Introduction

For thousands of years humans

believed in a geocentric model in which

everything rotated around Earth;

however, there was no theory to explain

why some planets appeared to move

backwards across the sky for short

periods (retrograde motion).

In the 2nd century, Ptolemy explained

these movements by proposing that

planets moved in epicycles. His theory

suggested that planets had small

circular orbits as they revolved around

the Earth. However, these epicycles

did not always match the planetary motion that was observed in the night sky. It was not until the 16th century that

Copernicus proposed a heliocentric model (sun-centred) and the retrograde motion could be explained because

planets orbit at different speeds and distances from the Sun.

The Sun

The Sun is at the centre of the solar system. As a main sequence star, energy is produced by the fusion of

hydrogen to helium. The Sun is composed of gases and can be divided into six layers - the core, radiation zone,

convective zone, photosphere, chromosphere, and corona.

In 2006, the International Astronomical Union (IAU) developed a new classification system for planets and objects

in the solar system. The three classes created were the planets, small solar system bodies, and dwarf planets.

The IAU classification of objects orbiting the around the Sun:

1. A "planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self￾gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round)

shape, and (c) has cleared the neighbourhood around its orbit.

2. A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its

self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round)

shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

3. All other objects except satellites (moons) orbiting the Sun shall be referred to collectively as "Small

Solar-System Bodies".

Planets

The eight planets all have counter-clockwise orbits with the orbital

planes being closely aligned with the Sun’s equator.

The four inner planets, Mercury, Venus, Earth, and Mars are all

terrestrial planets and are composed of heavier elements (Fe, Si,

Mg, S, and Ni).

The four outer planets, Jupiter, Saturn, Uranus, and Neptune are

all gaseous planets composed of mostly Hydrogen and Helium.

All of the outer planets are at least four times larger than Earth’s

diameter and Jupiter is 11.2 times larger. The planetary rings

are a unique feature to the outer gaseous planets, with the rings

of Saturn being the most famous and documented.

Dwarf Planets

With the addition of Makemake in 2008, there are now four official dwarf planets. Pluto was formerly considered

the smallest of the “nine traditional planets”, until being reclassified as a dwarf planet. Eris and Makemake are icy

bodies discovered in the Kuiper belt. Finally, Ceres, the largest asteroid in the asteroid belt, has enough mass to

produce a hydrostatic equilibrium (spherical shape). Astronomers estimate that with better telescopes and better

funding for research, more than 200 more dwarf planets may be proven to exist in the outer reaches of the solar

system.

Small Solar System Bodies (SSSBs)

SSSBs are irregular in shape and include;

 Asteroids composed of rock and metals located in the asteroid belt between Mars and Jupiter. There

are possibly over 1 000 000 asteroids greater than 1 km in size.

 Smaller asteroids less than 10 m in diameter called meteorites.

 Comets composed of ice. When comets approach the Sun, long tails of water vapour can be seen from

Earth. Comets may originate in the Kuiper belt (a great ring of debris similar to the asteroid belt but

consisting of icy material).

Outer Limits of the Solar System

Where are the outer limits of the solar system? The answer to this question has eluded astronomers. The

principle problem is how to define the limits – should it be the outer limit of the Sun’s particles or the outer limit of

the Sun’s gravitational influence? The influence of the Sun’s particles is known to be roughly four times that of

Pluto’s orbit; however the gravitational influence may be 100 times farther.

Beyond Neptune at 30AU, lies the Kuiper belt between 30AU and 50AU. The objects found in the Kuiper belt are

referred to as KBOs (Kuiper Belt Objects) and their research has emerged in the last 10 years.

The “scattered disc” is considered to be an extension of the Kuiper belt. Objects found in the scattered disc have

much higher eccentricities (more elliptical orbits)

than the KBOs. As a result they extend much farther

from the Sun, out to more than 100AU. These

objects are also inclined to the ecliptic plane, with

highly irregular orbits possibly due to gravitational

forces from Neptune. Eris, one of the small dwarf

planets, is located in the disc at 68AU.

The heliopause is the point at which the Sun’s solar

wind meets the interstellar medium or solar wind

from other stars. With estimates from Voyager I (a

satellite on the verge of leaving the solar system),

the edge of the heliopause has been determined to

be 153-158AU.

The Oort cloud is a spherical cloud that surrounds

the solar system. It was first proposed to exist in

1950 by Dutch astronomer Jan Oort and extends

from the Sun out to 100 000AU (1.87ly). The Oort

Cloud is considered to be the outer edge of the solar

system. Beyond the Oort cloud, the Sun’s physical

and gravitational effects become non-existent. The cloud is thought to be composed of billions of icy objects in

solar orbit. When a passing star disturbs the orbit of one of these bodies, the body enters the inner solar system

as a long period comet.

Solar System Formation

The nebular hypothesis maintains that the Solar System formed from the gravitational collapse of a fragment of a

giant molecular cloud. The cloud itself had a size of about 20 pc, while the fragments were roughly 1 pc (three

and a quarter light-years) across. The further collapse of the fragments led to the formation of dense cores 0.01–

0.1 pc (2,000–20,000 AU) in size. One of these collapsing fragments (known as the pre-solar nebula) would form

what became the Solar System. The composition of this region with a mass just over that of the Sun was about

the same as that of the Sun today, with hydrogen, along with helium and trace amounts of lithium produced by Big

Bang nucleosynthesis, forming about 98% of its mass. The remaining 2% of the mass consisted of heavier

elements that were created by nucleosynthesis in earlier generations of stars. Late in the life of these stars, they

ejected heavier elements into the interstellar medium.

Studies of ancient meteorites reveal traces of stable daughter nuclei of short-lived isotopes, such as iron-60, that

only form in exploding, short-lived stars. This indicates that one or more supernovae occurred near the Sun while

it was forming. A shock wave from a supernova may have triggered the formation of the Sun by creating regions

of over-density within the cloud, causing these regions to collapse. Because only massive, short-lived stars

produce supernovae, the Sun must have formed in a large star-forming region that produced massive stars,

possibly similar to the Orion Nebula. Studies of the structure of the Kuiper belt and of anomalous materials within

it suggest that the Sun formed within a cluster of stars with a diameter of between 6.5 and 19.5 light-years and a

collective mass equivalent to 3,000 Suns. Several simulations of our young Sun interacting with close-passing

stars over the first 100 million years of its life produce anomalous orbits observed in the outer Solar System, such

as detached objects. Because of the conservation of angular momentum, the nebula spun faster as it collapsed. As the material within

the nebula condensed, the atoms within it began to collide with increasing frequency, converting their kinetic

energy into heat. The centre, where most of the mass collected, became increasingly hotter than the surrounding

disc. Over about 100,000 years, the competing forces of gravity, gas pressure, magnetic fields, and rotation

caused the contracting nebula to flatten into a spinning protoplanetary disc with a diameter of ~200 AU and form a

hot, dense protostar (a star in which hydrogen fusion has not yet begun) at the centre.

At this point in its evolution, the Sun is believed to have been a T Tauri star. Studies of T Tauri stars show that

they are often accompanied by discs of pre-planetary matter with masses of 0.001–0.1 solar masses. These discs

extend to several hundred AU—the Hubble Space Telescope has observed protoplanetary discs of up to 1000 AU

in diameter in star-forming regions such as the Orion Nebula—and are rather cool, reaching only a thousand

kelvins at their hottest. Within 50 million years, the temperature and pressure at the core of the Sun became so

great that its hydrogen began to fuse, creating an internal source of energy that countered gravitational

contraction until hydrostatic equilibrium was achieved. This marked the Sun's entry into the prime phase of its life,

known as the main sequence. Main sequence stars derive energy from the fusion of hydrogen into helium in their

cores. The Sun remains a main sequence star today.

Formation of planets

The various planets are thought to have formed from the solar nebula, the disc-shaped cloud of gas and dust left

over from the Sun's formation. The currently accepted method by which the planets formed is known as accretion,

in which the planets began as dust grains in orbit around the central protostar. Through direct contact, these

grains formed into clumps up to 200 metres in diameter, which in turn collided to form larger bodies

(planetesimals) of ~10 kilometres (km) in size. These gradually increased through further collisions, growing at the

rate of centimetres per year over the course of the next few million years.

The inner Solar System, the region of the Solar System inside 4 AU, was too warm for volatile molecules like

water and methane to condense, so the planetesimals that formed there could only form from compounds with

high melting points, such as metals (like iron, nickel, and aluminium) and rocky silicates. These rocky bodies

would become the terrestrial planets (Mercury, Venus, Earth, and Mars). These compounds are quite rare in the

universe, comprising only 0.6% of the mass of the nebula, so the terrestrial planets could not grow very large. The

terrestrial embryos grew to about 0.05 Earth masses and ceased accumulating matter about 100,000 years after

the formation of the Sun; subsequent collisions and mergers between these planet-sized bodies allowed

terrestrial planets to grow to their present sizes.

When the terrestrial planets were forming, they remained immersed in a disk of gas and dust. The gas was

partially supported by pressure and so did not orbit the Sun as rapidly as the planets. The resulting drag caused a

transfer of angular momentum, and as a result the planets gradually migrated to new orbits. Models show that

temperature variations in the disk governed this rate of migration, but the net trend was for the inner planets to

migrate inward as the disk dissipated, leaving the planets in their current orbits.

The gas giant planets (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, the point

between the orbits of Mars and Jupiter where the material is cool enough for volatile icy compounds to remain

solid. The ices that formed the Jovian planets were more abundant than the metals and silicates that formed the

terrestrial planets, allowing the Jovian planets to grow massive enough to capture hydrogen and helium, the

lightest and most abundant elements. Planetesimals beyond the frost line accumulated up to four Earth masses

within about 3 million years. Today, the four gas giants comprise just under 99% of all the mass orbiting the Sun.

Theorists believe it is no accident that Jupiter lies just beyond the frost line. Because the frost line accumulated

large amounts of water via evaporation from infalling icy material, it created a region of lower pressure that

increased the speed of orbiting dust particles and halted their motion toward the Sun. In effect, the frost line acted

as a barrier that caused material to accumulate rapidly at ~5 AU from the Sun. This excess material coalesced

into a large embryo of about 10 Earth masses, which then began to grow rapidly by swallowing hydrogen from the

surrounding disc, reaching 150 Earth masses in only another 1000 years and finally topping out at 318 Earth

masses. Saturn may owe its substantially lower mass simply to having formed a few million years after Jupiter,

when there was less gas available to consume.

T Tauri stars like the young Sun have far stronger stellar winds than more stable, older stars. Uranus and

Neptune are believed to have formed after Jupiter and Saturn did, when the strong solar wind had blown away

much of the disc material. As a result, the planets accumulated little hydrogen and helium—not more than 1 Earth

mass each. Uranus and Neptune are sometimes referred to as failed cores. The main problem with formation

theories for these planets is the timescale of their formation. At the current locations it would have taken a

hundred million years for their cores to accrete. This means that Uranus and Neptune probably formed closer to

the Sun—near or even between Jupiter and Saturn—and later migrated outward (see Planetary migration below).

Motion in the planetesimal era was not all inward toward the Sun; the Stardust sample return from Comet Wild 2

has suggested that materials from the early formation of the Solar System migrated from the warmer inner Solar

System to the region of the Kuiper belt.

After between three and ten million years, the young Sun's solar wind would have cleared away all the gas and

dust in the protoplanetary disc, blowing it into interstellar space, thus ending the growth of the planets.

Subsequent evolution

The planets were originally believed to have formed in or near their current orbits. However, this view underwent

radical change during the late 20th and early 21st centuries. Currently, it is believed that the Solar System looked

very different after its initial formation: several objects at least as massive as Mercury were present in the inner

Solar System, the outer Solar System was much more compact than it is now, and the Kuiper belt was much

closer to the Sun.

Terrestrial planets

At the end of the planetary formation epoch the inner Solar System was populated by 50–100 Moon- to Mars￾sized planetary embryos. Further growth was possible only because these bodies collided and merged, which

took less than 100 million years. These objects would have gravitationally interacted with one another, tugging at

each other's orbits until they collided, growing larger until the four terrestrial planets we know today took shape.

One such giant collision is believed to have formed the Moon (see Moons below), while another removed the

outer envelope of the young Mercury.

One unresolved issue with this model is that it cannot explain how the initial orbits of the proto-terrestrial planets,

which would have needed to be highly eccentric to collide, produced the remarkably stable and near-circular

orbits the terrestrial planets possess today. One hypothesis for this "eccentricity dumping" is that the terrestrials

formed in a disc of gas still not expelled by the Sun. The "gravitational drag" of this residual gas would have

eventually lowered the planets' energy, smoothing out their orbits. However, such gas, if it existed, would have

prevented the terrestrials' orbits from becoming so eccentric in the first place. Another hypothesis is that

gravitational drag occurred not between the planets and residual gas but between the planets and the remaining

small bodies. As the large bodies moved through the crowd of smaller objects, the smaller objects, attracted by

the larger planets' gravity, formed a region of higher density, a "gravitational wake", in the larger objects' path. As

they did so, the increased gravity of the wake slowed the larger objects down into more regular orbits.

Asteroid belt

The outer edge of the terrestrial region, between 2 and 4 AU from Sun, is called the asteroid belt. The asteroid

belt initially contained more than enough matter to form 2–3 Earth-like planets, and, indeed, a large number of

planetesimals formed there. As with the terrestrials, planetesimals in this region later coalesced and formed 20–

30 Moon- to Mars-sized planetary embryos; however, the proximity of Jupiter meant that after this planet formed,

3 million years after the Sun, the region's history changed dramatically. Orbital resonances with Jupiter and

Saturn are particularly strong in the asteroid belt, and gravitational interactions with more massive embryos

scattered many planetesimals into those resonances. Jupiter's gravity increased the velocity of objects within

these resonances, causing them to shatter upon collision with other bodies, rather than accrete.

As Jupiter migrated inward following its formation (see Planetary migration below), resonances would have swept

across the asteroid belt, dynamically exciting the region's population and increasing their velocities relative to

each other. The cumulative action of the resonances and the embryos either scattered the planetesimals away

from the asteroid belt or excited their orbital inclinations and eccentricities. Some of those massive embryos too

were ejected by Jupiter, while others may have migrated to the inner Solar System and played a role in the final

accretion of the terrestrial planets. During this primary depletion period, the effects of the giant planets and

planetary embryos left the asteroid belt with a total mass equivalent to less than 1% that of the Earth, composed

mainly of small planetesimals. This is still 10–20 times more than the current mass in the main belt, which is about

1/2,000 the Earth's mass. A secondary depletion period that brought the asteroid belt down close to its present

mass is believed to have followed when Jupiter and Saturn entered a temporary 2:1 orbital resonance (see

below).

The inner Solar System's period of giant impacts probably played a role in the Earth acquiring its current water

content (~6×1021 kg) from the early asteroid belt. Water is too volatile to have been present at Earth's formation

and must have been subsequently delivered from outer, colder parts of the Solar System. The water was probably

delivered by planetary embryos and small planetesimals thrown out of the asteroid belt by Jupiter. A population of

main-belt comets discovered in 2006 has been also suggested as a possible source for Earth's water. In contrast,

comets from the Kuiper belt or farther regions delivered not more than about 6% of Earth's water. The panspermia

hypothesis holds that life itself may have been deposited on Earth in this way, although this idea is not widely

accepted.

Planetary migration

After the formation of the Solar System, the orbits of all the giant planets continued to change slowly, influenced

by their interaction with large number of remaining planetesimals. After 500–600 million years (about 4 billion

years ago) Jupiter and Saturn fell into a 2:1 resonance; Saturn orbited the Sun once for every two Jupiter orbits.

This resonance created a gravitational push against the outer planets, causing Neptune to surge past Uranus and

plough into the ancient Kuiper belt. The planets scattered the majority of the small icy bodies inwards, while

themselves moving outwards. These planetesimals then scattered off the next planet they encountered in a

similar manner, moving the planets' orbits outwards while they moved inwards. This process continued until the

planetesimals interacted with Jupiter, whose immense gravity sent them into highly elliptical orbits or even ejected

them outright from the Solar System. This caused Jupiter to move slightly inward. Those objects scattered by

Jupiter into highly elliptical orbits formed the Oort cloud; those objects scattered to a lesser degree by the

migrating Neptune formed the current Kuiper belt and scattered disc. This scenario explains the Kuiper belt's and

scattered disc's present low mass. Some of the scattered objects, including Pluto, became gravitationally tied to

Neptune's orbit, forcing them into mean-motion resonances. Eventually, friction within the planetesimal disc made

the orbits of Uranus and Neptune circular again.

In contrast to the outer planets, the inner planets are not believed to have migrated significantly over the age of

the Solar System, because their orbits have remained stable following the period of giant impacts.