How the solar system was assembled from a spinning cloud, why the inner planets are rocky and the outer ones gaseous, and the features of each planet, dwarf planet, asteroid and comet that exams ask about.
Our solar system consists of the Sun, eight planets, three recognised dwarf planets, a large number of satellites, and countless asteroids, meteoroids and comets. The Sun holds about 99.8 per cent of all the mass in the system. Everything else, including the Earth and every living thing on it, is built from the leftover 0.2 per cent.
That single statistic explains a great deal. It explains why every planet orbits the Sun rather than the reverse, why the orbits lie almost in one flat plane, and why the whole system turns in the same direction. To understand the solar system properly you have to start with how it was assembled.
How the solar system was formed
Several theories were proposed over three centuries, and examiners like to test who said what. Learn them as a progression rather than as isolated names.
| Theory | Proposed by | Core idea |
|---|---|---|
| Nebular Hypothesis (1796) | Immanuel Kant, developed by Laplace | The solar system was born from a giant interstellar cloud called a nebula. The theory had many flaws, but this one assumption turned out to be right. |
| Planetesimal / Tidal theory (1900) | Chamberlain and Moulton, later supported by Sir James Jeans and Sir Harold Jeffrey | A wandering star passed close to the Sun and pulled out a cigar-shaped extension of material, which condensed into planets. |
| Revised Nebular Hypothesis (1950) | Otto Schmidt (Russia) and Carl Weizsacker (Germany) | The Sun was surrounded by a solar nebula of hydrogen, helium and dust. Friction and collision formed a disc-shaped cloud, and planets grew by accretion. |
| Nuclear Disc Model (Neo-Laplacian) | Modern consensus | The currently accepted explanation, built on the Big Bang framework. It deals specifically with the formation of the solar system. |
The nuclear disc model, step by step
According to this model, the nebula began collapsing and forming a core some 5 to 5.6 billion years ago, and the Sun and planets took shape about 4.6 billion years ago. The age of the Earth is 4.543 billion years.
Formation sequence
- Collapse: the nebula became gravitationally unstable and began to fall in on itself, possibly triggered by shock waves from a nearby supernova.
- Snowball effect: gravity pulled dust and gas towards the centre. The centre grew denser and hotter, which increased its gravity, which pulled in more material still.
- Protosun: about 99.9 per cent of the material fell into the centre. There was no sunlight yet.
- Ignition: once the core became hot enough, nuclear fusion began and the Sun was born.
- Protoplanetary disc: the remaining 0.1 per cent flattened into a spinning disc around the Sun. This is where the planets formed.
- Accretion: dust grains collided and stuck together, growing into planetesimals a few kilometres across, then into protoplanets, and finally into planets as they swept up everything left over.
Why the inner planets are rocky and the outer ones gaseous
This is one of the most frequently asked conceptual questions in the topic, and it has two parts to the answer.
First, temperature at the time of formation. In the inner, hotter part of the solar nebula it was too warm for gases to condense into solid particles. Only substances with high melting points survived, so the planetesimals there were made mostly of silicates and metals. This hot, rocky material produced the terrestrial planets with iron and nickel cores. In the outer, cooler portion, water ice was the dominant component, which gave rise to the gas and ice giants.
Second, solar wind and gravity. The solar wind was most intense near the Sun, so it blew away large quantities of gas and dust from the surfaces of the inner planets. Because these planets are smaller, their weaker gravity could not hold on to the escaping gases. Farther out, the solar wind was too weak to strip the Jovian planets, and their much larger gravity retained thick hydrogen and helium atmospheres.
The iron catastrophe and planetary differentiation. When the Earth formed 4.5 billion years ago it was a uniform ball of hot rock. Radioactive decay and leftover primordial heat pushed the temperature higher until, after about 500 million years, it reached the melting point of iron, roughly 1,538 °C. This moment is called the iron catastrophe. Molten iron and nickel sank to the centre and became the early core, while lighter silicates, water and gases stayed near the exterior. This sorting by density is planetary differentiation, and it is the reason the Earth has a layered interior.
Components of the solar system
The solar system contains the Sun, eight planets, dwarf planets such as Pluto, Ceres and Eris, natural satellites, and countless minor planets, asteroids, meteoroids and comets.
Most large objects orbiting the Sun lie close to the plane of the Earth's orbit, which is called the ecliptic. The planets stay very near this plane. Comets and Kuiper Belt objects such as Pluto often sit at much greater angles to it. Planetary orbits are nearly circular, whereas many comets, asteroids and Kuiper Belt objects follow highly elliptical paths.
Models of the solar system
- Geocentric model: the Earth at the centre, proposed by Ptolemy.
- Heliocentric model: the Earth and planets revolving around the Sun. Nicolaus Copernicus was the first to develop a mathematically predictive heliocentric model, refuting Ptolemy.
- Spherical Earth: the idea was floated by Pythagoras around 500 BC and later validated by Aristotle around 340 BC.
- Kepler's three laws: (1) a planet's orbit is an ellipse with the Sun at one focus; (2) a line joining a planet and the Sun sweeps equal areas in equal time; (3) the square of a planet's orbital period is proportional to the cube of the semi-major axis of its orbit.
The planets
A planet is a celestial body moving in an elliptical orbit around a star. The eight planets divide into two clear groups.
- Inner or terrestrial planets: Mercury, Venus, Earth and Mars. They lie between the Sun and the asteroid belt. "Terrestrial" means Earth-like, because they are made of rock and metal and have relatively high densities.
- Outer or Jovian planets: Jupiter, Saturn, Uranus and Neptune. "Jovian" means Jupiter-like. They are larger, made of less dense material, and carry thick atmospheres of mostly hydrogen and helium.
All eight planets revolve around the Sun in the direction of the Sun's own rotation, which is counter-clockwise when viewed from above the Sun's north pole. Six of the eight also rotate on their axis in the same direction. Venus and Uranus are the exceptions, with retrograde or clockwise rotation.
| Planet | Surface temp (°C) | Rotation | Revolution | Distance (AU) | Diameter (km) | Moons |
|---|---|---|---|---|---|---|
| Mercury | +427 | 58 days | 87 days | 0.4 | 4,878 | 0 |
| Venus | +480 | 243 days | 224 days | 0.7 | 12,104 | 0 |
| Earth | +22 | 23 h 56 m | 365 days | 1.0 | 12,756 | 1 |
| Mars | -23 | 1.025 days | 687 days | 1.5 | 6,787 | 2 |
| Jupiter | -150 | 9.9 hours | 11.9 years | 5.2 | 1,40,000 | 95+ |
| Saturn | -180 | 10.7 hours | 29 years | 9.6 | 1,16,000 | 270+ |
| Uranus | -214 | 17 hours | 84 years | 19.2 | 51,000 | 28 |
| Neptune | -220 | 16 hours | 164 years | 30.0 | 48,000 | 16 |
One Astronomical Unit (AU) is the average distance between the Earth and the Sun, roughly 150 million km. Note that moon counts keep rising as new small satellites are discovered, so treat those figures as approximate.
Mercury
Mercury's surface is heavily cratered and resembles the Moon, which indicates it has been geologically inactive for billions of years. Having almost no atmosphere to retain heat, it shows the greatest daily temperature swing of any planet, from -173 °C at night to 427 °C during the day. It is smaller than Ganymede and Titan, the two largest moons in the solar system, but it is more massive than either. The MESSENGER spacecraft found evidence of pyroclastic flows and water ice at Mercury's poles.
Venus
Venus is the brightest planet and the third brightest object in our sky after the Sun and the Moon, which is why ancient literature called it the morning star and the evening star. Its brightness comes from the highest albedo among planets, caused by highly reflective sulphuric acid clouds.
It is called Earth's twin because of similar size, mass, proximity to the Sun and bulk composition, and because it too has high plateaus, folded mountain belts and numerous volcanoes. In every other respect it is hostile. Its atmosphere is about 96 per cent carbon dioxide, and surface pressure is 92 times that of the Earth, equivalent to being 900 metres underwater. It is the hottest planet in the solar system even though Mercury lies closer to the Sun, because of a runaway greenhouse effect. A day on Venus lasts 243 Earth days, which is longer than its year of 224 days.
Earth and the Moon
The Moon's diameter is only one-quarter that of the Earth and it lies about 3,84,400 km away. It is tidally locked to the Earth, meaning its orbital period matches its rotational period, both about 27 days. That is why only one side of the Moon is ever visible from the Earth.
The Moon is now believed to have formed through a giant impact, sometimes called the big splat. A body one to three times the size of Mars struck the young Earth and blasted a large mass of material into orbit, which coalesced into the Moon about 4.44 billion years ago. The Moon is a significant stabiliser of the Earth's axis. Without it, the tilt could vary by as much as 85 degrees, against the present steady 23.5 degrees.
Tidal friction between the oceans and the turning Earth is slowly braking our rotation, by about 1.4 milliseconds per century, and the Moon is drifting away at roughly four centimetres per year.
Mars
Mars is the Red Planet because of the reddish iron oxide covering its surface. It lost its magnetosphere about 4 billion years ago, so the solar wind interacts directly with its ionosphere and has thinned the atmosphere drastically. That atmosphere is now about 96 per cent carbon dioxide and less than 1 per cent as thick as the Earth's.
Of all the planets, Mars has the most Earth-like seasons, because its axial tilt of 25.19 degrees is close to the Earth's 23.5 degrees. Its landforms strongly suggest that liquid water once flowed there, but low atmospheric pressure now makes surface liquid water impossible, and the remaining water is locked in the two polar ice caps. Mars hosts Olympus Mons, the largest volcano and highest known mountain in the solar system at about 24 km, and Valles Marineris, one of the largest canyons. Its two irregular moons, Phobos and Deimos, are thought to be captured asteroids.
Jupiter
Jupiter is the largest planet, composed mostly of gas and liquid swirling in complex patterns, with no solid surface. Because it rotates once every ten hours, it bulges at the equator into an oblate spheroid. Its four large moons, Io, Europa, Ganymede and Callisto, are called the Galilean satellites because Galileo discovered them. Ganymede is the largest natural satellite in the solar system at 5,268 km across, larger than Mercury and three times the size of our Moon. The most recent probe to visit Jupiter is Juno.
Saturn
Saturn's density is less than that of water, so in principle it would float. Its rings are made of billions of ice particles and ice-covered rocks. Titan is its largest moon and the second largest in the solar system, and it is the only satellite anywhere in the solar system with a substantial, nitrogen-rich atmosphere.
Uranus and Neptune
Uranus is tipped over and spins on its side: its axis of rotation lies nearly in the plane of its orbit, so its poles sit where other planets keep their equators. Uranus and Neptune are together called the ice giants and the twins of the outer solar system, because they hold a higher proportion of ices such as water, ammonia and methane beneath thick hydrogen and helium atmospheres. Neptune has the strongest winds in the solar system at about 2,100 km per hour, followed by Saturn at 1,800 km per hour.
Other objects in the solar system
Asteroid Belt
Asteroids are remnants of planetary formation that failed to come together as a planet because of gravitational interference from Jupiter. They circle the Sun in a zone between Mars and Jupiter called the asteroid belt, lying between 2.3 and 3.3 AU from the Sun. They are made mainly of rocky and metallic minerals with some ice, and range from hundreds of kilometres across down to microscopic size.
Ceres, at 2.77 AU and 946 km across, is the largest asteroid. It is massive enough for its own gravity to pull it into a sphere, so it is classified as a protoplanet and a dwarf planet. Every asteroid other than Ceres is classified as a small solar system body. Vesta, at 525 km, is the second largest.
Kuiper Belt, Pluto and Charon
The Kuiper Belt is a great ring of debris similar to the asteroid belt, but consisting mainly of icy objects. It extends between 30 and 50 AU from the Sun. Pluto, at 39 AU, is the largest known Kuiper Belt object, and Charon is its largest moon. Discovered in 1930, Pluto was treated as the ninth planet until 2006, when the International Astronomical Union adopted a formal definition.
The IAU definition of a planet
- It orbits the Sun.
- It has sufficient mass to assume hydrostatic equilibrium, that is, a nearly round shape.
- It is not a satellite of another object.
- It has cleared the neighbourhood around its orbit. Pluto fails on this last count, since it shares its region with many other Kuiper Belt objects.
A meteoroid is a small rocky or metallic body travelling in space. A meteor is the streak of light produced when that body burns up on entering the Earth's atmosphere, which is what we call a shooting star. A meteorite is the fragment that survives the passage and lands on the surface. A comet is a body of ice and dust that develops a glowing coma and tail when it approaches the Sun.
Comets, meteor showers and small bodies
A comet is a body of ice, dust and rock that develops a visible structure only when it nears the Sun. Learn its three parts in order.
- Nucleus: the solid core of ice and dust, usually only a few kilometres across.
- Coma: the glowing cloud of gas and dust released as solar heat vaporises the ice.
- Tail: material blown off the coma. A comet has two tails, a dust tail and an ion tail, and both point away from the Sun regardless of the direction of travel, because they are pushed by radiation pressure and the solar wind. A comet leaving the Sun therefore travels tail first.
Comets are divided by orbital period. Short period comets, taking under 200 years, come mainly from the Kuiper Belt; Halley's Comet returns about every 76 years and is the best known. Long period comets come from the Oort Cloud, a spherical shell of icy bodies believed to surround the solar system far beyond the Kuiper Belt.
Meteor showers
When the Earth passes through the debris trail left along a comet's orbit, many meteors burn up in the atmosphere over a few nights. This is a meteor shower, and each one is named after the constellation from which the meteors appear to radiate. The Perseids in August and the Leonids in November are the most commonly cited. The showers recur on roughly the same dates each year because the Earth crosses the same debris trail at the same point in its orbit.
Small bodies: quick discrimination
- Asteroid: rocky or metallic, mostly between Mars and Jupiter.
- Comet: ice and dust, highly elliptical orbit, grows a coma and tail near the Sun.
- Meteoroid: a small body still travelling in space.
- Meteor: the streak of light as it burns in the atmosphere, that is a shooting star.
- Meteorite: the fragment that survives and reaches the ground.
- Near Earth Object (NEO): any asteroid or comet whose orbit brings it close to the Earth's orbit. These are tracked for impact risk.
How far the solar system reaches
The planets occupy only the inner part of the solar system. Neptune orbits at about 30 AU, but the Sun's gravity holds objects far beyond that. Three zones are worth fixing in order, moving outward.
- Asteroid Belt (2.3 to 3.3 AU): rocky and metallic bodies between Mars and Jupiter. Jupiter's gravity kept them from gathering into a planet.
- Kuiper Belt (30 to 50 AU): a flattened ring of icy bodies beyond Neptune. Pluto, Haumea and Makemake live here. It is the source of short period comets, which return in less than 200 years.
- Oort Cloud (roughly 2,000 to 1,00,000 AU): a vast spherical shell of icy bodies surrounding the whole system. It has never been observed directly and is inferred from the orbits of long period comets, which come in from every direction. Its outer edge is close to where the Sun's gravity stops dominating, about a quarter of the way to the nearest star.
Between the Kuiper Belt and the Oort Cloud lies the heliopause, where the solar wind is finally stopped by the gas between the stars. That boundary is covered in the article on the structure of the Sun. For exam purposes, remember that the solar system, measured by the reach of the Sun's gravity, extends far beyond the heliopause.
Five dwarf planets recognised by the IAU
- Ceres: in the Asteroid Belt, the only dwarf planet in the inner solar system. Visited by NASA's Dawn mission.
- Pluto: in the Kuiper Belt, with five moons, the largest being Charon. Visited by New Horizons in 2015.
- Haumea: Kuiper Belt, egg shaped because it spins very fast, once in about four hours.
- Makemake: Kuiper Belt, discovered around Easter 2005 and named after a Rapa Nui creator god.
- Eris: in the scattered disc beyond the Kuiper Belt. Nearly Pluto's size and more massive. Its discovery in 2005 forced the IAU to define the word planet in 2006.
Exploring the solar system: missions to remember
Questions on space missions are asked both in the geography and the science and technology papers. The missions below are the ones that changed what the textbooks say about the planets.
| Mission | Agency and year | What it did |
|---|---|---|
| Chandrayaan-1 | ISRO, 2008 | Its Moon Mineralogy Mapper confirmed water molecules on the lunar surface. |
| Mangalyaan (Mars Orbiter Mission) | ISRO, 2013 | Reached Mars orbit in September 2014. India became the first country to succeed at Mars on its first attempt, and the first Asian nation to reach Mars orbit. |
| Chandrayaan-3 | ISRO, 2023 | Landed the Vikram lander near the lunar south pole on 23 August 2023, making India the first country to land in that region. The landing point is named Shiv Shakti point. |
| Voyager 1 and 2 | NASA, 1977 | Grand tour of the outer planets. Voyager 2 is still the only craft to have visited Uranus and Neptune. Both have crossed the heliopause. |
| Cassini-Huygens | NASA, ESA and ASI, 1997 | Orbited Saturn from 2004 to 2017. Huygens landed on Titan in 2005, the most distant landing yet made. |
| New Horizons | NASA, 2006 | First flyby of Pluto in 2015, then of the Kuiper Belt object Arrokoth in 2019. |
| Juno | NASA, 2011 | Polar orbit of Jupiter since 2016, studying its interior, gravity and magnetic field. |
| DART | NASA, 2021 | Deliberately struck the asteroid moonlet Dimorphos in 2022 and shortened its orbit, the first test of deflecting an asteroid. |
Learn missions in pairs of target and first: first to Uranus and Neptune (Voyager 2), first to Pluto (New Horizons), first landing on Titan (Huygens), first landing near the lunar south pole (Chandrayaan-3), first Mars success on the first attempt (Mangalyaan). Most statement questions are built from such firsts.
Quick revision: six points
- The Sun holds about 99.8 per cent of the solar system's mass. The system formed about 4.6 billion years ago from a collapsing nebula, as described by the nuclear disc model.
- Near the young Sun only rock and metal could condense, so the inner planets are small and rocky. Beyond the frost line ices condensed and the giants grew large enough to hold hydrogen and helium.
- Venus and Uranus rotate in the retrograde direction. Venus is the hottest planet because of a runaway greenhouse effect, not because it is the closest.
- Ganymede is the largest moon in the solar system, Titan the second largest and the only one with a thick atmosphere.
- A planet must orbit the Sun, be nearly round and have cleared its orbit. Pluto fails the third test, so since 2006 it is a dwarf planet, along with Ceres, Haumea, Makemake and Eris.
- Short period comets come from the Kuiper Belt (30 to 50 AU); long period comets come from the far larger Oort Cloud.
Test yourself: 10 questions
Tap an option to answer. The explanation opens as soon as you choose.
Which of the following statements about our solar system is correct?
- AThe Sun contains 75 per cent of the mass of the solar system
- BVenus is the smallest planet in the solar system
- CJupiter has no natural satellites
- DThe Earth is the densest of all the planets in our solar system
Which two planets in the solar system show retrograde rotation, that is, they rotate in the direction opposite to the Sun's rotation?
- AVenus and Uranus
- BMercury and Mars
- CJupiter and Saturn
- DNeptune and Pluto
Venus is the hottest planet in the solar system despite Mercury being closer to the Sun. The primary reason is:
- AVenus rotates very slowly on its axis
- BA runaway greenhouse effect from a dense carbon dioxide atmosphere
- CVenus has a very high albedo
- DVenus has active volcanoes across its surface
Consider the following statements regarding asteroids: 1. Asteroids are rocky debris of varying size orbiting the Sun. 2. The asteroid belt lies between the orbits of Mars and Jupiter. 3. Ceres, the largest asteroid, is also classified as a dwarf planet. Which of the statements given above are correct?
- A1 and 2 only
- B2 and 3 only
- C1 and 3 only
- D1, 2 and 3
Which of the following is the largest natural satellite in the solar system?
- ATitan
- BEuropa
- CGanymede
- DThe Moon
The Galilean satellites of Jupiter are:
- ATitan, Rhea, Iapetus and Dione
- BPhobos, Deimos, Charon and Ceres
- CTriton, Nereid, Proteus and Larissa
- DIo, Europa, Ganymede and Callisto
Which of the following best explains why the inner planets are rocky while the outer planets are largely gaseous?
- AHigh temperatures near the Sun prevented gases from condensing, and intense solar wind stripped gases from the weaker gravity of the smaller inner planets
- BThe inner planets formed much later than the outer planets
- CThe outer planets absorbed gases released from the asteroid belt
- DThe inner planets lost their gases due to meteorite bombardment alone
The Moon is said to be tidally locked to the Earth. This means:
- AThe Moon does not rotate on its axis at all
- BThe Moon's orbital period is equal to its rotational period, so only one side faces the Earth
- CThe Moon's gravity has no effect on the Earth's oceans
- DThe Moon revolves around the Earth in exactly 365 days
Consider the following statements about Pluto: 1. It is the largest known object in the Kuiper Belt. 2. It lost its status as a planet in 2006 because it has not cleared the neighbourhood around its orbit. 3. Charon is its largest moon. Which of the statements given above are correct?
- A1 and 2 only
- B2 and 3 only
- C1 and 3 only
- D1, 2 and 3
Consider the following pairs of space missions and their achievements:1. Voyager 2 : only spacecraft to have visited Uranus and Neptune2. New Horizons : first flyby of Pluto3. Chandrayaan-3 : first landing near the lunar south poleWhich of the pairs given above are correctly matched?
- A1 only
- B1 and 2 only
- C2 and 3 only
- D1, 2 and 3
Answer writing
Question
Explain the formation of the solar system according to the Nuclear Disc Model. Why do the inner planets differ so sharply from the outer planets in composition and structure?
GS Paper I · 15 marks · 250 wordsModel answer
The Nuclear Disc Model, also called the Neo-Laplacian model, is the currently accepted explanation for the origin of the solar system. It builds on the older Nebular Hypothesis of Kant and Laplace, retaining its one sound assumption, that the system was born from a giant interstellar cloud of gas and dust called a nebula, while discarding its scientifically erroneous elements.
Sequence of formation. The nebula began collapsing and forming a core roughly 5 to 5.6 billion years ago, becoming gravitationally unstable possibly because shock waves from a nearby supernova rippled through it. Gravity drew dust and gas towards the centre, and as more matter fell in the centre grew denser and hotter, increasing its gravity further and producing a snowball effect. About 99.9 per cent of the material accumulated at the centre to form the protosun, which as yet emitted no sunlight. Once the core became hot enough, nuclear fusion ignited and the Sun was born, approximately 4.6 billion years ago.
The remaining 0.1 per cent of matter, orbiting the young Sun, flattened from a randomly shaped cloud into a protoplanetary disc. Within this disc, dust particles in the gas collided and clumped together. Through this process of accretion, microscopic grains built up into planetesimals a few kilometres across, then into protoplanets, and finally into full planets as they swept up the remaining dust and debris. Rocks that escaped capture survive today as asteroids, most of them orbiting between Mars and Jupiter.
Why the two groups differ. The contrast between the terrestrial and Jovian planets arises from two linked factors, both determined by distance from the Sun.
The first is temperature during formation. In the inner and hotter portion of the nebula, it was too warm for gases to condense into solid particles. Only substances with high melting points survived, so planetesimals there consisted mainly of silicates and metals. This hot rocky material produced Mercury, Venus, Earth and Mars, each with a dense iron and nickel core. In the outer and cooler portion, water ice was the dominant condensate, giving rise to Jupiter, Saturn, Uranus and Neptune.
The second factor is the interaction between solar wind and gravity. The solar wind was most intense close to the Sun and stripped large quantities of gas and dust from the surfaces of the inner planets. Because these planets are small, their weaker gravity could not retain the escaping gases. At greater distances the solar wind was far weaker, and the much larger Jovian planets had sufficient gravity to hold thick atmospheres of hydrogen and helium. Uranus and Neptune retained a higher proportion of ices such as water, ammonia and methane, and are therefore distinguished as ice giants.
Consequence for the Earth. A related process shaped the interior of our own planet. About 500 million years after formation, radioactive decay and primordial heat raised the Earth's temperature to the melting point of iron, an event called the iron catastrophe. Molten iron and nickel sank to the centre while lighter silicates, water and gases remained near the surface. This sorting by density, known as planetary differentiation, produced the layered structure of core, mantle and crust that underlies all subsequent physical geography.
Model answer: about 530 words. In the exam, keep the structure and trim the examples to fit the word limit.