Index

The Solar System

Formation and Evolution of Solar System

The Solar System

Our Sun is one of two hundred thousand million stars in our galaxy

The Solar System

The solar system is the sun and any objects in orbit around the sun or in orbit around an object orbiting the sun. 

Most prominent of these objects are the planets. Our solar system consists of the sun, the nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, satellites of the planets, numerous comets, asteroids, and meteoroids. The Sun is the richest source of electromagnetic energy; the planets, over 61 moons, the asteroids, comets, meteoroids and other rocks and gas all orbit the Sun. In addition there is dust and debris orbiting the sun, but we shall consider this insignificant.

If we consider only the planets, then the extent of the solar system is approximately 39 astronomical units. This would be the orbit of Pluto. However, if we consider comets, then the solar system is a much larger body. Cometary orbits may be on the order of 100,000 astronomical units. This is about one and one-half light years, or roughly one-third the distance to the nearest stars. To gain some insights into the size of the solar system consider the tip of your nose as the sun. Imagine the Earth to be one inch from your nose. Pluto would be a little more than one yard from your nose. The farthest comets would be more than one and one-half miles away.

The whole solar system orbits the center of our home galaxy, the Milky Way which is a spiral disk of 200 billion stars (more recent estimates have given numbers around 400 billion) and thousands of clusters and nebula, including at least almost all objects of Messier's catalog which are not galaxies on their own. Distribution of hydrogen clouds and radio astronomial investigations concluded that Milky Way is actually a spiral galaxy. It is about 30,000 light years from the center of the galaxy.

The Milky Way has two small galaxies orbiting it nearby, which are visible from the southern hemisphere. They are called the Large Magellanic Cloud and the Small Magellanic Cloud. The nearest large galaxy is the Andromeda Galaxy which is also a spiral galaxy like the Milky Way but is 4 times as massive and is 2 million light years away. Our galaxy, one of billions of galaxies known, is traveling through intergalactic space.

The location of sun in our galaxy  

Our galaxy was not recognised as a galaxy until the 20th century.

The planets, most of the satellites of the planets and the asteroids revolve around the Sun in the same direction, in nearly circular orbits. When looking down from above the Sun's North Pole, the planets orbit in a counter-clockwise direction. The planets orbit the Sun in or near the same plane, called the ecliptic. Pluto is a special case in that its orbit is the most highly inclined (18 degrees) and the most highly elliptical of all the planets.

There are many popular misconceptions concerning the size and scale of objects in the Solar System. These mostly have to do with a failure to realize the relative radii of planets and the Sun, and the failure to appreciate how large the outer solar system is relative to the inner solar system.

The Sun and the gas giant planets like Jupiter are by far the largest objects in the Solar System. The other planets are small specks on this scale.

Mass and Densities:

The Sun contains 99.85% of all the matter in the Solar System. The planets, which condensed out of the same disk of material that formed the Sun, contain only 0.135% of the mass of the solar system. Jupiter contains more than twice the matter of all the other planets combined. Satellites of the planets, comets, asteroids, meteoroids, and the interplanetary medium constitute the remaining 0.015%.

Almost all the solar system by volume appears to be an empty void which comprises the interplanetary medium. It includes various forms of energy and at least two material components: interplanetary dust and interplanetary gas. Interplanetary dust consists of microscopic solid particles. Interplanetary gas is a tenuous flow of gas and charged particles, mostly protons and electrons -- plasma-- which stream from the Sun, called the solar wind.

Solar Wind Variations

The solar wind is not uniform. Although it is always directed away from the Sun, it changes speed and carries with it magnetic clouds, interacting regions where high speed wind catches up with slow speed wind, and composition variations. The solar wind speed is high (800 km/s) over coronal holes and low (300 km/s) over streamers. These high and low speed streams interact with each other and alternately pass by the Earth as the Sun rotates. These wind speed variations buffet the Earth's magnetic field and can produce storms in the Earth's magnetosphere.

The Ulysses spacecraft has now completed one orbit through the solar system during which it passed over the Sun's south and north poles. Its measurements of the solar wind speed, magnetic field strength and direction, and composition have provided us with a new view of the solar wind.

The Advanced Composition Explorer (ACE) satellite was launched in August of 1997 and placed into an orbit about the L1 point between the Earth and the Sun. The L1 point is one of several points in space where the gravitational attraction of the Sun and Earth are equal and opposite. This particular point is located about 1.5 million km (1 million miles) from the Earth in the direction of the Sun. ACE has a number of instruments that monitor the solar wind and the spacecraft team provides real-time information on solar wind conditions at the spacecraft.

(Solar Wind Variation – text obtained from http://science.nasa.gov/ssl/pad/solar/sun_wind.htm .)

The Terrestrial Planets

As we study the planets, astronomers like to categorise the planets into two categories. The first category is the terrestrial planets. The second category is the Jovian planets. The terrestrial planets include Mercury, Venus, Earth, and Mars. These planets have solid surfaces, with the last three also having an atmosphere. Their size, radius, and density are all similar.

The Jovian Planets

The rest of the Jovian planets include Saturn, Uranus, and Neptune. They are much larger in radius than the terrestrial planets. Like Jupiter, these planets are gas giants as the majority of their volume is composed of gas. The structure and Saturn is fundamentally identical to the structure of Jupiter, although the liquid metallic hydrogen region is not as large in radius as that same region on Jupiter. Therefore the magnetic field of Saturn, although strong, is not as large as the field on Jupiter.

The Formation and Evolution of Solar System

Solar System formed about 4.6 thousand million years ago when a cloud of gas or dust in space was disturbed by a supernova of a nearby star. The explosion formed shock waves. Compression of the shock waves compressed the cloud of gas and dust, making it collapsed. It is because of  the gravitational forces that would like to collapse it overcome the forces associated with gas pressure that would like to expand it (the initial collapse might be triggered by a variety of perturbations---a supernova blast wave, density waves in spiral galaxies, etc.).This is known as The Nebular Hypothesis which was formed by Kant and Laplace in the 18th century.

 In the Nebular Hypothesis, a cloud of gas and dust collapsed by gravity begins to spin faster because of angular momentum conservation

As the cloud grew hotter and denser in the center, a disk of gas and dust surrounding it that was hot in the center but cool at the edges was formed. When disk got thinner and thinner, accretion occurs. In accretion, small particles gradually coalesce, clumping together to form a larger object, forming planets and moons.

The collapsing, spinning nebula begins to flatten into a rotating pancake

As the nebula collapses further, local regions begin to contract gravitationally on their own because of instabilities in the collapsing, rotating cloud

Material migrating gravitationally to the centre of the disk formed a diffuse globe, called the "proto-Sun". As more and more matter became incorporated into the proto-Sun and it became denser, its temperature increased. Eventually it became so hot that thermonuclear reactions.

 The center of the cloud,where planets like Earth formed, only rocky material could stand the great heat. Icy matter settled in the outer regions of the disk, where the giant planets like Jupiter formed. As the cloud continued to fall in, the center eventually got so hot that nuclear fusion was initiated, and it became a star. When the Sun first "turned on" it blew most of the gas and dust of the new solar system with strong stellar wind. This phase in the evolution of the Sun is called the T-Tauri phase.

The Eagle Nebula consists of gas and dust from which stars are born

The Orion Nebula is a region of starbirth occuring today.