How many Planets do we have

How many Planets do we have

In early times, people studied the night sky, by observing the objects with the naked eye, and then through telescopes. In the night sky, planets appear as bright specks of light, much like stars. They do not twinkle. While the stars show no visible change in their position in the sky, the planets seem to move or ‘wander’ with respect to the stars. Hence, they were named planets, which means ‘wanderers’ in Greek.

Unlike stars, planets do not emit light of their own. They appear bright like stars in the night sky because they reflect light from the sun (like the moon does). They are also closer to us than the stars and, therefore, the light coming to us from the planets is brighter and bigger (i.e., they are generally bigger blobs of light) and hence does not appear to twinkle.

Mercury, Venus, Mars, Jupiter, and Saturn are quite bright and can be seen with the naked eye. The other planets, namely Uranus and Neptune, are very faint and can be seen only with a telescope. Uranus can sometimes be seen with the naked eye. Mercury, Venus, Earth, and Mars are called inner planets because they are much closer to the sun as compared to Jupiter, Saturn, Uranus, and Neptune, which are called outer planets.  A few differences between inner planets and outer planets are given in Table.

Table: Differences between inner and outer planets

Let us now learn about each of these planets.

Mercury

This planet is not always visible to us as it is very close to the sun and is generally hidden in the sun’s glare. However, it can be seen as a bright spot of light during sunrise and sunset at a particular time of the year. This is the reason why, even though it is not a star, Mercury is known as the morning star or the evening star. It can be seen just before sunrise in the months of September and October and just after sunset in March and April. Mercury has no satellites. It is the smallest planet in our solar system.

Venus

This planet is the brightest object in the night sky after the moon. Since it is close to the sun, like Mercury, Venus too can be seen just before sunrise and just after sunset. Venus is also called the morning star or the evening star. Venus, like Mercury, has no satellites. It rotates from east to west, which is opposite to the Earth’s direction of rotation (west to east).

Earth

This is the planet that we live on. It has one natural satellite, the moon. The Earth takes 365.256 days to complete one orbit around the sun, which is one Earth year. We know that the Earth revolves around the sun, and that it also rotates about an axis, an imaginary line passing through the North and South poles. It takes 24 hours to complete one rotation. The axis of rotation of the Earth is tilted to the plane of its orbit. This is primarily responsible for the changes in season on the Earth. A thin layer of gases called the atmosphere envelops the Earth. Ninety nine per cent of the Earth’s atmosphere is made up of nitrogen and oxygen, and the remaining 1% contains other gases like argon, carbon dioxide, and water vapour.

Our Earth is unique! It is the only place we know where life exists.
The composition of its atmosphere and soil, its size, distance from the sun, and the presence of large amounts of water in liquid form on its surface make it different from any other planet.

Mars

This planet is visible to the naked eye. It looks like a bright orange-red star. Mars has two satellites—Phobos and Deimos. Both are dark, dusty, and irregular in shape. The Martian soil contains large amounts of iron oxide, which gives the planet a reddish look. Space probes have photographed deep channels on the surface of Mars, which suggest that water once flowed there in the distant past. The radius of Mars is a little more than half that of Earth. Its mass is about ten times lesser than the mass of Earth.

Jupiter

It is the largest planet in the solar system. After the moon and Venus, Jupiter is the brightest object in the night sky.
Jupiter can be easily seen with the naked eye. It looks like a bright star. With a good telescope, we can see its tinted cloud bands and the famous Great Red Spot, which is a great hurricane on Jupiter, twice the diameter of the Earth. Jupiter has four large moons (Io, Europa, Ganymede, and Callisto) and dozens of small ones.

Saturn

It is the second-largest planet in the solar system after Jupiter. The radius of Saturn is close to that of Jupiter, and its mass is about one-third the mass of Jupiter. Saturn is often called the ‘Ringed Planet’ because it is surrounded by distinctive rings of dust and rocks. Saturn has been known since prehistoric times. However, early astronomers could not see Saturn’s rings in much detail. Saturn has 25 satellites that measure at least 10 km in diameter and several smaller ones. Saturn’s biggest satellite, Titan, is even bigger than the planet Mercury. Some astronomers think that it may be able to support life.

Uranus

It was identified as a planet by the British astronomer William Hershel in 1781. Uranus can sometimes be seen with the naked eye, but not very easily. Like Saturn and Jupiter, Uranus too has rings. Uranus is about 2.9 x 109 km from the sun. It has five known satellites. The atmosphere of Uranus consists mainly of hydrogen, helium, and methane. Unlike any other planet in the solar system, Uranus spins on its side. Astronomers think that this is the result of a collision with a planet-sized object millions of years ago.

Neptune

It was first identified as a planet by the German astronomer Gottfried Galle in 1846. Neptune cannot be seen with the naked eye. It looks like a star through a pair of binoculars and like a small bluish circle through a powerful telescope. Neptune has two known satellites.

Pluto (a dwarf planet)

Pluto is so far away that it is very difficult to see it from the Earth. It was discovered in 1930 by Clyde Tombaugh of America. Pluto has an oval-shaped orbit, which takes it 249 years to go once around the sun. For about 20 years of its orbit, it is closer to the sun than the planet Neptune.