The Sun and the planets formed together 4.5 billion years ago from the solar nebula, a cloud of gas and dust. A blast wave from a distant supernova explosion probably triggered the solar nebula to collapse. The Sun developed at the centre, while the planets developed in a narrow disc around it. There are eight planets in our solar system. Mercury, Venus, Earth, and Mars are the four inner celestial bodies, all of which are mostly made up of rock. Jupiter, Saturn, Neptune, and Uranus are the four outer planets, all of which are massive planets made up mostly of gases or ice.
The Sun, moon, and bigger planets were visible to ancient astronomers with their naked eyes. Their observation and calculations of these bodies’ movements gave rise to the discipline of astronomy.
The volume of information on planets and smaller bodies’ movements, properties, and compositions has increased to enormous proportions today. The range of observational sensors has expanded far beyond the solar system to distant galaxies and the limit of the known universe. Except for Earth, all the planets were named after Greek and Roman gods and goddesses. The names Jupiter, Saturn, Mars, Venus, and Mercury were given thousands of years ago. The other planets were discovered considerably later, after the invention of telescopes.
Planets, unlike stars, do not sparkle. Stars are so far away that even through a telescope, they appear as identities of light inside the night sky. Because all the light is coming from a single source, its route is extremely vulnerable to environmental interference (i.e., their light is easily diffracted).
A planet is a natural satellite that
(a) Circles around the sun.
(b) Has sufficient mass to withstand multi-body constraints and acquire a hydrostatic equilibrium (nearly spherical) form.
(c) Has cleared the orbital region.
The word planet has been derived from the Greek word ‘plantes,’ which means wanderers. A planet can be defined as any moderately big natural body that travels in an orbit around the sun or another star and does not emit energy from inner nuclear fusion events.
The appearance of the planets in the solar system varies.
Even the giant planets are distinct, with Neptune and Uranus being an opaque blue and Jupiter and Saturn being largely beige with vivid red-brown belts.
Inside our solar system, the planets are so much closer. Although planets are much smaller than stars, since they are so close to us, planets appear to be around the same size as stars. Planets do not generate their own light. They reflect the sun’s light in the same way that our moon does.
Planets develop from particles hitting and sticking together in a disc of gas and dust as they orbit the star. Because the star’s wind sweeps away their gases and they are formed of heavier materials pulled by the star’s gravity, the planets closest to the star are likely to be rockier. Planets do not make their own light because they do not undergo nuclear fusion. Instead, they are illuminated by reflected light from a star. We perceive reflected sunlight when we observe planets in the sky at night, such as Venus, the so-called “Evening Star.”
There are eight planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and the hypothetical Planet Nine in the following order, starting closest to the sun and going outward. The Sun is at the centre of the solar system. Planets, unlike stars, do not sparkle as they do not produce light of their own.
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