The theories concerning the origin and evolution of the Solar System
are complex and varied, interweaving various scientific disciplines, from
planetary science. Over the centuries, many theories have been advanced as
to its formation, but it was not until the
eighteenth century that the development of the modern theory took shape.
With the dawn of the
the images and structures of other worlds in the solar system refined our
understanding, while advances in
nuclear physics gave us our first glimpse of the processes which underpinned
stars, and led to the first theories of their formation and ultimate
The current hypothesis of Solar System formation is the
nebular hypothesis, first proposed in 1755 by
Immanuel Kant and independently formulated by
The nebular theory maintains that 4.6 billion years ago, the Solar System formed
from the gravitational collapse of a gaseous cloud. This initial cloud was
likely several light-years across and played host to the birth of several stars.
Although the process was initially viewed as relatively tranquil, recent studies
of ancient meteorites reveal traces of elements only formed in the hearts of
very large exploding stars, indicating that the environment in which the Sun
formed was within range of a number of nearby supernovas. The shock wave from
these supernovas may have triggered the formation of the Sun by creating regions
of overdensity in the surrounding nebula, causing them in turn to collapse.
One of these regions of collapsing gas (known as the
nebula) would form what became the Sun. This region had a diameter of
between 7000 and 20,000 AU
and a mass just over that of the Sun (by between 0.1 and 0.001 solar masses).
Its composition was believed to be about the same as the Sun today: about 98%
helium present since the
and 2% heavier elements created by earlier generations of stars which died and
ejected them back into interstellar space (see
nucleosynthesis). As the nebula collapsed, conservation of
angular momentum meant that it spun faster. As the material within the
nebula condensed, the atoms within it began to collide with increasing
frequency, causing them to release energy as heat. The centre, where most of the
mass collected, became increasingly hotter than the surrounding disc.
 As the competing forces associated with gravity, gas pressure,
magnetic fields, and rotation acted on it, the contracting nebula began to
flatten into a spinning
protoplanetary disk with a diameter of roughly 200 AU
and a hot, dense
at the center.
stars, young, pre-fusing solar mass stars believed to be similar to the Sun
at this point in its evolution, show that they are often accompanied by discs of
These discs extend to several hundred AU and are rather cool, reaching only a
thousand kelvins at their hottest.
Eventually, the temperature and pressure at the core of the Sun became so great
that the hydrogen began to fuse, creating an internal source of energy which
countered the force of gravitational contraction. At this point the Sun became a
fully fledged star.
From this cloud and its gas and dust, the various planets are thought to have
formed. 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, which initially formed by direct contact into clumps between
one and ten kilometres in diameter, which in turn collided to form larger bodies
of roughly 5 km in size gradually increasing by further collisions by roughly 15
cm per year over the course of the next few million years.
The inner solar system was too warm for volatile molecules like water and
methane to condense, so the planetesimals which formed there were relatively
small (comprising only 0.6% the mass of the disc)
 and composed largely of compounds with high melting points, such
and metals. These
rocky bodies eventually became the
terrestrial planets. Farther out, the gravitational effects of
it impossible for the protoplanetary objects present to come together, leaving
Farther out still, beyond the
frost line, where more volatile icy compounds could remain solid,
able to gather more material than the terrestrial planets, as those compounds
were more common. They became the
captured much less material and are known as
because their cores are believed to be made mostly of ices (hydrogen compounds).
After 100 million years, the pressure and density of hydrogen in the centre
of the collapsing nebula became great enough for the
thermonuclear fusion, which increased until
hydrostatic equilibrium was achieved.
The young Sun's
solar wind then cleared away all the gas and dust in the
protoplanetary disk, blowing it into interstellar space, thus ending the
growth of the planets. T-Tauri stars have far stronger
winds than more stable, older stars.
Problems with the solar nebula model
One problem with this hypothesis is that of
angular momentum. With the vast majority of the system's mass accumulating
at the center of the rotating cloud, the hypothesis predicts that the vast
majority of the system's angular momentum should accumulate there as well.
However, the Sun's rotation is far slower than expected, and the planets,
despite accounting for less than 1 percent of the system's mass, thus account
for more than 90 percent of its angular momentum. One resolution of this problem
is that dust grains in the original disc created drag which slowed down the
rotation in the center.
Planets in the "wrong place" are a problem for the solar nebula model.
in a region where their formation is highly implausible due to the reduced
density of the solar nebula and the longer orbital times in their region.
Jupiters now observed around other stars cannot have formed in their current
positions if they formed from a "solar nebula" too. These issues are dealt with
by assuming that interactions with the nebula itself and leftover planetesimals
can result in
The detailed features of the planets are yet another problem. The solar
nebula hypothesis predicts that all planets will form exactly in the ecliptic
plane. Instead, the orbits of the
classical planets have various (but admittedly small) inclinations with
respect to the ecliptic. Furthermore, for the gas giants it is predicted that
their rotations and moon systems will also not be inclined with respect to the
ecliptic plane. However most gas giants have substantial axial tilts with
respect to the ecliptic, with
Uranus having a
98° tilt. The Moon
being relatively large with respect to the
Earth and other
moons which are in irregular orbits with respect to their planet is yet another
issue. It is now believed these observations are explained by events which
happened after the initial formation of the solar system.
Estimation of age
radiometric dating, scientists can estimate that the solar system is 4.6
billion years old. The oldest rocks on Earth are approximately 3.9 billion years
old. Rocks this old are rare, as the Earth's surface is constantly being
reshaped by erosion, volcanism and plate tectonics. To estimate the age of the
solar system scientists must use
which were formed during the early condensation of the solar nebula. The oldest
meteorites (such as the
Canyon Diablo meteorite) are found to have an age of 4.6 billion years,
hence the solar system must be at least 4.6 billion years old.
The planets were originally believed to have formed in or near the orbits at
which we see them now. However, this view has been undergoing radical change
during the late 20th century and the beginning of the 21st century. Currently,
it is believed that the solar system looked very different after its initial
formation, with five objects at least as massive as Mercury being present in the
inner solar system (instead of the current four), the outer solar system being
much more compact than it is now, and the
belt starting much farther in than it does now.
Impacts are currently believed to be a regular (if infrequent) part of the
development and evolution of the solar system. In addition to the Moon-forming
impact, the Pluto-Charon
system is believed to be the result of a collision between Kuiper Belt objects.
Other cases of moons around
and other Kuiper Belt obejcts are also believed to be the result of collisions.
That collisions continue to happen is evidenced by the collision of
Comet Shoemaker-Levy 9 with
1994, and the impact feature
Meteor Crater in the US state of
Inner solar system
According to the currently accepted view, the inner solar system was
"completed" by a
giant impact in which the young Earth collided with a
(being the "fifth" inner solar system object alluded to above). This impact
resulted in the fomation of the
Moon. The current
speculation is that this Mars-sized object formed at one of the stable Earth-Sun
Lagrangian points (either L4 or L5) and later drifted
away from that position.
Under the solar nebula hypothesis, the asteroid belt initially contained more
than enough matter to form a planet, and, indeed, a large number of
planetesimals formed there. However, Jupiter formed before a planet could
form from these planetesimals. Because of the large mass of Jupiter, orbital
resonances with Jupiter govern orbits in the asteroid belt. These resonances
either scattered the planetesimals away from the asteroid belt or held them in
narrow orbital bands and prevented them from consolidating. What remains are the
last of the planetesimals created initially during the formation of the solar
The effects of Jupiter have scattered most of the original contents of the
asteroid belt, leaving less than the equivalent of 1/10th of the mass of the
Earth. The loss of mass is the chief factor that prevents the asteroid belt from
consolidating into a planet. Objects with very large mass have a gravitational
field great enough to prevent the loss of large amounts of material as a result
of a violent collision. In the asteroid belt this usually is not the case. As a
result, many larger objects have been broken apart, and sometimes newer objects
have been forced out of the remnants in less violent collisions. Evidence of
collisions can be found in the moons around some asteroids, which currently can
only be explained as being consolidations of material flung away from the parent
object without enough energy to escape it.
The larger protoplanets were sufficiently massive to accrete gas from the
protoplanetary disk, and it is believed their mass distribution may be
understood from their positions in the disk, although such an explanation is too
simple to account for many other planetary systems. In essence, the first jovian
planetesimal to reach the critical mass required to capture helium gas and
subsequently hydrogen gas is most interior one, because - compared to orbits
farther from the Sun - here orbital speeds are higher, the density in the disk
is higher, and collisions happen more frequently. Thus Jupiter is the largest
jovian because it swept up hydrogen and helium gas for the longest period of
time, and Saturn is next. These composition of these two are dominated by the
captured hydrogen and helium gases (about 97% and 90% by mass, respectively).
The Uranus and Neptune protoplanets reach the critical size for collapse
significantly later, and thus captured less hydrogen and helium, which presently
makes up about only about 1/3 of their total mass.
Following gas capture, the outer solar system is now believed to have been
planetary migrations. As the gravity of the planets perturbed the orbits of
the Kuiper belt objects, many were scattered inwards by Saturn, Uranus, and
Neptune, while Jupiter often kicked those objects out of the solar system
altogether. As a result, Jupiter migrated inwards while Saturn, Uranus, and
Neptune migrated outwards. A major breakthrough in the understanding of how this
led to the current solar system structure occurred in 2004. In that year, new
computer models showed that if Jupiter started out taking fewer than two orbits
around the Sun for every time that Saturn orbited the Sun once, this migration
pattern would put Jupiter and Saturn into a 2:1
when the orbital period of Jupiter became exactly half that of Saturn's. This
resonance would have put Uranus and Neptune into highly elliptical orbits, with
there being a 50% chance that they would have exchanged places. The object which
ended up being outermost (Neptune) would then be forced outwards into the
belt as it initially existed.
The subsequent interaction between the planets and the Kuiper belt after
Jupiter and Saturn passed through the 2:1 resonance can explain the orbital
charactertistics and axial tilts of the giant outer planets. Uranus and Saturn
end up where they are due to interations with Jupiter and each other, while
Neptune ended up at its current location because that is where the Kuiper Belt
initially ended. The scattering of Kuiper belt objects could explain the
late heavy bombardment which occurred approximately 4 billion years ago.
Long after the solar wind cleared the gas out of the disk, a large population
of planetesimals remained behind, as yet unnaccreted by any planetary body. This
population was though to exist primarily beyond the outer planets, where
planetesimal accretion times were so long that a planet was unable to form
before gas dispersal. The outermost giant planet interacted with this
'planetesimal sea', scattering these small rocky bodies inwards, while itself
moving outwards. These planetesimals then scattered off the next planet they
encountered in a similar manner, and the next, moving the planets' orbits
outwards while the planetesimals moved inwards.
Eventually this planetary motion resulted in the 2:1 resonance crossing of
Jupiter and Saturn described above, and (so it is believed) Neptune and Uranus
were rapidly moved outwards to interact heavily with the sea of planetesimals.
The quantity of planetesimals being scattered inwards to reach the rest of the
Solar System increased hugely, and with many impacts upon all observed planetary
and lunar bodies. This period is known as the
Late Heavy Bombardment.
In this manner, the jovian planets (particularly Jupiter and Neptune)
gradually swept the disk clean of leftover planetesimals, "clearing
the neighbourhood" either by slinging them in the distant outer reaches of
Cloud (as far as 50,000
AU), or continually nudging their orbits into collisions with other planets
(or into more stable orbits like the
asteroid belt). This period of heavy bombardment lasts several
hundred million years, and is evident in the cratering still visible on
geologically dead bodies of the solar system. Planetesimals impacting Earth are
thought to have brought the Earth its water and other hydrogen compounds.
Although not widely accepted, some believe life itself may have been deposited
on Earth in this way (known as the
hypothesis). The current locations and contents of the Kuiper and Asteroid Belts
may be largely dependent on the Late Heavy Bombardment for transporting large
quantities of mass throughout the solar system.
Importantly, the bombardment and collisions of planetesimals and protoplanets
can explain unusual moons, moon orbits, axial tilts, and other discrepancies
from the originally very orderly motions. Excessive cratering of the Moon and
other large bodies, dated to this era of the Solar System, is also naturally
explained by the process. A
giant impact of a Mars-sized protoplanet is suspected of being responsible
for Earth's unusually large moon, whose composition and density is similar to
the Earth's mantle, and could simultaneously have altered Earth's rotation axis
to its present 23.5°
from its orbital plane.
In the solar nebula model, the only other way terrestrial planets can get
moons is by capturing them.
Mars' two tiny
low-altitude moons are clearly asteroids, and other examples of captured
satellites abound in the jovian systems.
Jupiter's regular gravitational interactions (see
orbital resonance) are also responsible for preventing the material which
once inhabited the
asteroid belt from accreting into another probably sizable terrestrial
planet. Most of that material has long since been thrown into eccentric orbits
and collided with something else; the total mass of the asteroid belt is now
less than a tenth of Earth's Moon.
Kuiper belt and Oort cloud
The Kuiper Belt was initially an outer region of icy bodies which lacked
enough of a mass density to consolidate. Originally its inner edge would have
been just beyond the outermost of Uranus and Neptune when they formed. (This is
most likely in the range of 15 - 20
A.U..) The outer edge was at approximately 30 A.U. The Kupier Belt initially
"leaked" objects into the outer solar system, and caused the initial planetary
The Jupiter-Saturn 2:1 orbital resonance caused Neptune to plow into the
Kupier belt, scattering most of the objects. Many of these objects were
scattered inwards, until they interacted with Jupiter and most often were placed
into highly elliptical orbits or even ejected outright from the solar system.
The objects which ended up in highly elliptical orbits form the
Closer in, some objects were scattered outwards by Neptune, and those form the
scattered disc, accounting for the Kuiper belt's present low mass. However,
a large number of KBOs, including Pluto, became gratitationally tied to
Neptune's orbit, forcing them into resonant orbits.
Moons have come to exist around most planets and many other Solar System
bodies. These natural
have come into being from one of three possible causes:
- co-formation from a proto-planetary disk (peculiar to the gas giants),
- formation from impact debris (given a large enough impact at a shallow
- capture of a passing object.
The gas giants tend to have inner moon systems which originated from the a
proto-planetary disk. This is indicated by the large sizes of the moons and
their proximity to the planet. (These attributes are impossible to achieve via
capture, while the gaseous nature of the primaries make formation from collision
debris another impossibility.) The outer moons of the gas giants tend to be
small and have orbits which are elliptical and have arbitrary inclinations.
These features are appropriate for captured bodies.
For the inner planets and other solid solar system bodies, collisions appear
to be the main creator of moons, with a percentage of the material kicked up by
the collision ending up in orbit and coalescing into one or more moons. The
Moon is believed to
have formed in this way.
After forming, moon systems will continue to evolve. The most usual effect is
modification due to
tides. This occurs due the tidal bulge that a moon creates in the atmosphere
and oceans of a planet, and to a lesser extent in the primary itself. If the
planet rotates faster than the moon orbits, the tidal bulge will constantly be
displaced ahead of the satellite. In this case, the gravity of the bulge will
cause the satellite to accelerate and slowly move away from the planet (as is
the case for the Moon). On the other hand, if the moon orbits faster than the
primary spins (or orbits against the spin), the bulge will stay behind the moon,
and the bulge's gravity will cause the moon's orbit to decay over time. (The
Phobos is slowly spiraling in towards
Mars for this
A planet can also create a tidal bulge in a moon, and this will slow down the
moon's rotation until its rotation period becomes the same as its revolution
period. Thus, the moon will keep one side of itself facing the planet, as is the
case for the Moon. This is called
locking and is present in many other moons in the Solar System such as
In the case of Pluto
Charon, both the primary and the satellite are tidally locked to each other.
Barring some unforeseeable accident, such as the arrival of a rogue
or star into its territory, astronomers estimate that the solar system as we
know it today will last another billion years or so, whereupon the Sun will
claim its first casualty, the Earth. As the
Sun brightens a
further ten percent beyond today's levels, its radiation output will increase,
gradually searing the Earth until its land surface becomes uninhabitable, though
life could still survive in the deeper oceans. Within 3.5 billion years, Earth
will attain surface conditions similar to Venus's today; the oceans will boil,
and all life (in known forms) will be impossible.
Five billion or so years from now, the hydrogen reserves within the Sun's
core will be spent, and it will begin to use those in its less dense upper
layers. This will require it to expand to eighty times its current diameter,
and, about 7.5 billion years from now, to become a
cooled and dulled by its vastly increased surface area. As the Sun expands, it
will swallow the planet Mercury. Earth and Venus, however, are expected to
survive, since the Sun will lose about 28 percent of its mass, and its lower
gravity will send them into higher orbits. Earth will be left a scorched cinder,
its land surface reduced to the consistency of hot clay by sunlight a thousand
times more powerful than today's, and its atmosphere stripped away by a
now-ferocious solar wind. The Sun is expected to remain in a red giant phase for
about a hundred million years.
During this time, it is possible that the watery worlds around Jupiter and
Saturn, such as
Europa, might achieve conditions similar to those required for current human
Eventually, the helium produced in the shell will fall back into the core,
increasing the density until it reaches the levels needed to fuse helium into
carbon. The Sun will then shrink to slightly larger than its original radius, as
its energy source has fallen back to its core, however, due to the relative
rarity of helium as opposed to hydrogen, the helium-fusing stage will only last
about 100 million years. Eventually it will have to again resort to its reserves
in its outer layers, and will regain its red giant form. This phase lasts only
100 million years, after which, over the course of a further 100,000 years, the
Sun's outer layers will fall away, ejecting a vast stream of matter into space
and forming a halo known (misleadingly) as a
This is a relatively peaceful event; nothing akin to a
which our Sun is too small to ever undergo. Earthlings, if we are still alive to
witness this occurrence, would observe a massive increase in the speed of the
solar wind, but not enough to destroy the Earth completely.
Eventually, all that will remain of the Sun is a
dwarf, a hot, dim and extraordinarily dense object; half its original mass
but only the size of the Earth. Were it viewed from Earth's surface, it would be
a point of light the size of Venus with the brightness of a hundred current
As the Sun dies, its gravitational pull on the orbiting planets, comets and
asteroids will weaken. Earth and the other planets' orbits will expand. When the
sun becomes a white dwarf, the solar system's final configuration will be
reached: Mercury will have long since ceased to exist; Venus will lie roughly a
third again farther out than Earth is now, and Earth's orbit will roughly equal
that of Mars today. Two billion years farther on, the carbon in the Sun's core
will crystallize, transforming it into a giant
Eventually, after trillions more years, it will fade and die, finally ceasing to
History of solar system formation hypotheses
During the late-19th
century the Kant-Laplace nebular hypothesis was criticized by
James Clerk Maxwell, who showed that if matter of the known planets had once
been distributed around the
Sun in the form of a
disk, forces of
differential rotation would have prevented the condensation of individual
planets. Another objection was that the Sun possesses less
angular momentum than the Kant-Laplace model indicated. For several decades,
most astronomers preferred the near-collision hypothesis, in which the
planets were considered to have been formed due to the approach of some other
star to the Sun. This near-miss would have drawn large amounts of matter out of
the Sun and the other star by their mutual
forces, which could have then condensed into planets.
Objections to the near-collision hypothesis were also raised and, during the
nebular model was improved such that it became broadly accepted. In the modified
version, the mass of the original
protoplanet was assumed to be larger, and the angular momentum discrepancy
was attributed to
magnetic forces. That is, the young Sun transferred some angular momentum to
protoplanetary disk and
planetesimals through Alvén waves, as is understood to occur in
The refined nebular model was developed based entirely on observations of our
own solar system, because it was the only one known until the mid 1990's. It was
not confidently assumed to be widely applicable to other
planetary systems, although scientists were anxious to test the nebular
model by finding of
protoplanetary disks or even planets around other stars, so-called
Stellar nebula or
protoplanetary disks have now been observed in the
nebula, and other
star-forming regions, by astronomers using the
Hubble Space Telescope. Some of these are as large as 1000 AU in diameter.
As of November 2006, the discovery of over 200
has turned up many surprises, and the nebular model must be revised to account
for these discovered planetary systems, or new models considered. There is no
consensus on how to explain the observed 'hot Jupiters,' but one leading idea is
planetary migration. This idea is that planets must be able to migrate
from their initial orbit to one nearer their star, by any of several possible
physical processes, such as orbital friction while the protoplanetary disk is
still full of hydrogen and helium gas.
In recent years, an alternative model for the formation of the solar system,
the Capture Theory, has been developed. It is claimed that this model explains
features of the solar system not explained by the