The Solar System Have you ever looked up at the night sky and wondered what was out there? You may see stars, moons, or even planets, but can you define each of these objects? What is the difference between a moon and a planet? How are they different from asteroids, meteoroids, comets, and the other kinds of objects in the solar system? Discover Science: Geocentric vs. Heliocentric For many centuries, most people assumed the Sun and other stars orbited—or moved around—our planet, Earth. This is called the geocentric model of the solar system. (Geo- means geocentric “Earth,” so means “Earth-centered.”) Ancient peoples saw the Sun rise in the east and set in the west with the various bright lights of the planets also moving across the night sky. This motion gave the false impression that celestial objects moved around Earth. The invention of the telescope in the early 1600s gave scientists a much more accurate view of space from Earth. Using measurements made while looking through telescopes, scientists such as Galileo Galilei demonstrated the truth of the heliocentric model. The Sun is the center of the solar system, with the planets orbiting the Sun and the stars remaining stationary (not moving). What is in the solar system? To ancient peoples, only five planets (Mercury, Venus, Mars, Saturn, and Jupiter) and the Moon were visible to the naked eye. Unmanned spacecraft such as the Voyager on the left have traveled beyond the edges of our solar system, revealing new views of old worlds and surprising views of newly discovered worlds. Today we know that our solar system consists of our Sun, planets, dwarf planets (smaller planets), moons, asteroids, comets, meteoroids, and a host of frozen worlds beyond Neptune. We also know that Earth and its solar system are part of the Milky Way galaxy, which is one of many galaxies in the universe. 1 The Solar System The Sun The Sun has enough mass that its gravitational pull holds the planets and other objects in the solar system in orbit. The Sun rotates, or spins, about its axis like all the other stars located in the Milky Way galaxy. A star is an extremely hot, dense mass of gases. As these gases burn, the star gives off visible light as well as other forms of energy and charged particles. Most of the energy that reaches our planet, including light and heat, comes from the Sun. The Sun is actually medium-sized compared to other stars. Still, it is the most massive object in the solar system. The gravitational attraction between celestial objects decreases with smaller size and greater distances and increases with greater size and smaller distances. The Sun, in turn, is pulled by the massive gravitational forces of the center of the Milky Way, which cause stars and stellar systems to orbit around the center of the galaxy. The Sun is even larger when compared to the planets. In fact, the Sun’s diameter is more than 100 times greater than Earth’s. Find a spherical object, such as an orange, and measure its diameter. (You can do this by cutting it in half.) If the orange represents Earth, what size of object would represent the Sun? Planets Inner Planets Within our solar system are eight major planets grouped by proximity to the Sun. The inner planets closest to the Sun are small and rocky: Mercury, Venus, Earth, and Mars. The outer planets are the gas giants: Jupiter, Saturn, Uranus, and Neptune. 2 The Solar System The Inner Planets Mercury, the smallest planet, has a cratered surface that looks like the Moon. Named for the swift messenger of the gods, Mercury is the fastest planet to orbit the Sun, with a year that is only 88 days long. Venus, the second planet from the Sun and similar in size to Earth, is covered in a thick atmosphere of carbon dioxide with temperatures that make Venus the hottest planet. Known for the longest day in the solar system, Venus spins so slowly that by the time it has orbited the Sun, it has spun almost completely on its axis making its day longer than its year. Earth, the third planet from the Sun, is noted for its life, water covering three-quarters of its surface, and its one moon. Mars, the fourth planet from the Sun, is called the red planet, due to iron oxide (rust) in the soil. Mars is a cold, lifeless, rocky desert where water once flowed in ancient times. Mars is known for its polar ice caps, having the largest volcano and the longest valley in the solar system, and its two moons. You will find the asteroid belt (large space rocks) between Mars and Jupiter. The Outer Planets Jupiter, Saturn, Uranus, and Neptune are the four gas giants that make up the outer planets. They are characterized by freezing temperatures, many moons, and ring systems. Jupiter, the largest of the planets, has the most moons (63 and counting) and the giant red spot, a famous storm that has lasted over 400 years. Saturn is known for having the largest ring system and has close to 60 moons. Uranus, a gas giant, has 27 moons and is tipped on its side in its orbit around the Sun, the result of an ancient impact. The last major planet is a blue gas giant named aptly after Neptune, the Roman god of the sea, and having 13 moons. All the gas giants have ring systems, although Saturn’s is the most prominent. The Solar System Planets to Scale How do you think the planets compare in size? The largest planets are the gas giants in the outer solar system. Jupiter is the largest, followed by Saturn, Uranus, and Neptune. Earth is the largest of the inner planets, followed by Venus, which is almost the same diameter as Earth. Mars is next, at about half that diameter. The smallest planet is Mercury, at about one-third the diameter of Earth. Pluto is no longer a major planet. Astronomers once identified nine planets in the solar system. In addition to Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, astronomers considered tiny Pluto to be a planet. However, in 2006, the International Astronomical Union (IAU) officially defined planet as a celestial object that meets all three of the following criteria: Pluto does not meet the third part of this definition, due to the large number of other frozen worlds that share its orbit. Because Pluto is so small, its gravity is not strong enough to clear smaller objects from its orbit. Since 2006, astronomers have considered Pluto a dwarf planet. Pluto is part of a vast grouping of frozen worlds beyond Neptune called the Kuiper Belt. Astronomers have named several other dwarf planets in the solar system, including Eris, which is nearly as large as Pluto. The largest asteroid in the asteroid belt, Ceres, has also been reclassified as a dwarf planet due to its large size. There may be dozens more dwarf planets awaiting discovery! 1. The object is in orbit around the Sun. 2. The object has a nearly spherical shape. 3. The object has cleared the neighborhood around its orbit of smaller objects. The Solar System Do all planets take the same amount of time to rotate (spin on its axis) and revolve (orbit the Sun)? Like the Sun, each planet spins on an imaginary axis. Planets complete their rotations in different amounts of time. Earth rotates once every 24 hours, or 1 day. In contrast, Jupiter takes only 10 hours to complete one rotation. Venus takes 243 days to rotate once! Planets that are farther from the Sun take longer to complete one revolution. Neptune takes more than 160 times longer than Earth to orbit the Sun! Earth orbits the Sun in about 365 Earth days, or 1 year. The orbital paths of the planets are not perfect circles. The planets’ orbits are elliptical, or oval shaped. This means the planets are sometimes closer to and sometimes farther from the Sun during their orbits. (dwarf planet) Not all solar system objects rotate in the way in which the Sun would appear to rise in the east and set in the west, as Earth does. Venus, for example, rotates backward, called retrograde rotation, so that the Sun rises in the west and sets in the east. Uranus spins on its side, the result of an ancient collision. Planets are generally in the same plane and thus appear in the night sky along a path called the ecliptic. Sometimes the slower planets are passed by Earth and appear to move backward for a short while before progressing forward again. Planets do not shine like stars but instead reflect the sunlight to observers on Earth. Ancient Greek observers thought these moving points of light that shifted among the fixed constellation patterns were “wandering stars,” which they called planets. 5 The Solar System Moons About 180 discovered moons orbit the planets of our solar system. Moons are smaller than the planet they orbit and are held in place by the pull of gravity. The gas giants attract scores of moons. Jupiter, the largest planet in the solar system, not only has the most moons (more than 60), but also has some of the largest moons in the solar system: Io, Europa, Ganymede, and Callisto (pictured above). Ganymede is the largest moon in our solar system and is even bigger than the planet Mercury. These are known as the Galilean moons because they were discovered by the Italian scientist Galileo Galilei.