The Universe - Where Did They Come From?
The Nebular Theory - Formation of the Solar System
To figure out how the solar system formed,we need to put together what we have learned. There are two other important features to consider. First, all the planets orbit in nearly the same flat, disk-like region. Second, all the planets orbit in the same direction around the Sun. These two features are clues to how the solar system formed.
The nebula was drawn together by gravity.
A Giant Nebula
Scientists think the solar system formed from a big cloud of gas and dust, called a nebula. This is the solar nebula hypothesis. The nebula was made mostly of hydrogen and helium. There were heavier elements too. Gravity caused the nebula to contract (Figure below). These nebulas may be formed from material leftover from the big bang or recycled from previous supernovas.
As the nebula contracted, it started to spin. As it got smaller and smaller, it spun faster and faster. This is what happens when an ice skater pulls her arms to her sides during a spin move. She spins faster. The spinning caused the nebula to form into a disk shape.
This model explains why all the planets are found in the flat, disk-shaped region. It also explains why all the planets revolve in the same direction. The solar system formed from the nebula about 4.6 billion years ago.
Formation of the Sun and Planets
The Sun was the first object to form in the solar system. Gravity pulled matter together to the center of the disk. Density and pressure increased tremendously. Nuclear fusion reactions begin. In these reactions, the nuclei of atoms come together to form new, heavier chemical elements. Fusion reactions release huge amounts of nuclear energy. From these reactions a star was born, the Sun.
Meanwhile, the outer parts of the disk were cooling off. Small pieces of dust started clumping together. These clumps collided and combined with other clumps. Larger clumps attracted smaller clumps with their gravity. Eventually, all these pieces grew into the planets and moons that we find in our solar system today.
The outer planets — Jupiter, Saturn, Uranus, and Neptune — condensed from lighter materials. Hydrogen, helium, water, ammonia, and methane were among them. It's so cold by Jupiter and beyond that these materials can form solid particles. Closer to the Sun, they are gases. Since the gases can escape, the inner planets — Mercury, Venus, Earth, and Mars — formed from denser elements. These elements are solid even when close to the Sun.
If you look at the sky on a dark night, you may see a meteor, like in the Figure below. A meteor forms a streak of light across the sky. People call them shooting stars because that's what they look like. But meteors are not stars at all. The light you see comes from a small piece of matter burning up as it flies through Earth's atmosphere.
Meteors burning up as they fall through Earth's atmosphere.
Before these small pieces of matter enter Earth’s atmosphere, they are called meteoroids. Meteoroids are as large as boulders or as small as tiny sand grains. Larger objects are called asteroids; smaller objects are interplanetary dust. Meteoroids sometimes cluster together in long trails. They are the debris left behind by comets. When Earth passes through a comet trail, there is a meteor shower. During a meteor shower, there are many more meteors than normal for a night or two.
A meteoroid is dragged towards Earth by gravity and enters the atmosphere. Friction with the atmosphere heats the object quickly, so it starts to vaporize. As it flies through the atmosphere, it leaves a trail of glowing gases. The object is now a meteor. Most meteors vaporize in the atmosphere. They never reach Earth’s surface. Large meteoroids may not burn up entirely in the atmosphere. A small core may remain and hit the Earth’s surface. This is called a meteorite. Meteorites provide clues about our solar system. Many were formed in the early solar system (the Figure below). Some are from asteroids that have split apart. A few are rocks from nearby bodies like Mars. For this to happen, an asteroid smashed into Mars and sentup debris. A bit of the debris entered Earth’s atmosphere as a meteor.
The Mars Rover, Opportunity, found a metal meteorite on the Red Planet.
Source: Open Education Group Textbooks - Earth Science