solar system

The Solar System

Earth Science

1st Semester

• The sun is the central hub of a rotating wheel of planets, their moons, and many other smaller celestial objects, such as comets and asteroids.

• The sun is estimated to contain 99.85% of the mass of our solar system.

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• The planets traveling outwards from the sun are as follows: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

• Pluto is no longer considered a planet.

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• Due to the sun’s gravity, all planets move in an elliptical orbit in the same direction around the sun.

• The closer a planet is to the sun, the faster it travels in its orbit.

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The Planets: An Overview

• The planets fall into two groups – the terrestrial planets, and the Jovian planets.

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• The terrestrial planets include Mercury, Venus, Earth, and Mars.

• They are relatively small and rocky.

• Since they are closer to the sun they are also known as the inner planets.

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• The Jovian planets include Jupiter, Saturn, Uranus, and Neptune.

• They are huge planets made primarily of gas.

• Since they are further from the sun, they are known as outer planets.

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• Size is the most notable difference between the terrestrial and Jovian planets.

• Density, chemical makeup, and the rate of rotation are other ways in which the two groups of planets differ.

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• The planets have different densities because of their different chemical compositions.

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The Interiors of the Planets

• The substances that make up the planets are divided into three groups based on their melting points.

• Gases: Hydrogen and Helium – have melting points near absolute zero (-273 C or 0 kelvin)

• Rocks: made from compounds of silicates and metallic iron, both of which have melting points above 700C.

• Ices: Include ices made from compounds of ammonia, methane, carbon dioxide, and water.

• These particular ices have intermediate melting points.

• Terrestrial planets are dense and consist mostly of rocky and metallic substances.

• They have very little gas and ice.

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• The Jovian planets are less dense because they contain:

• Large amounts of gases like hydrogen and helium.

• Ices made from water, ammonia, and methane.

• The outer planets cores contain rocky and metallic materials.

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The Atmospheres of the Planets

• A planet’s ability to hold onto an atmosphere depends on its mass and temperature.

• Small, relatively warm planetary bodies, with small surface gravity cannot hold much gas.

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• Therefore, terrestrial planets have very thin atmospheres.

• Jovian planets however, have very thick atmospheres composed of hydrogen, helium, methane and ammonia.

• They are able to retain thick atmospheres due to their large mass, and low temperatures.

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Formation of the Solar System – Nebular Theory

• Nebula are clouds of dust and gas in space.

• These thin, gaseous clouds begin to rotate in space and collapse in upon themselves.

• As they continue to contract, they begin to spin faster.

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• According to the nebular theory, the sun and planets formed from just such a rotating disk of dust and gases.

• As the speed of rotation increased, the center of the disk began to flatten out and increase in temperature.

• The sun eventually formed in this location.

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Planetesimals

• Planets began to grow as solid bits of matter began to collide and clump together in a process known as accretion.

• The colliding matter formed small irregularly shaped bodies known as planetesimals.

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• As the collisions increased, the planetesimals grew in size, eventually growing large enough to exert a gravitational pull on surrounding objects and pull them in.

• In this fashion the planetesimals added more mass and grew into true planets.

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• In the inner solar system, only metals and rocks with high melting points and high densities could remain.

• The sun is too hot for materials with low melting points to stay solid.

• The sun does not attract items with low densities towards it.

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• Materials with low melting points and low densities (like gases) would be forced into the outer solar system due to the sun’s heat and the solar wind.

• In the outer solar system, the colder temperatures allowed materials with low melting points to form up as ice.

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• The Jovian planets were able to form from accumulating solid bits of matter but also from large quantities of ice.

• Eventually, the Jovian planets grew so large that they were able to attract even the lightest gases like hydrogen and helium.

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