The 8 Planets in our Solar System

**A revolution is the time it takes for a planet to orbit the sun. It defines the length of 1 year. **Rotation is how long a planet takes to rotate on its own axis. It defines the length of 1 day. Retrograde clockwise rotation– Venus and Uranus. Prograde is counterclockwise rotation – all the rest.

These are the inner planets of our solar system Terrestrial means “of land” – these planets are small, dense, and rocky. The inner planets include Mercury, Venus, Earth, and Mars.

It takes more than 58 days for one rotation – this means that a day on Mercury is equal to 58 Earth days. It takes 88 days for one revolution around the sun (so its year is 88 Earth days long). Mercury is the smallest planet in our solar system. Its diameter is only 4,879 km. The distance from New York to Los Angeles is 3,942 km. Mercury has no moons and no rings. It has no substantial atmosphere.

It is only 3.2 light-minutes from the sun (35,977,320 miles). So, it takes 3 minutes, 12 seconds for light from the sun to reach Mercury. In 2004, the Messenger mission was launched to orbit and learn about Mercury (arrived in 2011). Since Mercury is smaller than Earth, its gravitational force is less. If you weigh 100 pounds on Earth, you would weigh 39 pounds on Mercury (take your weight and multiply it by 0.39).

In many ways, it is very much like Earth. It is often referred to as Earth’s twin. It has no rings and no moons. Venus is a little smaller than Earth – its diameter is 12,104 km. It is 6 light minutes away from the sun (67,232,228 miles ). This means that light from the sun takes 6 minutes to reach Venus. It takes 243 Earth days for one rotation. It makes one revolution of the sun in 224 days. This means a day is longer than a year! Venus has the densest atmosphere of all the planets – made mostly of carbon dioxide. It has the hottest surface of any planet in the solar system. To find your weight on Venus, multiply your weight by 0.91.

It takes 23 hours, 56 minutes for one Earth rotation. The Earth goes through 1 revolution in 365 days, 6 hours (not being exactly 365 days is why we have leap years). It is often called the blue planet – it is home to a vast variety of life because of the presence of water. Earth is the 5 th largest planet. You could fit about 1,300 Earths inside of Jupiter and over 1,000,000 inside the sun. Its diameter is 12,756 km. Earth has one moon, no rings, and its atmosphere is mostly nitrogen and oxygen.

Earth is 8.3 light-minutes away from the sun (92,956,944 miles) so light takes 8 minutes, 18 seconds to reach Earth.

Mars is referred to as the red planet because of iron in the soil. It is the most studied planet other than Earth. It takes hours for one rotation. Mars does one revolution of the sun in 1 year, 322 days. Mars is the 7 th largest planet. Its diameter is 6,794 km. Mars has no rings but 2 moons (Phobos and Deimos). Its atmosphere is very thin – it’s made of mostly carbon dioxide. It is 12.7 light-minutes from the sun (141,610,211 miles). The gravity on Mars is similar to that of Mercury, to calculate your weight, multiply your Earth weight by 0.38

Mars is home to the biggest volcano in our solar system – it’s called Olympus Mons. It’s about the size of Arizona and 3 times the height of Mount Everest. It is no longer active though…..

In 1997, the Sojourner (part of Pathfinder mission) landed on Mars. It was about the size of a microwave. In 2004, the twin rovers Spirit and Opportunity landed on Mars. Spirit worked for more than 6 years but it last communicated with Earth in March Opportunity continues to operate even today – it just past its 9 th anniversary exploring the red planet.

In August 2012, Curiosity landed on Mars. It is twice as long and 4 times as heavy as Spirit/Opportunity. It has a nuclear battery so it can go further than the other rovers that relied on solar energy.

Asteroids are small, rocky bodies that revolve around the sun. They range in size from a few meters to more than 900 km in diameter. Most asteroids revolve around the sun in the main asteroid belt - a wide region between the orbits of Mars and Jupiter.

The outer planets are called “gas giants” because they are very big and have no known solid surfaces. They include Jupiter, Saturn, Uranus and Neptune.

Jupiter is the first of the “outer planets.” The great red spot is similar to a hurricane system that has lasted for centuries. Like the sun, it is huge and made mostly of hydrogen and helium. Jupiter and the sun are the solar system’s 2 main bodies. In terms of mass, the rest of the solar system is insignificant! The sun has 1000 times more mass than Jupiter.

Jupiter (continued) Clouds of water, methane, and ammonia make up the outer part of Jupiter’s atmosphere. The beautiful colors are probably due to small amounts of organic materials. Its distance from the sun is 43.3 light-minutes (483,674,368 miles). Its diameter is 142,984 km. It is the largest planet. A rotation is 9 hours and 50 minutes. But its revolution is 11 Earth years, 313 Earth days long. It has a few rings and many moons (63?). Since it’s such a big planet, its gravity is strong. To calculate your weight on Jupiter, multiply by 2.6

Saturn’s rings are the largest of all the planets (as wide as 21 Earths placed side by side). They are made of icy particles. Saturn has 1000s of rings (7 mains ones). Scientists think helium is falling out of the atmosphere and sinking to its core- meaning it is still forming! The other main gas is hydrogen. It is the least dense of the planets (it could float in water). Saturn is 1.3 light-hours from the sun (884,304,000 miles). Its diameter is 120,536 km. It is the 2 nd largest planet. To find your weight on Saturn, multiply your weight on Earth by 1.07

It has a large number of moons (close to 60?). A rotation on Saturn takes 10 hours, 30 minutes and a revolution is 29 Earth years, 155 Earth days long. In October the Cassini probe was launched to explore Saturn and its moons. Cassini flew by Jupiter in 2000 and arrived at Saturn in 2004.

The gases in its atmosphere absorb the red part of sunlight very strongly, giving Uranus its featureless blue- green color. It has several rings (11?) and 27 moons. Its atmosphere is mostly hydrogen and helium but its interior is made of heavier elements and possibly rocky materials. Unlike most planets, it is tipped over on its side. This is likely because of a large object crashing into Uranus.

Uranus is 2.7 light-hours from the sun (1,783,510,000 miles). A rotation on Uranus takes 17 hours, 14 minutes. Its revolution takes 83 years, 274 days. Its diameter is 51,118 km – it’s the 3 rd biggest planet. Its gravity is less than that of Earth – to find your weight, multiply by 0.9

The shape of Pluto’s orbit causes it to cross the orbit of Neptune. This causes Neptune it to become the more distant from the sun for a few years. Neptune has 13 moons and 6 rings. Its atmosphere is made of hydrogen, helium, methane, ammonia and water.

It is 4.2 light-hours from the sun (2,793,880,000 miles). Neptune takes 16 hours and 7 minutes for a rotation. One revolution around the sun takes a whopping 163 years, 263 days. In 1613, Galileo observed Neptune when it happened to be very near Jupiter, but he thought it was just a star. Neptune was later “discovered” in Its diameter is 49,528 km - it’s the 4 th biggest planet. To find your weight on Neptune, multiply by 1.15

Pluto was demoted to a “dwarf planet” in August of 2006, mostly because of its size. It is only half the size of Mercury (the next smallest planet in the solar system). Its moon, Charon, is more than half Pluto’s size. To calculate your weight on Pluto, multiply your Earth weight by 0.05 Its rotation is 6 days, 10 hours. Its revolution is 248 years. Most of the time, it is farther from the sun than the 8 planets. It is 5.5 light-hours away (3,690,600,000 miles). There are currently 2 other dwarf planets, Ceres and Eris. The IAU decided planets must orbit the Sun, they need to have enough gravity to pull themselves into a ball, and they must have cleared out all other objects from their orbit.

The Kuiper Belt is a disk-shaped region past Neptune containing many small icy bodies. It is now considered to be the source of some comets.

Past the Kuiper Belt is an area called the Oort Cloud. It is not really a cloud, but an area containing about a trillion comets. These comets do not orbit the Sun in a ring but each comet can arrive in the solar system from any direction and orbit the Sun.

- An object discovered in the Oort Cloud in 2003 was named Sedna. It is the most distant body known that orbits our Sun. It takes 11,500 years for 1 orbit around the sun.

Solar system vs. Galaxy Many solar systems make up a galaxy (there are billions of stars in a galaxy) Our solar system does not have a name but our galaxy is the Milky Way