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The Solar System

Posted 18th May 2009 at 06:13 by Ẋʼn

Most of the information in the following post is gleaned from the book Teach Yourself Planets, with additional information and images from other sources. Hopefully the idea behind this post is to provide an at-a-glance summary of the most pertinent information contained therein.

Local Geography

The Solar System is centred around the Sun, which is a G class star located approximately halfway from the core in the main disk of the Milky Way, which is believed to be a barred spiral galaxy. The Sun is one of approximately 200 to 400 billion stars in the Milky Way.

A map of the Milky Way, showing the location of the Sun and major features

TYP is slightly out of date, as it was published before the IAU defined a "planet" as an object that orbits the Sun, is large enough for its gravity to make it round, and has cleared the neighbourhood. Therefore contrary to TYP, the Solar System consists of eight standard planets and five dwarf planets.

The Solar System as it is currently defined

Because of the truly enormous distances and masses involved in planetary science, scientists use the Astronomical Unit (AU) when measuring distances within the Solar System, which is equal to 150,000,000Km, the average distance from the Earth to the Sun. For masses, multiples of 6e24 kg, or approximately one Earth mass, is the measurement used.

Kepler's Laws

Planetary orbits are not perfect circles - rather, they are ellipses, with two foci positioned on the long axis of the ellipse such that the sum of the distances between any point on the ellipse and its two foci remains constant. It is illustrated below:

The more elongate the ellipse, the further apart are the two foci and hence the greater the ratio between the distance between the foci and the size of the ellipse's long axis. This ratio is called the eccentricity of the ellipse. Conversely, when the two foci coincide the eccentricity is zero and the ellipse is simply a circle.

Although not labelled on the above diagram, the point on the orbit closest to the Sun is called the perihelion while the corresponding point furthest from the Sun is called the aphelion.

Kepler's second law of planetary motion states that, for a given planet, a line drawn from the planet to the Sun will always sweep out the same amount of area over a given time interval. This is illustrated below under section B of the diagram.

Planetary orbits and planetary spin

The orbits of all the known planets lie very nearly in the same plane, known as the ecliptic, and they all orbit in the same direction, anti-clockwise if one is looking down at the Sun's (or the Earth's) North pole. This anti-clockwise motion is known, somewhat confusingly, as prograde motion, while its reversal is called retrograde.

Because of the vast scale of the Solar System and the fact that the planets lie so close to the ecliptic, it is possible to represent the Solar System in 2 dimensions fairly accurately.

All planets rotate on their axis, but none of them perfectly perpendicular to its orbit. The degree to which a planet deviates from this is known as its axial inclination.

Most of the smaller objects within the Solar System share this pattern of prograde motion close to the plane of the ecliptic. Such objects include asteroids, Kuiper Belt Objects and short-period comets. Long-period comets from over 10,000 AU away can come in from any angle and pass around the Sun in either direction.

Formation of the Solar System

The formation of the Solar System began when a fragment of a giant molecular cloud collapsed due to Self-gravitation. This cloud fragment consisted mainly of hydrogen with traces of various ices, carbon and particles of silicates (specks of rock). The vast majority of this material ended up forming the Sun in the centre. As the cloud contracted, it condensed into a ever-faster spinning disk due to conservation of angular momentum, the same principles that speeds up a spinning ice skater when she bring in her limbs.

An image detailing the formation of the Solar System

Within the protoplanetary disk, the grains of dust were able to stick together into larger and larger clumps, until after about 10,000 years, the grains of dust had accumulated into globules a little more than a centimetre across. Approximately 100,000 years later, the aforementioned globules had agglomerated into planetesimals averaging 10km in diameter. Due to their mutual prograde motion and size, gravitational attraction between these bodies began to play a significant role. The largest planetesimals grew the fastest due to being able to attract more mass than their rivals. Within a further fews tens of thousands of years, the largest planetesimals had grown to a few thousand kilometres across, destroying most of the smaller ones in the process.

An artist's impression of two planetesimals colliding

At this stage the Solar System was mainly composed of bodies known as planetary embryos, of which there were perhaps a few hundred. Collisions between these bodies were extremely rare compared to previous times, due to the much larger distances. This meant that the formation of an Earth-sized planet would have taken maybe 50 million years or so through chance collisions of planetary embryos.

A planetary embryo undergoing impacts from smaller bodies

These collisions, known as giant impacts, would have been enough to either completely shatter the two bodies, thus starting the process again, or would have liberated enough energy to completely melt the impacted body. This molten state would have allowed heavier materials to sink inwards and lighter materials to sink upwards. THis process is known as differentiation.

Evidence suggests that the Earth's Moon owes its existance to such a collision, in which much of the debris from the shattered impactor plus some ejecta from the Earth ended up in orbit, where it eventually coalesced into the Moon.

A diagram illustrating how the Moon formed

At this stage the planets are more or less fully formed, having had their internal structure differentiated by melting impacts and the impact of the body that was to form the Earth's Moon representing the last of the planetary embryo-sized impactors. But there were still a great many asteroids and similar bodies cluttering the solar system, and bodies such Mercury and the Moon still bear the scars of these violent times. In fact, it is possible to tell the age of a planetary surface by the amount of cratering it possesses, since the frequency of impacts has been dropping off since the formation of the Solar System. This is known as the cratering timescale.

A diagram detailing crater formation

The processes and timescales described above apply to the terrestrial planets, but what about the gas giants? Beyond the orbit of Mars, temperatures were low enough for frozen water, a highly abundant substance, to condense directly from the solar nebula. The outer planets began as rock balls of several Earth masses covered in a layer of ice. Once such a planet had reached about 10 Earth masses it became a very effective gravitational scavenger of hydrogen and other gases directly from the nebula. Jupiter and Saturn were able to capture significantly more gases, hence their stupendous size.

Cross-sections comparing the internal structures of the giant planets

The presence of the massive planet Jupiter had a disruptive effect on planetary formation between its orbit and that of Mars, with the effect that collisons betweent he planetesimals were too violant for any significant form of accretion to occur. That material that would have otherwise formed planetary embryos and/or a planet instead formed asteroids (also known as the minor planets), in a region sometimes referred to as the asteroid belt, although asteroids are by no means limited to this area. Due to the recent IAU ruling mentioned earlier in this article, one of the former asteroids in this region, Ceres, is now classified as a dwarf planet.

A diagram showing the Asteroid Belt, as well as the Trojans which congregate at Jupiter's L4 and L5 Lagrangian Points.

A cutaway showing the specualtive interior composition of Ceres

Beyond the orbit of Neptune, the distances between the planetesimals were too great to allow the formation of large planets. Between 1992 and 2002 several hundred icy bodies were discovered beyond Neptune, mostly orbiting between 30 and 50 AUs from the Sun in a region known as the Kuiper Belt. It is know known that Pluto (and it's moon Charon) is merely one of the largest of a class of bodies known as Kuiper Belt Objects (KBOs). It is also suspected that one of Neptune's moons, Triton, is a captured KBO.

Composition showing Kuiper Belt Objects (Sedna, Pluto and Quaoar) with Earth and the Moon, all to scale

Comets consist of a mixture of ice, carbonaceous material and rock dust measuring a few tens of kilometres in diameter or less. They can develop spectacular tails of gas and dust when their highly eccentric orbits bring them close enough to the sun for the temperatures to vapourise the water, carbon monoxide and other materials otherwise trapped as ice. Short-period comets are probably small KBOs that have been gravitationally scattered inwards, and will survive only a few solar passages before all their volatiles are expended. Long-period comets fall into the inner Solar System from a reservoir come forty thousand AUs from the Sun known as the Oort Cloud.

Scaled illustration of the planets, inner Solar System, Outer Solar System and the Oort Cloud respectively

This blog post took way too fucking long. Might have to split up future entries a little.
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Total Comments 5


  1. Old Comment
    NecroCommie's Avatar
    What are thou?
    Posted 19th May 2009 at 15:49 by NecroCommie NecroCommie is offline
  2. Old Comment
    Ẋʼn's Avatar
    I'm not sure I understand your question.
    Posted 19th May 2009 at 18:06 by Ẋʼn Ẋʼn is offline
  3. Old Comment
    Coggeh's Avatar
    Wow cool post , 3rd time reading it trying to understand some stuff but really interesting .

    Keep it up .
    Posted 27th July 2009 at 21:42 by Coggeh Coggeh is offline
  4. Old Comment
    Coggeh's Avatar
    Why exactly is ceres a dwarf planet and not an asteroid , is it because of the sheer size of it ?
    Posted 28th July 2009 at 16:21 by Coggeh Coggeh is offline
  5. Old Comment
    Ẋʼn's Avatar
    It fits the IAU definition.
    Posted 28th July 2009 at 22:09 by Ẋʼn Ẋʼn is offline
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