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The outer planets - Jupiter, Saturn, Uranus and Neptune are called the giant planets or Jovian planets. They are mainly made up of hydrogen, helium and ice. Compared to the terrestrial planets, they contain little iron and rock. Each of the giant planets has several satellites. They also have rings around them. However, only Saturn's large, bright rings can be easily seen through a small telescope.
All planets revolve round the sun in close elliptical paths. The nine planets in the order of increasing distances from the sun are given below:Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.
Mercury
Mercury is the smallest planet in the solar system and it is nearest to the sun. It is a bare, rocky planet not much larger than our moon. There is no water or air on mercury. By day it is hotter than the hottest desert. At night, it is freezing cold. The ground is covered with craters formed by the crashing meteorites/rocks.
Although only one third the size of the earth, mercury is very dense. The surface of mercury is similar to the earth's moon. Everything we know about mercury comes from one space probe, Mariner 10, which was sent in the year 1973, 1974 and again in 1975. With no atmosphere or water to erode them, the craters look as fresh as the day they were formed.
Venus
The second planet from the sun is Venus. Venus is almost the same size as the earth, but it is surrounded by thick clouds of hot, poisonous gases. The surface of Venus is generally plain, dotted with mountains, valleys, craters and volcanoes, which may or may not be extinct.
It is the brightest object in the night sky other than the moon. It is visible in the early morning in the eastern sky or in the early evening in the western sky. It appears very bright because it reflects about 75 percent of the light, which it receives from the sun.
This planet is also known as earth's twin. The thick atmosphere on Venus has made the pressure in its atmosphere 90 times greater than on earth. Its atmosphere mainly consists of carbon dioxide. Carbon dioxide traps the heat causing green house effect. The temperature increases to 480oC. The atmosphere contains nitrogen, water vapor and traces of gases like sulphur. Everything we know about this planet is from the four Russian Venera probes. Like earth, Venus has a rocky mantle and a crust. Core contains iron, which may be partly in the molten stage.Earth
Earth is the only planet in our solar system, which is surrounded by oxygen, the life giving gas. It is the only planet to have large quantities of water. Earth's atmosphere contains oxygen, nitrogen, carbon dioxide and water vapour.
Earth is a medium sized planet with a thick atmosphere. The inside of the earth is still hot. The molten outer core of iron generates electrical currents which may be the cause of this planets magnetic field. The first primitive life forms may have developed in the oceans.
Mars
It is called the red planet. The rocks on Mars are full of rusted iron. This is why the planet looks red. Huge storms of dust can also make the sky look pink. Mars has two very odd little moons called Deimos and Phobos. They look like giant potatoes and are pitted with craters. The space probe Mariner-9 that orbited Mars discovered channels that look like dried-up riverbeds.
Like earth Mars has a 24-hour day. Mars has a weak magnetic field. The Martian atmosphere is only one hundredth the density of earth. The presence of cracks and canyons indicate past earthquakes. In 1976, two Viking space probes were set out to investigate mars.
Jupiter
Jupiter is a huge ball of gases. It is the biggest planet in the solar system.
Jupiter has a huge red spot on its surface. Jupiter has more than 20 moons that spin around it.
Saturn
A ringed planet like Jupiter, Saturn is a giant ball of gases with a rocky core.
From earth it looks as if three wide rings surround Saturn. In fact, these rings are made up of thousands of narrow rings. Each ring of Saturn is made up of dust and tiny bits of ice and rock. Saturn has more than 30 moons, more than any other planet in the solar system. A blanket of nitrogen gas surrounds Titan, the biggest of them.
Uranus
Uranus is the only planet in the solar system to orbit the sun tipped over on its side. The axes of most planets are almost perpendicular (at an angle of 90o) to the planets paths around the sun. But the axis of Uranus is tilted 98o from the perpendicular position, so that it is almost level with the planet's path around the sun.
Uranus has at least 11 narrow rings. It has more than 21 moons.
Neptune
Neptune is another gas giant. Some of the fastest winds in the solar system blow across its surface.
The winds can reach speeds of over 2000 km an hour. Therefore, Neptune is said to be the windiest planet in the solar system. Neptune has 13 moons circling it.
Pluto
Pluto is the ninth planet and is the most distant planet of all. It has one moon that is half as big as the planet.Note:
The number of moons of the planets keeps changing i.e., with the imporvement in observing techniques, many new moons may be discovered.
Pluto
Some important facts and figures about the planets are given below:
| Name of planet | Distance from sun | Time taken for one revolution around sun | Time taken to turn once on its axis | Diameter of planet | Mass of planet compared to earth taken as 1 |
|---|---|---|---|---|---|
| Mercury | 58X106 km | 88 days | 58 days | 4880 km | 0.055 |
| Venus | 108x106 km | 225 days | 243 days | 12100 km | 0.8 |
| Earth | 150x106 km | 365 1/4 days | 24 hours | 12760 km | 1 |
| Mars | 228x106 km | 687 days | 24 hours 37 minutes | 6780 km | 0.1 |
| Jupiter | 778x106 km | 11 3/4 years | 9 hours 50 minutes | 142800 km | 318 |
| Saturn | 1427x106 km | 29 1/2 years | 10 hours 14 minutes | 120000 km | 95 |
| Uranus | 2870x106 km | 84 years | 10 hours 49 minutes | 50800 km | 15 |
| Neptune | 4504x106 km | 165 years | 15 hours 48 minutes | 48600 km | 17 |
| Pluto | 5900x106 km | 248 years | 6 days 9 hours | 6000 km | 0.002 |
