Planets, Satellites, and Dwarf Planets (UPSC Notes)

Solar System/Image Credit: HaticeEROL

Planets

Planets are celestial bodies that do not generate heat and light. They revolve around the stars and are lit by their light. Earth where we live is one such example of planets.
The word "planet" originated from the Greek word planetai, which means wanderers in English.
There are eight planets in our solar system which move around the Sun in a fixed elliptical orbit.
Eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
These planets are divided into two groups. 1. Terrestrial Planets and 2. Jovian Planets
Terrestrial planets are dense rocky bodies and are called earth (terra) like planets. These are Mercury, Venus, Earth, and Mars. They are also called inner planets.
Jovian Planets are gigantic and are gaseous in composition with a large number of satellites. These have similar features to that of Jupiter (Jove). Thus called Jovian Planets.
Terrestrial and Jovian planets are separated by an asteroid belt.

Mercury

Mercury is the smallest planet in our solar system. It is just slightly larger than the Earth’s moon.
It is the closest planet to the sun at a distance. The average distance between Sun and Mercury is about 58 million km (36 million miles) or 0.39 AU.
It takes 59 days to complete one rotation on its axis and 88 days to complete one revolution around the sun.
Mercury has a thin atmosphere that is composed mostly of oxygen, sodium, hydrogen, helium, and potassium.
The temperature of mercury ranges from 430 C to -180 C.
It has no satellite.

Venus

Venus is slightly smaller than Earth. Due to the similarity in their size, Venus is also known as Earth's twin planet.
The average distance between Venus and Sun is 108 million km (67 million miles) or 0.72 AU.
It takes 243 Earth days to spin on its axis, which means one day on Venus is equal to 243 Earth days.
Its atmosphere is made up of carbon dioxide and nitrogen, with clouds of sulfuric acid droplets.
Venus spins backward (retrograde rotation) when compared to other planets, which means that the sun rises in the west and sets in the east on Venus.
Venus has no natural satellites.
Venus is known as the morning and evening star as it can be seen through naked at the time of sunrise and sunset on Earth.

Earth

Earth is the third planet from the sun at a distance of about 150 million km (93 million miles) or 1 Astronomical Unit (AU).
A day on Earth is equal to 24 hours.
Earth's atmosphere is made up of Nitrogen (78%), Oxygen (21%), and other gases.
Earth has one natural satellite - Moon.
Earth is the heaviest planet in terms of mean density.

Mars

Mars is the fourth planet from the sun. The average distance between Mars and the Sun is 228 million km (142 million miles) or 1.52 AU.
Mars takes slightly more than one day to complete a rotation on its axis. (24 hour 37 minutes).
On another side, it takes 687 days to complete its one revolution around the Sun.
Mars has a thin surface-atmosphere made up of Carbon di Oxide, Nitrogen, and Argon.
Mars has two natural satellites: 1. Phobos and 2. Deimos. These are named after twin sons of Greek Gods Ares.
It is known as the Red planet due to iron minerals found in Martian soil.

Jupiter

Jupiter is the largest planet in our solar system.
It is the fifth planet from the sun at a distance of about 778 million km (484 million miles) or 5.2 Astronomical Units (AU).
It takes only 10 hours to rotate on its axis and 4333 days to revolve around the Sun.
It is a gas giant planet, but may have a solid inner core about the size of Earth.
Jupiter's atmosphere is made up of Hydrogen and Helium.
It has 79 discovered satellites. These are named after smaller Roman and Greek gods.
It has a faint ring system discovered by Voyager I in 1979.

Saturn

Saturn is the second-largest planet after Jupiter. It is known for its well-defined ring system.
It lies at a distance of 1.4 billion km (886 million miles) or 9.5 AU.
Saturn rotation period is about 10 hours and 45 minutes, where it takes 29.4 years to revolve around the Sun.
Like Jupiter, its atmosphere is made up of Hydrogen and Helium.
It has 82 known satellites. These are named after Greek and Roman gods.
It is also the least dense planet in our solar system.

Uranus

It is the seventh planet in our solar system. It is at a distance of 2.9 billion km (1.8 billion miles) or 19.19 AU.
It takes around 17 hours to complete a rotation on its axis and 84 Earth years to complete one revolution around the Sun.
It is known as Ice Giant as 80% of planet mass is made up of hot dense fluid material made up of water, methane, and ammonia.
It has 27 known satellites. These are named after the characters of William Shakespeare and Alexander Pope.
It also has a faint ring system.

Neptune

Neptune is the eighth planet in our solar system. It is farthest and coldest planet in our solar system.
It takes around 16 hours to complete a rotation on its axis and 164.8 Earth years to complete one revolution around the Sun.
It is known as the sister planet to Uranus due to its similar composition.
It has 13 known satellites. These are named after various sea gods and nymphs of Greek mythology.
It has six faint rings.

Asteroids

Asteroids are rocky heavenly objects that revolve around the Sun in a belt system known as Asteroid Belt.
More than 150 asteroids have moons.
Ceres is the first and largest asteroid discovered in 1801 by Italian astronomer Giuseppe Piazzi.

Dwarf Planets

Dwarf Planets are heavenly objects which are not large enough to be categorized as planets. They do not dominate their region and usually do not have their own satellites.
There are five known dwarf planets in our solar system. These are Ceres, Pluto, Haumea, Makemake, and Seden.

Satellites

Satellites are heavenly objects which revolve around planets as planets move around the star.
Moon is the natural satellite of Earth.
Saturn has the largest number of identified natural satellites (82) with Jupiter at second position with 79 natural satellites.
Ganymede (moon of Jupiter) is the largest natural satellite in our solar system. It is the ninth-largest body in our solar system. Even larger than Mercury.
Titan, a satellite of Saturn, is the 10th largest body in our solar system. It is also larger than Mercury.