Solar System Planets

The Planets in the Solar System

In ancient times, people studied the lights in the sky, and how they moved.
One explanation for this was that Earth is the center of the universe, but some lights seemed to move differently from the stars. These lights were called "planets" which mean "wanderers". Later, it was proposed that the planets, with the earth, went around the sun instead. This explained the motion of the planets better.

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​The Solar System

A planet is a large body that orbits a star.
A moon is a body that orbits a planet.
A Solar System consists of a star*, as well as all the planets, moons, and other bodies travelling around it.
*some stellar systems have two or more stars.

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The Solar System

​A 3D Model of the Oort Cloud

One of the things that keep planets in their orbits is gravity.
The more massive an object is, the greater its gravity. Also, the farther objects are, the smaller the gravity.
The other thing keeping the planets in orbit is inertia. Inertia keeps an object in its current position or motion.
Without inertia, the planets will just fall into the sun. Without gravity, they will just move in straight lines.

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Planets and Orbits

Mercury, Venus, Earth, and Mars are called the inner planets.
They have similar sizes, and rocky structures. They have few moons, and rotate slowly. None of them have rings.

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​The Inner Solar System

Between the orbits of Mars and Jupiter lies the asteroid belt. Asteroids are rocky or metallic objects that orbit the sun.
The largest asteroid is 1/4 the size of the moon.
Sometimes, asteroids can cross Earth's orbit.

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Asteroids

Diameter: 4880 km
Distance from Sun: 57.9M km
Day: 59 days
Year: 88 days
It has no moons. The side facing the sun can melt zinc, while the opposite side can drop to -170 ⁰C.

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Mercury

Diameter: 12100 km
Distance from Sun: 108.2M km
Day: 243 days
Year: 225 days
It has no moons. It's atmosphere is carbon dioxide that is 90 times denser than Earth's. Venus has many volcanoes.

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Venus

Diameter: 12750 km
Distance from Sun: 149.6M km
Day: 23h 56m 4s
Year: 365.24 days
It has one moon. Its atmosphere supports a wide variety of life. Earth has a strong magnetic field and tectonic activity.

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Earth

Diameter: 6800 km
Distance from Sun: 227.9M km
Day: 23h 37m 12s
Year: 687 days
It has two moons. It also has seasons. Mars has a thin atmosphere of carbon dioxide.

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Mars

The planets beyond the asteroid belt, Jupiter, Saturn, Uranus and Neptune, are the outer planets.
They are called gas giants.
They have a small, metallic core and a thick atmosphere.

​They all have rings and many moons.
They spin rapidly, and their orbits are farther apart.

​The Outer Planets

Beyond the outer planets are the smaller, icy worlds. One of the largest is Pluto. It was once known as the ninth planet. In 2003, a slightly larger planet was found beyond Pluto. The IAU reclassified Pluto as a dwarf planet in 2006.

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Dwarf Planets

Diameter: 143000 km
Distance from Sun: 778.4M km
Day: 9h 55m
Year: 12 years
Has at least 95 moons. It is the largest planet. Its atmosphere is mostly hydrogen and helium. The great red spot is a 300 years old storm. Its most well-known moons are Ganymede, Europa, and Io.

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Jupiter

Diameter: 120500 km
Distance from Sun: 1.43B km
Day: 10h 40m
Year: 29 years
Has at least 146 moons. Its atmosphere is mostly hydrogen and helium. It has huge storms and jet streams. It has a huge ring system. Its moon, Titan, is the only moon with a cloudy atmosphere.

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Saturn

Diameter: 51000 km
Distance from Sun: 2.87B km
Day: 17h 14m
Year: 84 years
Has at least 27 moons and 11 rings. Its atmosphere includes methane, which is why it appears blue. Its moon, Miranda, looks broken apart.

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Uranus

Diameter: 49500 km
Distance from Sun: 4.5B km
Day: 16h 7m
Year: 165 years
It has at least 16 moons. Its atmosphere is mostly hydrogen, helium, and methane. Possibly, has an ocean. Its moon, Triton, is larger than Pluto and has ice volcanoes.

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Neptune

A Comet is a ball of ice and rock that orbits the sun. As a comet approaches the sun, the ice is warmed and forms a cloud of gas. The solar radiation pushes the cloud away and the comet appears to have a tail.
Comets may come from a) Kuiper Belt - contains around 70,000 large asteroids or b) Oort Cloud - a huge cloud around the solar system.

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​Other Objects

Meteoroids - small rocky or metallic objects that orbit the sun.
Meteor - a meteoroid that enters the Earth's atmosphere. It appears as a streak as it breaks apart and burns in the atmosphere.
Meteorite - a meteor that did not break up in the atmosphere and has hit the Earth's surface.

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​Other Objects