Although Saturn, Uranus, and Neptune are also very massive planets, their gravitational forces are about the same as Earth. This is because the gravitational force a planet exerts upon an object at the planet's surface is proportional to its mass and to the inverse of the planet's radius squared. A Day on Each of the Planets A day is the length of time that it takes a planet to rotate on its axis (360°). A day on Earth takes almost 24 hours. The planet with the longest day is Venus; a day on Venus takes 243 Earth days. (A day on Venus is longer than its year; a year on Venus takes only 224.7 Earth days). The planet with the shortest day is Jupiter; a day on Jupiter only takes 9.8 Earth hours! When you observe Jupiter from Earth, you can see some of its features change. The Average Orbital Speed of the Planets As the planets orbit the Sun, they travel at different speeds. Each planet speeds up when it is nearer the Sun and travels more slowly when it is far from the Sun (this is Kepler's Second Law of Planetary Motion).