We live in a mammoth universe surrounded by an infinite number of stars and other celestial bodies. This universe is said to have originated as a result of a massive explosion about 14 billion years ago, called ‘The Big Bang’.
In the beginning, the entire universe was a condensed very hot, small, and dense structure (called a “singularity”). This singularity was very unstable as pressure mounted up and it exploded forming stars, groups of stars called galaxies and stellar systems containing planets along with stars. As a result of this explosion, the universe is still expanding today, but getting colder as well.
A star is a massive ball of plasma (very hot gas) held together by gravity. It radiates energy in the form of heat, light etc., due to nuclear reactions happening inside it. A planet is an object like Earth that orbits a star and is large enough to have sufficient gravity to make it almost spherical in shape. It is smaller than a star, and it does not make light. Objects that orbit planets are called satellites. A star and everything which orbits it is called a star system.
After the Big Bang, the cloud of dust was not uniformly distributed across ‘the universe’. The areas where the dust particles concentrated attracted more materials from the surrounding cloud due to t the gravitational force of attraction. Such thicker clouds spreading across very large areas were named as ‘Nebula’ and consisted mainly of hydrogen. As the nebula grew further, there were localised clumps of gases. These clumps attracted more materials and became heavier to develop into stars, while the whole of the nebula developed into a galaxy or cluster of stars.
Before the stars developed within the nebula the core of each gas clump was surrounded by rotating disk of gases and dust around a gas core. As the universe cooled down the rotating disc of gases condensed and clung together at various areas to form planets and the gas core developed into a star.
The star system containing our planet earth is called solar system after the star which is in the centre of this system, Sun (word “solar” derives from the Latin “sol,” which means the sun). The Sun is orbited by planets, asteroids, comets and other things. An asteroid is an object in the Solar System that travels around the Sun, but smaller than a planet.
The name “asteroid” means “like a star” in the ancient Greek language because ancient people thought they are small stars. They are a swarm of small bodies in between the orbits of mars and Jupiter, which revolve around the sun. There is an opinion that the name “planetoid” (“like a planet”) would be a better name. Most asteroids in the Solar System are in the asteroid belt between Mars and Jupiter. Many are not in the main asteroid belt. The ones that come close to Earth are called Near-Earth asteroids.
A comet is a ball consisting mostly ice that moves around in outer space, often described as “dirty snowballs”. The orbital inclinations of comets are usually high and not near the ecliptic where most solar system objects are found. Those are very far away from the Sun, but some comets come near enough to Earth for us to see at night. They have long “tails” because the Sun melts the ice and points directly away from the Sun because it is blown by the solar wind. The hard centre of the comet is the nucleus.
There are eight planets in the Solar System. From closest to farthest from the Sun, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The first four planets called terrestrial planets or inner planets are mostly made of rock and metal, and mostly solid. The last four planets called gas giants and are mostly made of gas. Even though they are made of gas, they have much more mass than the terrestrial planets. All the nine planets of the solar system revolve around the sun in elliptical orbits in the counter-clockwise direction. The time taken by the planets to revolve around the sun increases with their distance from the sun. They also rotate on their own axes. Except for Venus and Uranus rotates counter-clockwise as seen from above the North Pole; that is to say, from west to east. Uranus rotates “on its side,” 90 degrees away from its orbital motion while Venus rotates backwards compared to the other planets, both likely due to an early asteroid hit which disturbed its original rotation.
Pluto had been called a planet since it was discovered in 1930, but in 2006 astronomers meeting at the International Astronomical Union decided on the definition of a planet, and Pluto did not fit. Instead, they defined a new category of dwarf planet, into which Pluto did fit, along with some others. These small Planets are sometimes called plutinos.
A satellite is a smaller body which moves around a larger body held in orbit by gravitation. The term is commonly used for moons which go around planets. Six of the planets and three of the dwarf planets in our solar system are orbited by moons. For example, Moon is the satellite of Earth.
The Sun contains 99.9 percent of the Solar System’s mass and is mostly made out of hydrogen and helium. The Sun is about 1.3 million times larger than earth and a hundred times as wide as the Earth. It has a mass of 1.9891×1030 kg, which is 333,000 times the mass of the Earth.
The Earth is the third planet in the solar system by the distance from the sun and fifth in size. It is the only planet known to have life on it and was formed around 4.5 billion years ago. As every planet, the earth rotates on its own axis and revolves around the sun. As the earth orbits the sun, it completes one rotation every 23.9 hours. This is called as a day. It takes 365 ¼ days to complete one trip around the sun. The extra quarter of the day presents a challenge to our calendar system, which counts one year as 365 days. To keep our yearly calendar consistent with our orbit around the sun, every four years we add one day and is called leap day, and the year is called the Leap year. While the Earth revolves around the Sun once (one “year”) for every 365¼ times and it rotates once (one “day”), Plane of rotation is slightly tilted from the axis of rotation. The seasonal changes on Earth are because of the 23.4-degree axial tilt of its rotation and slightly elliptical path around the Sun.
The Earth is the only planet in our Solar System that has a large amount of liquid water. About 71% of the surface of the Earth is covered by oceans. Because of this, it is sometimes called the “Blue Planet”.
The Earth is generally 150,000,000 kilometres away from the Sun (this distance is named an “Astronomical Unit”). The Earth moves along its way at an average speed of about 30 km a second. The closest distance Earth is to the sun, or perihelion, is 146 million km (91 million miles) and the farthest or aphelion is 152 million km (94.5 million miles). It takes light (or any other electromagnetic radiation) just over eight minutes to travel from Sun to Earth.
The Moon is Earth’s only satellite. Our moon is about a quarter the size of the Earth. The gravity on the moon is one-sixth of the Earth’s gravity. The moon is an average of 384,400 km away from earth and is drifting away from Earth at the rate of four centimetres each year. The Moon revolves around the Earth in about every 27⅓ days. As the Earth goes round the Sun at the same time, the changing light of the Moon takes about 29½ days to go from dark to bright to dark again. Because the earth is moving as well as rotating on its own axis as it orbits the sun- from our perspective the moon appears to orbit us every 29 days.