Our Solar System

When we look out into the vastness of the void, we are overwhelmed the sheer size of it. Tucked away into a little corner, is us, on a small blue marble, suspended in a sunbeam. The Sun and the family of 8 planets, stranded in a forgotten corner of the Milky Way galaxy is what we call home.

However, we have yet to uncover the workings of our neighbourhood. Only in the past few centuries or so, of our millennia long existence did we start discovering the truths. Let us embark on that journey, to explore the place we call the Solar System.


About 5 billion years ago, the place we call the Solar System was a beautiful nebula, or a cloud of gas called the Solar Nebula. Then, about 4.568 billion years ago, something began to happen. The cloud began to collapse into a spinning disk of matter which rotated faster and faster.

The centre of the disk accreted more and more matter which resulted in a rise in temperature. The centre soon began to glow. The disk continued to flatten out. Small grains of dust and rocks attracted each other in some pockets of the disk forming planetesimals.

As the planetesimals accreted more and more matter, the centre grew hotter to form a protostar. Due to high temperatures near the protostar, only metals and silicates survived in the vicinity of the protostar, forming the terrestrial planets we call Mercury, Venus, Earth and Mars.

The nebula collapses into a spinning disk with a protostar at its centre

On the contrary, in the outer reaches of the Solar System, temperatures were cool enough to allow the formation of icy materials. These ices were more plentiful than the metals and silicates, which allowed them to accrete large amount of gases, forming super thick atmospheres. This gave birth to gas giant planets.

Millions of years passed and the protostar at the centre was hot enough to allow thermonuclear fusion to take place at its core. A star was born. We call it the Sun. Gradually, the planetesimals coalesced to form full fledged planets.

The Sun was now a main sequenced star. The solar wind from the Sun blew away the remaining disk of debris, thus putting an end to planetary formation.

Structure and Composition

The Solar System consists of the Sun, a star which accounts for 99.86% of mass in the system. It is orbited by 8 planets and thousands of minor bodies called asteroids, meteors and comets. The entire system is surrounded by a body of comets called the Oort Cloud which extends 100,000 AU from the Sun.

All the bodies in the Solar System, orbit the Sun on a plane called the ecliptic. Kuiper Belt objects, however tend to orbit at angles to the plane. When viewed from Earth’s North Pole, all bodies orbit the Sun in a counter clockwise direction, again, with the exception of a few Kuiper Belt Objects.

Astronomers divide the Solar System into 3 regions, namely Inner and Outer Solar System and the Trans Neptunian Region. The Inner Solar System consists of the rocky worlds, the planets upto Mars. The boundary of this zone is the asteroid belt beyond which lies the Outer Solar System with the gaseous planets from Jupiter to Neptune. Finally we arrive at a region called the Trans Neptunian Region which consists of many comets, meteors, asteroids and dwarf planets like Pluto which many of you are still protesting is a planet.

The Solar System

The Outer Planets in the Solar System possess mini systems of their own,as they are orbited by a tens of satellites which we informally call moons. Also, a unique property of  the four giant planets, are the planetary rings – thin bands of tiny particles that orbit them in unison. Most famous example being that of Saturn. Most of the large moons are in synchronous rotation, with one face permanently turned toward their planet. Somewhat like our moon.

One might notice a clear demarcation line between the rocky and gaseous worlds, with 4 rocky planets in the beginning and 4 gaseous planets later on. This is due to the fact that gases and icy substances are vaporised in the regions closer to the Sun than the asteroid belt.

The boundary in the Solar System beyond which the gaseous and icy substances do not vaporise is known as the frost line, which lies roughly 5 AU from the Sun.

Let us study our Solar System part by part beginning from the Sun to the system’s edges.

Inner Solar System: The rock worlds

The Inner Solar System, comprises of the 4 inner planets i.e. Mercury, Venus, Earth and Mars. Terrestrial and rocky in Nature, these planets have little to no satellites. Mercury and Venus have none, while Earth has one and, Mars has two.

The inner planets are rocky and terrestrial, composed mostly of silicates and metals. The inner planets are very close to each other, as compared to the outer planets . In fact, the  region occupied by all planets fro Mercury to Mars is less than the distance between Jupiter and Saturn.

Moreover, the inner planets are small and dense. With a radius of 12,756 km, Earth is the largest inner planet while Mercury is the densest planet.

Except for Mercury, all inner planets have an atmosphere of their own which generates various weather patterns. Mercury has an extremely thin temporary atmosphere, that is constantly renewed by solar wind.  All inner planets have impact craters and tectonic surface features, such as rift valleys, volcanoes, mountains etc.

In the Solar System, Mercury is the closest planet to our Sun and the smallest planet in terms of size. Its magnetic field is only about 1% that of Earth’s, and it’s very thin atmosphere results in extremely hot temperatures during the day (up to 430°C) and freezing temperatures at night (as low as -187 °C). This is because the thin atmosphere cannot retain the Sun’s heat at night. It gets that cold, even after being so close to the Sun!!


It has no satellites of its own and is comprised mostly of iron and nickel. Mercury is one of the densest planets in the Solar System. A day on Mercury is about 60 days on Earth while a year is only 88 Earth days.

Venus, is about the same size as Earth, which is why it is sometimes called Earth’s sister planet. However that is where the similarity ends. Venus has a thick toxic atmosphere that traps heat, making it the hottest planet in the Solar System. Even hotter than Mercury, which is closer to the Sun. This atmosphere is composed of 96% carbon dioxide, along with nitrogen and a few other greenhouse gases. Such gases trap heat, which makes the planet so hot. Dense clouds in the atmosphere comprise of sulphuric acid and other corrosive compounds, instead of water. It rains sulphuric acid on Venus!!

The surface is mostly marked with volcanoes and deep canyons. And after all these hellish features, Venus is named after the Roman god of love!!! A Venusian day is 243 Earth days long and a year is 224 Earth days long. This means a day on Venus is longer than a year!!

Earth is the third planet from the Sun and our home. Earth is the largest inner planet, and unique in the sense that it is the only planet known to support life as we know it. Earth’s magnetosphere aka magnetic field protects the planet from dangerous solar radiation and the greenhouse gases in the atmosphere helps keep the Sun’s warmth in, which is essential for life to thrive.

Unlike other terrestrial planets, Earth has tectonic plates which move about to create surface features like mountain ranges, volcanoes and rift valleys. Earth has a heavy metal core which powers its magnetic field. Earth’s atmosphere contains oxygen which is necessary for the life processes of majority of the organisms. We have only one satellite which we call the Moon.

Mars is the fourth planet from the Sun, and is also known as the “Red Planet” due to its reddish appearance. This redness is due to the rusting of iron present in its soil. Mars has two satellites called Phobos (27 km) and Deimos (13 km). Due to their very small size, scientists think that they are asteroids captured by Mars’ gravity.

Much of Mars’ surface is very old and filled with craters. but there do exist interesting features on its surface. These include Olympus Mons, the largest mountain in the Solar System which rises some 21,229 m above the surface, and a giant canyon called Valles Mariner is which is 4000 km long and reaches depths of up to 7 km. Olympus Mons is almost 3 times taller than Mount Everest!!

Olympus Mons: The largest mountain in the Solar System

The poles of the planet contain dry ice or frozen carbon dioxide and traces of water which change in size in accordance to the seasons. Mars is half the size of Earth and has a weaker magnetic field.

Mars’ thin atmosphere has led some astronomers to believe that the surface of the planet once had water which existed as liquid and formed oceans as we see on Earth. However, no such thing is observed today. A day on Mars is about the same as a day on Earth but a year is almost two years on Earth.

Outer Solar System: Home of the giants

The outer solar system consists of the gas giants, Jupiter, Saturn, Uranus and Neptune. What sets them apart from the inner planets is there sheer size, ring systems and a collection of moons. The outer planets are fully made of gas, which means we cannot land on them. These planets have no surface!!

Jupiter is the fifth planet from the Sun and the largest planet in our Solar System. It spins very fast, which means that one day on Jupiter is only 10 Earth hours. However a year on the planet is almost 12 Earth years. Jupiter is mostly made up of hydrogen and helium, and is though to have a rocky core that is about Earth’s size. The planet has about 67 satellites, a faint ring system and is raging with storms. One storm on Jupiter, called the Great Red Spot has been raging non-stop for 300 years and is 3 times the size of the Earth!!

Saturn is the sixth planet from the Sun. Like Jupiter, its atmosphere is mostly hydrogen and helium, and it also rotates quickly resulting in a day that is just 10.7 hours long. A year however is 29.7 years on Earth due to the time taken to circle the Sun. Saturn is best known for its beautiful rings. Within and around these rings orbit 62 known satellites of the planet. The rings are made of rocks and ice particles, some as big as a house. Saturn is also famous for the hexagon shaped storm on its north pole.

Hexagonal storms on Saturn’s north pole

Discovered by William Herschel in 1781, Uranus lies far beyond Saturn and is the seventh planet from the Sun. A day on the planet is as long as 17 hours and one year takes 84 years on Earth. It has 27 known moons and a faint ring system made of dark rocks. The Uranian atmosphere contains water, methane, ammonia, hydrogen and helium and might have a rocky core at its centre. Very little is known about this planet as only one spacecraft has ever visited it. That was the Voyager 2 probe in 1986. Uranus has a mute appearance, as the whole planet is covered in clouds. However scientists are sure that beneath the clouds, rage ferocious storms.

Neptune is the eighth and the farthest planet from the Sun. It is the smallest gas giant and was discovered by Urban Le Verrier and Johann Gottfried Galle in 1846. The Neptunian system consists of 14 moons and a dark ring system. The atmosphere consists of water, ammmonia, methane, hydrogen and helium which may have a possible Earth-sized core at the centre. Even Neptune was visited by only one probe which was the Voyager 2 sent by NASA in 1989. Neptune is the windiest planet in our Solar System and features a giant storm called the Great Dark Spot.

Uranus and Neptune are called the ice giants due to extremely cold temperatures on both these planets.

Trans-Neptunian Region: A frozen corner

Beyond Neptune lies what we call the Trans-Neptunian Region. Here lies the Kuiper belt, a belt of dwarf planets, asteroids and comets. There have been more than a thousand objects discovered in the Kuiper Belt. Famous dwarf planets like Pluto lie in this belt.

The Kuiper Belt was theorised to exist by Gerard Kuiper in 1951. After the discovery of Eris and few other bodies, scientists confirmed its existence and thus demoted Pluto from the status of a planet to a dwarf planet in August of 2006. This was because Pluto lied in the Kuiper Belt and had not cleared its orbital path of other objects, like an ideal planet should.

But Pluto is not the only one out there in the Kuiper belt. Sedna, Quaoar, Makemake, Haumea, Orcus, and Eris are some of the major icy bodies in the belt. Some of them even have satellites of their own.

Pluto, as seen by New Horizons spacecraft

Spectrographic studies conducted of the region indicate that its members are primarily composed of ices: a mixture of light hydrocarbons (such as methane), ammonia, and water ice. Further observations have also shown KBOs to come in a broad range of colours, ranging from neutral grey to deep red.

Till now, only one probe, the New Horizons probe has explored Pluto and is now about to explore some other Kuiper Belt Objects or KBOs.

As we move further, we arrive upon a region known as the heliosphere. This structure is created when the solar wind slams against the interstellar medium (remember, space is not entirely empty).

The Voyager 1 probe detected a short lived beep when it passed the heliosphere. This marked the region where the solar wind collided with the interstellar medium. Currently, the Voyager probes are thought to have crossed this region into interstellar space.

The Oort Cloud: The boundary of the Solar System

The Oort Cloud is a giant cloud of comets thought to surround and orbit the Sun at a distance of 2,000 AU to as far as 50,000 AU. Some astronomers even suggest that the cloud extends upto 3 light years from the Sun! The Cloud is thought to be comprised of two regions, an inner cloud shaped like a disc (2000-20,000 AU) and an outer cloud shaped like a sphere (20,000-50,000 AU). The existence of the Oort Cloud was proposed by Jan Oort in 1950.

The location of the Oort Cloud with respect to other objects

The outer Oort cloud may have trillions of objects larger than 1 km, and billions that measure 20 kms in diameter. This is where comets are thought to originate. Its total mass is not known, but considering the masses of some famous comets we have seen so far it is thought to have a mass of roughly 3×1025kilograms or five Earths.

The composition of the Oort Cloud is a mystery as no spacecraft has ever visited this region. However, based on the analyses of comets that enter the Solar System, the vast majority of Oort Cloud objects are thought to be composed of icy volatiles such as water, methane, ethane, carbon monoxide, hydrogen cyanide, and ammonia.

Beyond the Oort Cloud lies interstellar space, the abode of other star systems just like ours. Some stars have companions and families of planets while other wander lonely. Someday we may find another Earth amongst these stars.

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