Our Solar System is made up of eight planets in orbit around a medium-sized yellow star our Sun. There are also a variety of other objects, including moons, comets, asteroids, and planetoids. Of the eight planets, four are known as inner planets, because they orbit closer to the Sun. The four outer planets are much farther away. A close look at both groups of planets reveals similarities among the planets in each group. It also notes the differences between inner and outer planets. The eight planets of our Solar System orbit the Sun, a medium-sized yellow star. The Inner Planets The inner planets are also called terrestrial planets due to their solid surfaces. They are small and have few, if any, moons. These planets include Mercury, Venus, Earth, and Mars. The closest planet to the Sun is Mercury. The surface of this small, rocky planet is marked with craters of all sizes. It moves quickly around the Sun, hurtling along at 31 miles per second (50 km/s), with an orbit of only 88 Earth days. Since Mercury is the closest planet to the Sun, passing as close as 29 million miles (47 million kilometers) from the Sun s surface, temperatures can rise as high as 800 F (427 C). However, the planet has very little atmosphere. As a result, there is no greenhouse effect in which atmospheric gases trap solar energy near a planet s surface. Consequently, temperatures during the night can be as low as -290 F (-179 C). Mercury s surface lacks the protection provided by a thick atmosphere, so its surface is heavily cratered. Mercury is a small planet just a little bigger than Earth s Moon. However, it is very dense, thanks to its core, which is primarily composed of molten metal. Because of its high density, it has a significant amount of gravity for such a small planet more than 1/3 of Earth s gravity.
Moving farther away from the Sun, the next planet is Venus. In contrast to Mercury s thin atmosphere, Venus has a thick, cloudy atmosphere of carbon dioxide and sulfuric acid that traps the Sun s energy close to the planet s surface. This causes temperatures on Venus s surface to climb higher than 880 F (471 C). This is hot enough to melt the metals from which space probes are constructed, which has made it quite difficult to explore Venus s surface. Volcanoes on Venus s surface and its stormy, acidic atmosphere present additional barriers to exploration. The surface of Venus is covered in volcanoes, while its stormy atmosphere swirls overhead. Venus s orbit around the Sun takes about 225 Earth days, nearly three times the length of time it takes Mercury to orbit the Sun. It rotates slowly in the opposite direction as Earth, the only inner planet to spin in this direction. Although it is about the same size and density as Earth, Venus does not have a similar magnetic field. This is due to its very slow spin, which does not provide the motion necessary to set up the motion in Venus s core that it would take to generate a magnetic field. The third planet from the Sun is Earth, which is orbited by a single moon. Earth is home to an incredible diversity of living things. This life is made possible by the interactions of several factors. Earth s distance from the Sun, its abundance of water, its spin, and its tilt all contribute to the conditions that make life possible. Earth is close enough to the Sun to receive Earth has the conditions necessary for life. light and warmth, and Earth s atmosphere both protects it from extremely high temperatures and holds enough warmth to protect it from extremely low temperatures. The range of temperatures on Earth allows life to flourish on land and in water. Earth s tilt accounts for the uneven heating of the planet, which causes weather and seasons. Earth s spin
causes the motion within its molten core that produces a strong magnetic field. Mars, the fourth planet, is dry and barren. It exhibits a reddish color due to the amount of iron in its crust, which can form orange and red-colored compounds. While Mars is only about half the size of Earth and has no currently active magnetic field, it does share some similarities with Earth. It is tilted on its axis, and so has seasons. It has two moons Phobos and Deimos that orbit the planet. It also has polar ice caps as Earth does, and these include some water ice. In addition, precipitation has been observed by the Phoenix Mars Lander mission. Mars has a red color due to iron compounds in its crust. The Outer Planets Beyond Mars lies the asteroid belt, and beyond this are the outer planets. These planets are also knows as gas giants, since they are not small and rocky, but are large and made of gases. In general, they also have more objects in orbit around them, including moons and other particles. The outer planets are Jupiter, Saturn, Uranus, and Neptune. Jupiter is the first of the gas giants. It is the largest of the planets, having more than 1300 times the volume of Earth. Jupiter has Jupiter has four large moons, two of which four large moons and many smaller ones, (Io and Europa) can be see here. Its Great Red Spot is a storm in the atmosphere. along with rings made up of smaller particles. Its magnetic field is much stronger than Earth s, due to the fact that Jupiter spins more quickly on its axis. Its path around the Sun takes nearly 12 Earth years, partially due to its longer orbit and partially because it travels about half as fast as Earth does. Jupiter and its atmosphere are both composed mostly of hydrogen and helium, like the Sun. Winds in the atmosphere cause its striped appearance.
Saturn is known for its large bands of vibrant rings, which are thousands of kilometers in diameter. The thousands of individual rings are made up of a variety of different-sized particles, from tiny dustsized particles to huge chunks of matter. Each ring has its own orbit around Saturn, and each one orbits at a slightly different speed. Gaps between bands of rings are caused by moons that orbit in the midst of the rings. Other than its rings, Saturn is much like a smaller Jupiter. It has many moons, is made up of hydrogen and helium, and has a volume and a magnetic field many times as great as Earth s. Of all the gas giants, Saturn has the most vibrant rings. Although it has rings and several moons like the other outer planets, Uranus is unique in several ways. It rotates in the opposite direction of the Earth and the other gas giants. Instead, it rotates east to west like the inner planet Venus. Also, its axis nearly lines up with the plane of its orbit, so that it looks like it is lying on its side. Its magnetic field is similarly askew, in that it is not centered on the center of the planet but is tilted 60 degrees from the rotation of its axis. Also, Uranus atmosphere contains methane gas in addition to the hydrogen and helium found in the atmospheres of Jupiter and Saturn. This makes Uranus look blue through a telescope. Although the planet is traditionally classified as a gas giant, the majority of its mass is made up of liquid water, methane, and ammonia. This has given rise to the term ice giant in reference to Uranus and Neptune. Neptune appears blue due to the presence of Neptune is the farthest planet from the Sun, methane in its atmosphere. orbiting at a distance of about 2.8 million miles from our star. This gives it an extremely long orbit of about 165 Earth years. Like Uranus, it appears blue in color due to methane in its atmosphere, has an irregular magnetic field, and
has an icy liquid core. Neptune also has several moons, as well as the rings common to all the outer planets. Beyond Neptune lie the outermost objects in our Solar System, including those in the icy Kuiper Belt.