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1. Orbit the Sun. 2. Must be a “round” shape as a result of its gravity. 3. The object must clear their orbital path of debris. (This is where Pluto failed) Planets are large objects that circle a star and do not produce light of its own. We can see other planets because they reflect sunlight. They all rotate on an axis that last one “day.” They all revolve around the Sun, forming their own year. “Planet” means wanderer. The inner planets of our solar system are Mercury, Venus, Earth, and Mars. All four planets are close to the Sun and are like Earth in size, density, and in their mostly rocky make-up. Because of their similarities they are known as the Mercury is the Roman messenger of the gods. Mercury is the closest planet to the Sun and the second smallest of the nine planets, just a little larger than our moon. It has almost no atmosphere. Temperatures vary from 425* C during the day, to -183* C during the night. That’s a larger difference than melting Lead and freezing Carbon Dioxide! One Mercury year lasts about 3 Earth months. That means that if you are ten years old on Earth, you would be 40 years old on Mercury! Venus is the Roman goddess of love and beauty. Venus is about the same size as Earth and is often referred to as our sister planet. It has a dense atmosphere with layers of thick sulfuric acid clouds and carbon dioxide, keeping the surface temperature at about 450* Celsius. It is the hottest planet in our solar system. The air pressure is 90 times greater than Earth’s air pressure. Venus has no known moons. The atmosphere is made mainly of nitrogen, oxygen, and carbon dioxide. Water covers about ¾ of the surface. The surface temperature rarely goes below -30* C or above 43* C. It is the only planet in our solar system on which life is known to exist. Earth has only one moon, called “Luna.” It is believed that an object the size of Mars struck the Earth and created our moon. It only takes 1.3 seconds for light to reflect off Luna and reach the Earth! Mars is named after the Roman god of war. Mars has two moons that are smaller than our moon. Mars appears red because of a rust-like dust blowing around on the surface. Temperatures range from -124*C to -31*C, or always below freezing. The Martian day is only 41 minutes longer than Earth’s, and the planet has four seasons, just like us! Mars has the largest volcano in our solar system, called Olympus Mons. Mars has two moons, Phobos, meaning “fear” and Deimos, meaning “terror.” Olympus Mons is a huge extinct volcano nearly three times the height of the tallest mountain on Earth. The reason that the volcano was able to get so big is simple. Unlike the Earth, which the surface is made up of plates that are moving, Mars does not have plate tectonics. On Earth, the plates move over a “hotspot” that form a series of volcanoes and mountains. Hawaii, for instance, is a island chain formed from the plate movements over a hotspot. On Mars, this doesn’t happen, and the volcanic eruptions The next four planets, Jupiter, Saturn, Uranus, and Neptune, all have characteristics in common. They are all much larger than the inner planets; Jupiter having a diameter over eleven times that of Earth! They are made up mostly of substances that would be gases on Earth. For this reason, they are referred to as the gas giants. Jupiter is the Roman ruler of the gods. Jupiter is the fifth planet from the Sun. It is the largest planet in our solar system. It is made up mostly of hydrogen in gas and liquid form and helium. The gases probably surround a small, molten Iron core. Jupiter’s atmosphere has many cloud bands and the Great Red Spot, a huge circular storm that’s lasted for centuries! Jupiter has a set of rings, but they are so thin and dark, they are almost invisible. Jupiter has 58 known moons! Io, one of Jupiter’s moons, is the most volcanically active object that we know of in our solar system. Its surface is regularly covered in This huge storm has lasted centuries and is as big three Earths! Saturn is the Roman god of agriculture. Saturn is the sixth planet from the Sun and is the second largest planet in the solar system. It is best known for its beautiful rings. Saturn has 31 moons, the largest being Titan. Saturn is made up of similar elements to Jupiter. Strong winds sweep across Saturn constantly, reaching speeds of 1,100 mph! Uranus was the Roman god of the skies and husband to Earth. Uranus is the seventh planet from the Sun. Uranus is unique, for it rotates on its side. It is made mainly of thick, green gases. It has 11 rings and 15 moons. The atmosphere is hydrogen and helium. Temperatures on Uranus are very cold, around - 215*C. Miranda, one of Uranus’ moons, has been called the most bizarre object in the solar system. Scientists think that the patchwork appearance of its surface resulted when the moon was torn apart by collisions and then Neptune is the Roman god of the seas. Neptune is the eighth planet in our solar system. It is the smallest of the gas giants. Like Jupiter and Uranus, its rings are too faint to see. It has 8 moons, one which being Triton. Neptune’s face has swirling blue and white features. No! Not that Pluto! Pluto is the god of the underworld. Pluto is the ninth planet in our solar system, or at least it usually is! Pluto has an unusual orbit around the Sun. For about 20 years, it is the 8th planet, as it was from 1979-1999. For about 227 years, it is the 9th planet. Pluto is also the smallest planet in our solar system. It is about half the size of our moon. Not much is known about Pluto, since no space probe has visited it, although one is on the way. Pluto has one known moon, Charon. Because it is so far away from the solar system’s heat source, the Sun, the temperature on Pluto probably doesn’t Classifying Pluto is a difficult task for astronomers. It is terrestrial, or Earth-like, like the inner planets, but is obviously the outer-most outer planet. It is currently classified as a dwarf planet. There are currently five official dwarf planets. Pluto, Ceres, Eris, Makemake, and Haumea - for now! Planet Distance from Sun (in millions of km) Diameter (in km) Length of year (in Earth time) Length of day (in Earth time) # of known moons Mercury 58 4,880 88 days 59 days 0 Venus 108 12,100 225 days 243 days 0 Earth 150 12,756 365 days 24 hours 1 Mars 228 6,784 687 days 24.6 hours 2 Jupiter 778 143,200 11.9 years 10 hours 58 Saturn 1,429 120,000 29.5 years 10 hours 31 Uranus 2,875 51,400 84 years 18 hours 15 Neptune 4,504 49,500 165 years 18 hours 8