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Inner Planets The eight planets in our solar system are divided into two groups: inner planets and outer planets. The inner planets are closest to the sun. They are Mercury, Venus, Earth and Mars. These planets are also called the terrestrial planets because they are most similar to Earth. Mercury is a small planet which is the closest to the Sun. Since it is between the Sun and Earth, it is often hidden in the Sun’s glare. The Sun appears nine times larger on Mercury than on Earth. The sun covers this planet in deadly radiation. Mercury is a ball of rock that has craters, hills, plains and mountains. The days and nights on Mercury are long—the time between one sunrise and the next is 180 Earth days. Mercury is the speed demon of the Solar System, however, because it takes only 88 days to orbit around the Sun. Venus is second from the Sun and has an orbit twice as big as Mercury. Venus is sometimes called the morning or evening star because it appears just after sunset and before sunrise. With sunlight reflecting off its dense cloud cover, Venus is brighter than anything in the sky except for the Sun and moon. Because of its location between the Sun and Earth, Venus goes through phases as does our moon. Venus is a hostile place. Its atmosphere is carbon dioxide (CO2). The upper clouds are poisonous sulfuric acid. Its surface temperature is approximately 900 degrees. The atmosphere alone would crush you! Earth, the third planet from the Sun, is the largest of the inner planets. It is the only planet known to have liquid water and to support life. Earth’s atmosphere absorbs and reflects the right amount of solar energy to keep the planet at the correct temperature for living things such as humans to survive. Earth has one moon. It takes 24 hours to rotate on its axis and 365 days to orbit the Sun. Mars is a small rocky planet with a thin atmosphere. It is the fourth planet from the Sun, and about half the size of Earth. A year on Mars is nearly twice as long as a year on Earth. It takes 687 Earth days for Mars to complete one orbit around the Sun. Mars is sometimes called the Red Planet because its soil is dark reddish brown. Mars has two moons and the largest volcano in the solar system, names Olympus Mons. Dust storms on Mars last for months and affect the whole planet. Space probes have found deep valleys and sedimentary rocks, which causes them to believe that Mars may have had flowing water in the past.