Solar System: Definition, Planets, Galaxy and the Universe

The solar system comprises the Sun, eight planets, stars from the Milky Way galaxy orbiting the Sun, 210 planetary satellites, and countless asteroids. Comets and other ice things formed of incredibly tenuous gases and dust particles are found in the solar system.

The solar system as we know it is almost (4.6) billion years old. The space family is vast and expanding. Over the years, there have been new studies on the solar system. Let us learn more about Solar System the planets with a diagram.

What is a Solar System?

Our solar system consists of the sun and the celestial bodies revolving in it. It is composed of many heavenly bodies like comets, asteroids, meteors, and planets. The gravitation force of attraction between the sun and the other bodies around it holds our solar system in its place. The sun lies at the approximate centre of the solar system. Like all other planets in the solar system, Earth revolves around the sun. The eight planets in the increasing order of distance from the Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

The vast majority of our solar system’s mass lies in the sun, with most of the remaining mass in Jupiter. Mercury, Venus, Earth, and Mars are terrestrial planets primarily composed of rock and metal. These are the four innermost planets. The four outer system planets consisting of Jupiter, Saturn, Uranus, and Neptune are giant planets. These planets are substantially more massive than the terrestrials.

Jupiter and Saturn are the two largest planets of our solar system. These are called gas giants since they are composed mainly of hydrogen and helium. Uranus and Neptune, the two outermost planets, are called ice giants. These are substances like water, ammonia, and methane, which have relatively high melting points compared to the volatiles, i.e., hydrogen and helium. The circular orbits of all the eight planets lie within the ecliptic, which is a flat elliptical disc.

Planets in the Solar System

Let us learn about all the planets there in our solar system and their properties.

Sun

Sun is a star that lies at the centre of our solar system. It is the star closest to the Earth and is responsible for the existence of life on the Earth. The temperature and density at the sun’s centre are quite high, enough to sustain the nuclear fusion of hydrogen to helium. Due to these reactions, a large amount of energy in the form of heat and light is emitted continuously from the sun. The energy received from the sun is the main source of energy on the Earth.

Planets

Planets are the heavenly bodies that move around a star in a certain elliptical orbit. Our solar system consists of \(8\) planets. Although initially there were \(9\) planets, in 2006, a new definition of a planet was adopted in the International Astronomical Union (IAU). The farthest planet from the sun, pluto, did not fit that definition, and hence it was removed from the list of planets. It is now defined as the dwarf planet.

These planets move around the sun in a definite path called an orbit. The time taken by a planet to complete one revolution around the sun is known as its period of revolution. The period of revolution of a planet increases with its distance from the sun. A planet also rotates about its axis, and the time taken to complete one rotation around its axis is known as the rotation period. Some planets have small celestial bodies revolving around them, and these are known as their natural satellites. 

For example, the moon is the natural satellite of the Earth. However, various man-made space probes have also been sent around the moon and mars to study their structure. These are called artificial satellites.

These planets resemble stars in their structure but differ in composition. Unlike stars, planets do not have their light; they reflect the rays of light falling on them from the nearby star. Stars twinkle, but the planets do not, and this helps us differentiate between a star and a planet in the night sky. Planets are continuously moving, and their position to the stars keeps changing.

Mercury

Mercury or Budh is the planet that is closest to the sun. It is very difficult to observe mercury in the sky because of its proximity to the sun. The sun’s glare makes it hard to locate mercury in the sky. Mercury can only be observed very close to the horizon, either before sunrise or sunset. It is better visible in open places away from trees and buildings. Mercury is the smallest planet in our solar system, and it has no natural satellite.

Venus

Venus or Shukra is the second closest planet to the sun. It is the Earth’s planetary neighbour. Since it is the brightest planet, it is quite clearly visible in the night sky. Venus is referred to as a morning or evening star because it is often visible before sunrise in the eastern sky and after sunset in the western sky. Venus does not have a natural satellite. Unlike the Earth and other planets, Venus does not rotate around its axis from west to east, but it rotates from east to west.

Earth

Earth or Prithvi is the third planet from the sun. Life is known to exist only on our planet due to the special environmental conditions prevalent on the Earth. These special conditions include its appropriate distance from the sun, its temperature and the presence of water, a suitable atmosphere, and the protective ozone layer around it. Due to the reflection of light from water bodies and land, the Earth appears blue-green when viewed from outer space. Earth’s axis of rotation is not exactly perpendicular; it’s tilted, and it is the reason behind the change of seasons on the Earth. Earth has a natural satellite called the moon, which is visible regularly in the night sky.

Mars

Mars or Mangal is Earth’s next-door neighbour and fourth in line from the sun. It has two small natural satellites- Phobos and Deimos. It is known as the red planet owing to its reddish appearance.

Jupiter

Jupiter or Brihaspati is the largest planet in our solar system. It is big enough to fit \(1300\) earth inside it. However, the mass of this giant planet is almost \(318\) times the mass of our Earth. It spins very quickly on its axis. Its faint rings make it easy for us to recognize it in the sky. Jupiter has \(79\) known natural satellites, and four of these are easily visible using a telescope.

Saturn

Saturn or Shani is the second-largest planet in our solar family. It has a yellowish appearance and beautiful rings around it. Although these rings are not visible to the naked eye, we can observe them using a small telescope. Saturn has the least density among all the planets; its density is less than water. It has \(82\) known natural satellites.

Uranus

Uranus or Arun is the second last planet in our solar system, and we need a large telescope to look at it. Uranus rotates from east to west like Venus. Its orbital motion appears to roll on its side because of its highly tilted rotational axis. It has \(27\) known natural satellites.

Neptune

Neptune or Varun is the farthest planet from the sun. It can only be seen with the help of a large telescope. It is slightly smaller than Uranus, and it has \(14\) known natural satellites.

Asteroids

The large gap between the orbits of Mars and Jupiter is occupied by numerous small objects called Asteroids. These objects, like the planets, revolve around the sun and can only be seen through a large telescope.

Comets

Comets are members of our solar system that revolve around the sun in highly elliptical orbits. The period of revolution of a typical comet is very long compared to that of planets. A comet generally has two parts: a bright head and a long tail. The length of its tail increases as it approaches the sun. The direction of the comet’s tail is away from the sun. Comets appear periodically through space, and one such is Hailey’s comet appears every \(76\) years; it was last reportedly seen in 1986.

Meteors and Meteorites

A meteor is a small object that occasionally enters the atmosphere of the Earth at really high speeds. They suffer large frictional force due to their motion through the atmosphere, which causes them to heat up. Due to this heat, they begin to glow and evaporate quickly; that is why sometimes, on a night, we see bright streaks of light passing through the night sky. We call them shooting stars, but in fact, these are the meteors. This bright streak only lasts for a short duration of time because the tiny meteors evaporate due to the excessive heat. However, large meteors can reach the Earth before they can evaporate completely. These large meteors that reach the surface of Earth are called a meteorite. These meteorites help scientists study the nature of the material that led to the solar system’s formation.

Stars

A star is a type of astronomical body that consists of a luminous spheroid of plasma held together by gravity. The Sun is the closest star to Earth. Many other stars can be seen with the naked eye at night, but their enormous distances from Earth cause them to appear as fixed points of light.

Constellations

A constellation is a group of stars that has been named and resembles a specific shape in the sky. These stars are a long way away from Earth. They have nothing to do with one another. Some stars in a constellation may be very close to one another, while others may be very far away. However, if you draw lines in the sky between the stars like a dot-to-dot puzzle and use your imagination, the picture will resemble an object, animal, or person.
Throughout history, different cultures have given different names and numbers to constellations based on what they thought they saw. There are currently 88 officially recognised constellations.
Some of the famous constellations are Ursa Major, Cassiopeia, Orion, Canis Major, Centaurus.

Summary

Our solar system consists of the sun and the celestial bodies revolving in it. It is composed of a large number of heavenly bodies like comets, asteroids, meteors, and planets. The gravitation force of attraction between the sun and the other bodies around it holds our solar system in its place. Planets are the heavenly bodies that move around a star in a certain elliptical orbit. Our solar system consists of 8 planets.

These planets resemble stars in their structure but differ in composition. Unlike stars, planets do not have their light; they reflect the rays of light falling on them from the nearby star. The large gap between the orbits of Mars and Jupiter is occupied by numerous small objects called Asteroids. Comets are members of our solar system that revolve around the sun in highly elliptical orbits. The period of revolution of a typical comet is very long compared to that of planets. A meteor is a small object that occasionally enters the atmosphere of the Earth at really high speeds.

FAQs

Q.1. What are asteroids?
Ans: Asteroids are the small heavenly bodies present between the orbits of Mars and Jupiter that revolve around the sun.

Q.2. How many planets are there in our solar system?
Ans: There are total \(8\) planets in our solar system.

Q.3. Which is the smallest planet in our solar system?
Ans: Mercury is the smallest planet in our solar system.

Q.4. Which is the largest planet in our solar system?
Ans: Jupiter is the largest planet in our system.

Q.5. Why is life possible only on Earth?
Ans: Life is possible on Earth due to its appropriate distance from the sun, ambient temperature, water, atmosphere, and the ozone layer.

NCERT Solutions For Class 8 Science Science Chapter 17: Stars And The Solar System

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