Our Earth and The Solar System

The solar system

(The sun has a magnetic field, the heliosphere, which envelops our entire solar system)

Our Solar System

The solar system is dominated by the sun and it accounts for almost 99.9% of the matter of the whole system. It is the source of light and heat for our earth. Scientists believe that the Sun has been formed from a moving cloud of gases, which is called Nebula. The sun and the planets were born out of this cloud. The force of gravity has created them. For over millions of years, these balls of dust and gas are moving around the sun. The Sun by virtue of its mass and weight, controls the movement of the planets. This force is called the force of gravity.

Sun is at the Centre of the solar system with eight planets (i.e. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune), satellites, asteroids, meteors and comets. They all move around the Sun. Till 2006, there were nine planets in the solar system. Pluto was the farthest planet from the sun. In 2006, the International Astronomical Union (IAU) adopted a new definition of a planet. Pluto and other celestial bodies like Ceres, 2003 UB313 does not fit this definition. It is no longer considered as a planet of the solar system.

The sun

It is believed that the sun was born about 5 billion years ago. Since that time, it is continuously emitting huge amount of heat and light, and it is expected to glow in a similar manner for the next 5 billion years or so. The sun is essentially a sphere of hot gases. The disc is called photosphere. The layers of gas above the photosphere are extremely hot, but their densities are extremely low. These layers are very faint and are not visible in the presence of strong light from the disc of the sun. At the time of the total solar eclipse, when the light of the sun’s disc is completely cut off, the outermost layer becomes visible. It appears like a crown round the sun. This layer is called Corona.

Our Sun

The radius of the sun is almost 100 times of the radius of our earth and its mass is about a million times the mass of our earth. The sun is the nearest star from us. The sun is the source of almost all energy on the earth. In fact, the sun is the main source of heat and light for all the planets. The temperature of the bright disc is about 6000 K. At the center of the Sun, the temperature is about 15 million K. The pressure is also extremely high. All these conditions are right for fusion reactions.

In the sun, four protons (nuclei of hydrogen atoms) combine to form a helium nucleus. The mass of the helium nucleus is slightly smaller than the combined mass of four protons. So there is a loss of mass in the process. This loss of mass is the source of sun’s energy. According to Einstein’s equation i.e. E=mc² , E is the energy produced when a mass, m, is lost. The quantity, c, is the velocity of light. Most stars produce their energy by the process of fusion.

The sun is about 150 million km away from the earth. As light travels at a speed of 300,000 km/s., the light of the sun takes about 8.3 light minutes to reach us. The light of the nearest star (Proxima or Alpha centuari) to the sun reach earth in about 4.3 light years.

The planets

Planets are actually spinning around the Sun. They have their own elliptical path of movement known as the orbits. Movement around its own axis is called rotation and around the sun is revolution. They have their own speed also. Planets, unlike stars, have no light or heat of their own. They are lit by the light of the stars. That’s why, stars twinkle whereas planets do not. The word ‘planet’ comes from the Greek word “Planetai” which means ‘wanderers’. Planets keep changing their positions with respect to the stars. 

Mercury (Budha)

Mercury

It is nearest and the smallest planet of the solar system i.e. nearly of the same size and mass as the moon. Mercury takes 88 days for one orbit around the sun and 59 days for one spin on its axis. It has no satellite. There is no atmosphere on mercury. The surface of mercury is rocky and mountainous. One side of the surface facing the sun receives maximum heat and light. The surface of this planet does not receive sunlight or heat on its other side. One part of mercury, therefore, is very hot while the other part is very cold. Because Mercury is very close to the sun, it is difficult to observe it, as most of the time it is hidden in the glare of the sun. However, it can be observed in September and October just before sunrise in the eastern sky as a morning star. It is also seen in the western region of the sky immediately after sunset in March and April as an evening star.

Venus (Shukra)

Venus

Venus has no moon or satellite of its own. It rotates on its axis is somewhat unusual i.e. from east to west. Venus shows phases just like the moon. Its size is nearly the same as that of the earth. The mass of Venus is nearly 4/5 times that of the earth. It takes 255 days for one orbit around the sun and 243 days for one spin on its axis. Venus has an atmosphere that consists of mainly carbon dioxide. Minute quantities of water vapour and oxygen have also been found on Venus. The cloudy atmosphere of Venus sends back almost 3/4th of the sunlight that falls on it. That is why Venus could be seen as the brightest object in the sky after the sun and the moon. Sometimes Venus appears in the eastern sky before sunrise and sometimes it appears in the western sky just after sunset. Therefore it is often called a morning or an evening star. However the surface of Venus is much hotter than the earth but not as hot as that of mercury. Though mercury has no atmosphere, Venus is the hottest planet in our solar system.

The Earth (Prithvi)

Our earth

Our earth rotates from west to east. Our earth is more or less like a sphere, which is slightly flattened in the north and south. Slightly flattened or tapered at the poles, the earth is best to be described as Geoid which means earth like shape. Earth is the 3rd nearest planet to the sun. In size it ranks 5th. In size and make up the earth is almost identical to Venus. Due to presence of water and landmasses the earth appears blue-green in color from the space. Therefore, it is called blue planet. The earth is the only planet where some special environment conditions are responsible for the existence and continuation of life. These include just the right distance from the sun, so it has the right temperature range, the presence of water, soil, minerals, suitable atmosphere and a blanket of ozone. 

Earth’s Natural Satellite – The Moon (Chandra)

A small body revolving around a bigger body is called its satellite. The moon is a satellite of the earth. Planets are the satellites of the sun. They are natural satellites. Like planets, satellites also have no light or heat of their own. As the Moon revolve round the earth, and also around the sun its position changes daily in relation with the Sun. As a result, the relative positions of the moon keep on changing everyday. That is how moon appears different each night. The Moon takes almost the same time, 27 days and 7 hours to complete both the movements i.e. rotation and revolution. Thus it always presents the same side towards our earth. The moon has no atmosphere. It has no water.

Our Moon

On a full moon day, the earth lies in between the moon and the sun. So the full face of the moon is visible to us. On a new moon day, the moon lies in between the earth and the sun. So sunlight falls on that face of the moon, which is on the other side of the earth and we cannot see it. As we know the moon completes one revolution around the earth in 27 days and 7 hours. But, in the meanwhile the earth moves ahead in its orbit. Thus from the earth, the moon appears to complete one revolution between one new moon day and the next, in 29 ½ days. Nearly one-half of the Moon is always illuminated by the sunlight. But, on the day following the new moon day, the illuminated part of the moon that is visible from the earth is crescent in shape. The various shapes of the bright part of the moon as seen during a month are called phases of the moon. 

The diameter of the moon is only 1/4th of that of the earth. The Moon is about 384,400 km away from the earth. The light reflected by the moon reaches us in just one and a quarter seconds. Neil Armstrong, Edwin Aldrin and Collins set foot on the Moon for the first time on July 21, 1969. They found that the moon’s surface is dusty and barren. There are many craters of different sizes. It also has a large number of steep and high mountains. These cast shadows on the moon’s surface.

Mars (Mangal)

Mars

Mars is almost half of the size of the earth. The diameter of Mars is slightly more than half of that of the earth but its mass is only 1/10th of that of the earth. It takes 687 days for one orbit around the sun and 1 day for one spin on its axis. The atmosphere of Mars is much thinner than that of the earth. It perhaps consist mainly nitrogen. Traces of oxygen have also been found. It appears slightly reddish and, therefore, it is also called the red planet. Mars has two small natural satellites named Phobos and Deimos.

The Mars is visible from the earth for most part of the year. However, it is best situated for observation when it is opposite the sun’s position in the sky with respect to the earth. On these days it is closer to the earth as well. Astronomers have noticed certain changes on the surface of Mars that gave them idea that water may also be present on the planet and it may have life in some form. However, so far, no evidence of water or life has been found on it although possibility of their existence is still being investigated.

Jupiter (Brihaspati)

Jupiter

Jupiter is the largest planet of the solar system. It takes 11 years and 11 months for one orbit around the sun and 9 hours, 56 minutes for one spin on its axis. It has 16 satellites. It has faint rings around it. Its most distinguishing feature is the great red spot. It is believed that it is a complex storm in the atmosphere of the planet ranging for a few hundred years. Its mass is more than the combined mass of all other planets. Because of its large mass, Jupiter exerts a strong gravitational pull on other objects which pass by it. A comet passing close to it is likely to experience this pull and stray from its original path. In 1994, the comet, Shoemaker-levy, actually broke up into several pieces when it came too near this planet. The pieces then collided with the planet.

Jupiter’s bright appearance in the sky after Venus and occasionally Mars is due to its thick atmosphere that reflects most of the sunlight falling on it. It is believed that Jupiter mainly consists of hydrogen and helium in gaseous form. Its cloud like outer regions consists of methane in gaseous form while ammonia is present in crystalline form.

Saturn (Shani)

Saturn

Beyond Jupiter is Saturn which appears yellowish in colour. What make it unique in the solar system are its three beautiful rings. It takes 29 years, 5 months for one orbit around the sun and 10 hours, 40 minutes for one spin on its axis. It has 18 satellites. Saturn is the least dense among all the planets. Its density is less than that of water. It is similar in size, mass and composition to Jupiter. It is however cooler than the Jupiter.

Uranus

Uranus

Uranus was the 1st planet to be discovered with the help of a telescope. William Herchel discovered the planet in 1781. Hydrogen and methane have been detected in the atmosphere of Uranus. Like Venus, Uranus also rotates from east to west. Except Venus and Uranus, all other planets rotate in the same direction in which they revolve. The most remarkable feature of Uranus is that it has highly tilted rotational axis. As a result, in its orbital motion it appears to roll on its side. It takes 84 years for one orbit around the sun and 17 hours, 14 minutes for one spin on its axis. It has 17 satellites. 

Neptune

Neptune

Neptune discovered by Sir William Herchel on the basis of the law of gravitation given by Sir Issac Newton which he gave almost 180 years before it. Neptune takes 164 years for one orbit around the sun and 16 hours 7 minutes for one spin on its axis. It has 8 satellites. Uranus and Neptune are quite faint and cannot be seen with the naked eye. That is why only six planets were known in ancient times. These two planets have been discovered only after telescopes came into use in astronomy. 

Asteroids (meaning starlike)

Asteroid Belt

There are numerous tiny bodies found between the orbits of Mars and Jupiter, which also revolve around the Sun. These bodies are called Asteroids. Each asteroid has its own orbit and the orbits of all of them are spread over a large distance forming a band. A few asteroids have elongated orbits going beyond the orbit of Jupiter. The size of asteroids varies from barely a pebble to a few hundred kilometres. The largest asteroid is Cere, has a diameter of about 1,000 km. Scientists believe that asteroids are the pieces of matter that somehow could not get assembled in the form of a planet. The asteroids are small rocky bodies. While making their rounds the asteroids often collide and break into smaller pieces. These smaller pieces or Meteorites sometime fall on to the earth. They crash to the ground and create craters. Meteor crater in Arizona, USA and Lonar Lake in the state of Maharashtra, India are good examples of that.

Meteors and Meteorites

A Meteorite from Mars

Meteors are very small stone-like objects that are revolving around the sun. Their existence becomes known only when some of them occasionally enter the earth’s atmosphere. At that time it has a very high speed. Due to the friction in the atmosphere it heats up. It glows and evaporates quickly. The path of the meteor, therefore, appears as bright streaks of light in the sky. The bright steak lasts for a very short time. These are commonly known as shooting stars, although they are not stars. 

Some meteors are large and so they can reach the earth before they evaporate completely. The body that reaches the earth is called a meteorite. They can be examined in the laboratory. They give us an idea of the composition of the material from which the solar system was formed.

Comets

A Comet

Comets revolve around the sun in highly elliptical orbits. However, their period of revolution round the sun is usually very long. They become visible from the earth only when they come close to the sun. A comet appears generally as a bright head with a long tail. The length of the tail grows in size as it approaches the sun. The tail disappears again when the comet move away from the Sun. The tail of a comet is always directed away from the sun. Many comets are known to appear periodically. One such comet is Halley’s comet, which appears after nearly every 76 years. It was last seen in 1986. The study of comet tails has shown the existence of molecules of carbon, nitrogen and hydrogen. Since these molecules can help to form complex molecules necessary for the origin of life, some scientists have suggested that the seeds of life on the earth were brought by comets from the outer space.

Meteor Showers

Meteor Showers

When the earth crosses the tail of a comet, swarms of meteors are seen. They are known as meteor showers. Some meteor showers occur at regular intervals each year.