The Universe & Solar System

The universe has a foamy structure made of filaments, superclusters, and galaxy groups; visible matter is only 5% of its total content.

• Normal (visible) matter comprises just 5% of the universe; the rest is unseen.
• Dark matter accounts for 27% and Dark energy for 68% of the total universe.
• The visible matter of the universe is primarily composed of Hydrogen (72–75%) and Helium (23–26%); other elements make up <2%.
• The visible universe is concentrated in parts of a complex scaffold surrounding a network of enormous cosmic voids (vast empty spaces).

The universe contains 1011 galaxies and an equal number of stars in each; galaxies are divided into Normal Galaxies and Radio Galaxies.

• Lyman Alpha Blobs — heavy cluster bodies found in the universe.
• Andromeda Galaxy — also known as Messier 31 or M31; it is the nearest galaxy to the Milky Way.
• The Milky Way Galaxy — the galaxy in which our Solar System exists; contains around 400 billion stars.
• Sirius — also known as the Dog Star; the brightest star in Earth's night sky. It is gradually moving closer to the Solar System, which will result in increased brightness over the next 60,000 years.
• Proxima Centauri — the closest star to the Sun.
• Super Nova — a primitive star that exploded and created the universe.
• Neutron Star — a star of very small radius, typically 30 km, and very high density. Thought to form from the gravitational collapse of a massive star remnant after a supernova explosion, provided the star is insufficiently massive to produce a black hole.
• Asteroids — small fragments of rock and dust that keep revolving around the Sun between the orbits of Mars and Jupiter.
• Pulsar — objects that look like stars blinking on and off but are not actually stars; a pulsar is a highly magnetized rotating neutron star.

The Sun accounts for 99.86% of all the mass in the Solar System and is the Star at its centre; it is 13 lakh times bigger than Earth.

• The Solar System is dominated by the Sun, which accounts for almost 99.9% of the matter of the whole system (the accepted precise figure is 99.86%).
• Scientists believe the Sun has been formed from a moving cloud of gases called a nebula.
• For over millions of years, balls of dust and gas moved around the Sun; gravity (the force of gravity) controls the movement of planets.
• The Sun is at the centre with 8 planets — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune — plus their satellites, dwarf planets, asteroids, meteors and comets.
• Till 2006 there were nine planets; Pluto was the farthest planet. In 2006 the International Astronomical Union (IAU) adopted a new definition; Pluto and celestial bodies like Ceres and 2003 UB313 do not fit this definition and are no longer considered planets.
• The Sun is the father of the Solar System and a main-sequence star.
• It produces temperatures and densities in its core high enough to sustain nuclear fusion of Hydrogen into Helium.
• It releases energy mostly as electromagnetic radiation peaking in visible light.
• The Sun has a higher abundance of elements heavier than hydrogen and helium.
• Temperature at the corner (surface/photosphere): 6000°C; at the centre: 2,000,000°C.
• The Sun also rotates on its own axis.

The four inner planets — Mercury, Venus, Earth, Mars — are Terrestrial Planets made of rock and metal with no or few moons.

• Mercury is the nearest planet to the Sun.
• Revolution period: 88 days — fastest revolution in the Solar System.
• Fastest planet in terms of motion.
• It has no satellite.
• No chances of life; temperature is very high due to proximity to the Sun.
• Symbol: ☿ | Colour: Gray | Type: Terrestrial Planet.

• Venus is the brightest planet of the entire Solar System.
• Also called Earth's Twin Planet.
• Revolution period: 225 days.
• Atmosphere contains 97% CO₂.
• Hottest planet in the Solar System; temperature: 475°C.
• Contains clouds made of Sulphuric Acid.
• Also known as the Evening Star and Morning Star (visible in the evening after sunset and in the morning before sunrise).
• Symbol: ♀ | Colour: Yellowish/White | Type: Terrestrial Planet | Atmosphere: Dense.

• Distance from Sun: 14.96 Cr. km.
• Earth's surface is covered with water 71.96%.
• Revolution period: 365 days and 3/4th of a day.
• Speed of Revolution: 29.8 km/sec.
• Rotation period: 23 hrs 56 minutes 4 seconds.
• Polar Radius: 6457 km; Equatorial Radius: 6371 km.
• Highest point: Mount Everest (8848.86 m).
• Deepest point: Mariana Trench, Pacific Ocean (10,994 m).
• Light reaches from Sun to Earth in 8 minutes 18 seconds.
• Symbol: 🜨 | Colour: Blue & Green | Type: Terrestrial Planet | Natural Satellite: Moon.

• Red Planet — iron oxide found in soil imparts red colour.
• Has big deserts in its surface.
• Atmosphere contains CO₂, NO₂ and their inert gases.
• Second smallest planet in the Solar System.
• Distance from Earth to Mars: 50,679,452 km; weather is cold.
• Distance of Mars from Sun: 142 million miles.
• Distance of Earth from Sun: 93 million miles.
• Two moons: Deimos and Phobos.
• Symbol: ♂ | Colour: Reddish | Type: Terrestrial Planet | Moons: 2.

The four outer planets — Jupiter, Saturn, Uranus, Neptune — are Gas/Ice Giants with many moons and ring systems; Jupiter and Saturn are primarily Hydrogen & Helium, Uranus & Neptune are Ice Giants with water, ammonia and methane.

• Biggest planet of the Solar System.
• Atmosphere contains Methane, Ammonia and Hydrogen.
• Jupiter is 11 times heavier than Earth.
• Contains 79 natural moons (earlier 63); largest moon: Ganymede (also the largest moon in the entire Solar System).
• Temperature: −123°C.
• Has a giant red spot.
• Symbol: ♃ | Colour: Brownish-Orange | Type: Gas Giant | Special Feature: Giant Red Spot.

• Contains 7 rings.
• Has 82 natural moons — highest number of moons among planets (as recorded in the notes).
• Second largest planet after Jupiter.
• Largest satellite: Titan.
• Also known as "Galaxy like Planet".
• Has the least density in the entire Solar System.
• Temperature: −178°C.
• Revolution period: 29.5 years.
• Last planet visible to the naked eye.
• Symbol: ♄ | Colour: Yellowish-Golden | Type: Gas Giant | Special Feature: 7 Rings.

• Contains 5 rings around it — Alpha, Beta, Gamma, Theta and Epsilon.
• Discovered by William Herschel on 13th March, 1781.
• Mainly made up of gases, with a solid core of rock and metal.
• Distance from Sun: 2.8 Billion km.
• Temperature: −219°C.
• Third largest planet in the Solar System.
• Primary composition: Hydrogen and Helium (like Jupiter), but contains more ices such as water, ammonia and methane.
• It is green due to the high amount of Sulphur.
• Moons: 27 | Symbol: ⛢ | Colour: Blue-Green | Type: Ice Giant | Rings: 5.

• Revolution time: 165 years.
• Distance from Sun: 4.95 Billion km.
• Eighth and farthest known planet from the Sun.
• One day on Neptune takes about 16 hours.
• Temperature: −216°C.
• Mostly consists of Hydrogen, Helium with trace amounts of Methane, water and Ammonia.
• Coldest Planet in the Solar System.
• Moons: 14 (known).
• Symbol: ♆ | Colour: Deep Blue | Type: Ice Giant | Special Name: Coldest Planet.

Memorise the revolution periods and moon counts — these are direct SSC CGL questions every year.

• Jupiter and Saturn have the maximum number of moons.
• Moons are natural satellites that revolve around planets.

Pluto was reclassified as a Dwarf Planet by the IAU in 2006; there are 7 dwarf planets in the Solar System.

• Pluto was discovered in 1930 by Clyde Tombaugh.
• It was considered the 9th planet of the Solar System.
• Since 2006, the IAU announced that Pluto is a dwarf planet.
• Three attributes of a planet (IAU definition):

1. It is in orbit around the Sun. 2. It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape). 3. It has cleared the neighbourhood around its orbit.

• Pluto does not have all three attributes, so it was declared a dwarf planet.
• Temperature: −233°C.
• Atmosphere: Nitrogen (mainly) with minor amounts of Methane and Carbon Monoxide.
• Symbol: ♇ | Colour: Brownish | Moons: 5 (known) | Discovered in: 1930.

SSC CGL repeatedly tests planet superlatives — biggest, hottest, coldest, nearest, farthest, brightest, smallest.

• Gas Giant Planets: Jupiter, Saturn, Uranus & Neptune.
• All four outer planets also have ring systems.
• The Sun contains more than 99.8% of the total mass of our Solar System.
• On the Moon, you weigh 1/6th of your weight on Earth.
• The word planet comes from the Greek word Planetai, meaning wanderers — planets keep changing positions with respect to stars.
• Planets have no light or heat of their own — they are lit by the Sun; planets do not twinkle, stars do.