Nature of the Universe-Chapter Six

We are going to discuss our solar system. A solar system consists of a star and the objects gravitationally bounded to it, including the planets, comets, etc. We will study stars themselves in the second semester. In this chapter, we will briefly introduce the formation of a solar system, the overview of our solar system and the Earth, our home planet.

Formation of a Solar System
It is impossible to go back in time to see how our solar system formed, nor can we create a solar system as an experiment. Besides, there is only one solar system that we can examine in details. Then, how do we know? To be honest, we can only guess. But it is an educated guess. The guess should satisfy all known laws of nature and it must be natural, in other words, it cannot be the result of some special initial arrangements.

We believe that about 5 billion years ago, there was a large cloud of gas and dust. It contracted by its own gravity. In general, the cloud would also rotate. According to a physical law, if an object rotates and there is no external force, it will keep on rotating. Upon contraction, the rotation of the cloud and the collisions between the particles in cloud would flatten the cloud to a disk, called the accretion disk.

The contraction did not stop, it never stops. Materials at the center of the disk contract to form a star, our Sun. Materials not near the center will contract by themselves and form the planets. The rotation of the original gas and dust cloud becomes the revolutions of the planets around the Sun and the rotation of the Sun itself. The overall time taken is about one billion years.

Overview of Our Solar System
At the center of our solar system locates our Sun. It is about 1000 times more massive than the sum of all other objects in our solar system. There are nine major planets. Started from the planet nearest to the Sun, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Their relative sizes are shown in the figure below.

Courtesy NASA.

In terms of length scales, the size of the planets are nothing compared to their distances to the Sun. By definition, the average distance between the Earth and the Sun is called one astronomical unit, 1 A.U., which is about 1.5x10¹¹ meters. The ratio of the diameter of the Earth to 1 A.U. is about 1:10,000. Here shows the size of the orbits. Apart from the major planets, there are asteroids or minor planets, comets and many other things in our solar system.

The Earth


	Courtesy NASA.

We have an intimate knowledge on our Earth. We will lay out what we know about the Earth as a planet. By studying the Earth and other planets, we will understand both better.

The mass of the Earth is about 6x10²⁴kg. Its radius is about 6400km. So, light will travel 7.5 turns around the Earth in one second. The height of the highest mountain is about 9000m, very small compared to the radius. The density is about 5 times that of the water.

Apart from many artificial satellites, the Earth has only one natural satellite, the Moon. We will talk about the Moon in the next chapter.

The innermost region of the Earth is called the core, which is very hot. It consists of iron and nickel. The inner core is believed to be solid and the outer core is liquid. The major heat source of the core is the natural radioactivity in the Earth. Outside the core is the mantle, which is partially melted. The outermost layer is the crust, which is solid.

However, the crust is not a rigid sphere. It is segmented into plates, thousands of kilometers in extent but only about 50 km thick. The plates float on top a hot and soft layer and this theory is called the plate tectonics.

The rotation of the core generates the magnetic field of the Earth. The magnetic axis does not coincide with the spin axis and it does not pass through the Earth's center.

The magnetic field of the Earth will trap the charge particles generated by the Sun. When they collide with molecules and atoms in the air, the molecules and atoms will emit light. These are called the aurora.

Courtesy NASA.

We have detected magnetic field and aurora in some other planets. It means that those planets, like ours, have an atmosphere. Our atmosphere contains 78% nitrogen, 21% oxygen and some other less common gases, like argon and carbon dioxide. The atmosphere is not static. The atmospheric turbulence makes the stars twinkle. It also scatters light, more on the blue light and less on the red. Thus, blue light cannot travel very far without changing direction in the atmosphere. Red light, in contrast, can travel much farther. During sunset, we see a red Sun because the red light can go directly to us and almost all the blue light is scattered away.

Tide should be the most well known phenomenon due to the Moon. (Werewolves not counted.) From the figure below, we can see that the Moon exerts a stronger attractive force on the water in the near side of the Earth, less on the Earth itself and even less on the water in the far side. Relative to the center of the Earth, the effective forces are as shown in figure (b) below. They lead to two bulges of water on the Earth's surface. Therefore, we have two high tides per day.

In addition to the Moon, the Sun also attracts water on Earth. When the Moon is new or full, the effects from the Moon and the Sun will reinforce each other. The high tides will be higher than the high tides at other times.