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The Origin Of The Solar System
Observation and Theory
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Like any theory, a theory of the origin of the solar system must comply with
observations of its various properties and characteristics. The suggested theory must
explain why:
1. All the planets lie within approximately one plane.
2. All the planets revolve around the Sun in a counterclockwise direction.
3. The Sun and most of the planets rotate in a counterclockwise direction, and
explain the reason for the planets that do not.
4. There are two categories of planets, terrestrial (Mercury, Venus, Earth, and Mars)
and Jovian (Jupiter, Saturn, Uranus, and Neptune).
5. All the objects making up the solar system presumably formed at about the same
time, about 4.6 billion years ago.
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Solar Nebula Theory (Nebular Hypothesis) – The most accepted theory of the origin
of the solar system is the solar nebula theory. The solar nebula is defined as the cloud
of gas and dust from which the solar system formed.
Stages of Development
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Stage One 1 – Initially, a slowly counterclockwise-rotating cloud of gas and dust,
which may be part of a much larger nebula (a giant molecular cloud), began to
gravitationally collapse.
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This explains why all planets revolve around the Sun counterclockwise and why
the Sun and most of the planets rotate counterclockwise 2.
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This collapse may have been triggered in a number of ways:
1. Supernova Outburst – A nearby star exploded as a supernova, sending out
shockwaves that initiated the collapse of the cloud of interstellar gas and dust.
2. Stellar Winds and Radiation Pressure – Stellar winds (ejected streams of
charged particles, mostly protons and electrons) and radiation pressure (the
pressure exerted by the emitted electromagnetic radiation) from hot stars near
the cloud of interstellar gas and dust initiated compression.
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3. Density Waves – Density waves are the result of gravitational variations
within a galaxy. These waves spiral outward from the center at a constant
rotational rate somewhat slower than the rotation of stars and clouds of
interstellar gas and dust. Density waves do not consist of moving matter, but
are a moving pattern of compression that is overtaken by the faster moving
interstellar material. When a cloud of interstellar gas and dust moves through
the compressional waves, it piles up and becomes dense enough to
gravitationally collapse.
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Stage Two – As the nebula collapsed under its own gravity, three things occurred:
1. Increase Rotation Rate 3 – An increase in rotation rate of the nebula resulting from
the conservation of angular momentum. This is an important physical quantity
because all experimental evidence indicates that angular momentum is rigorously
conserved in our Universe – it can be transferred, but it cannot be created or
destroyed. Moreover, angular momentum is a constant, and velocity and radius
are inversely related. Therefore, the decrease in the radius of the solar nebula due
to its collapse, must spin faster in order to conserve angular momentum.
2. Disk-Shaped Formation 4 – The more rapid spin increased centrifugal forces, and
caused the nebula to flatten into a disk.
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This explains why all the planets lie in approximately one plane 5.
3. Temperature and Pressure Increase – Gravitational energy caused an increase in
temperature. At the center, a spherical assemblage developed, where the
temperature and pressure continued to rise until it finally reached the point where
nuclear fusion of hydrogen occurred. At this stage, the Sun was “born” and this
spherical assemblage of matter began its “life” as a star.

Stage Three 6 – Temperature differences between the warm inner regions and the cool
outer regions of the disk determined what kinds of material could condense to form
planets – this is referred to as the temperature-condensation sequence. (Condensation
is the formation of solid or liquid particles from a cloud of gas.) Thus, the higher
temperature in the inner region where the terrestrial planets eventually formed
produced solids characterized by metals, silicates, and high temperature oxides.
Volatiles – elements or compounds with low vaporizing temperatures – in this region
remained in the form of gases. In contrast, the lower temperature in the outer region
where the Jovian planets eventually formed (beyond the frost line) was characterized
by volatiles in the solid phase. More specifically, ices of water, ammonia, methane,
carbon dioxide, etc. Although the outer region of the disk contained condensates of
all kinds (metals, rocks, and ices), ices were nearly three times more abundant than
metals and rocks because of the overall greater abundance of hydrogen compounds in
the solar nebula.

This temperature-dependent condensation primarily explains why there are two
major categories of planets, terrestrial 7 and Jovian 8.
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Stage Four 9 – The solid grains resulting from the condensation then built up by
“gentle” low velocity collisions and sticking together to form larger objects called
planetesimals. Initially, the particles stuck together through electrostatic forces, but
as the particles grew in mass, gravity began to aid in their bonding together,
accelerating their growth. This process of growing by colliding and sticking is called
accretion.
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Asteroids and comets are now recognized as surviving examples of planetesimals.
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Stage Five 10 – Planetesimals continued to collide and gradually built up into larger
bodies. When a planetesimal diameter exceeded several hundred miles, it would have
been massive enough to dominate its orbit and gravitationally attract other
approaching planetesimals, thereby continuing to rapidly grow in size. In this way,
the terrestrial planets and the cores of Jovian planets formed.
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Stage Six 11 – As the planets grew, their gravitational reach extended farther and
farther, causing an increase in violent impacts. The late stages of the solar system’s
formation may have been characterized by planet-sized objects with non-established
orbits. Some of these objects were probably ejected from the solar system. However,
a few of these planet-sized objects may have impacted the planets and can explain
some of their peculiarities. In particular, such catastrophic events may explain
Mercury’s deficiency in silicate material (rock), the highly inclined rotation axis of
Uranus, and the formation of the Earth’s moon. (All discussed later.)
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These violent impacts explain the many craters observed on the Moon and other
planetary objects of the solar system.
Stage Seven – The slow-orbiting massive cores of the proto-Jupiter and proto-Saturn
gravitationally attracted large quantities of gas (mainly hydrogen and helium) left
over from the solar nebula. As a result, both planets became quite massive,
developing thick atmospheres, and with increasing depth and thus pressure, layers of
liquid. However, since Uranus and Neptune reside in a region where apparently the
solar nebula’s density was significantly less, the formation of their cores at their
current distance from the Sun is highly implausible. According to the so-called Nice
model, Uranus and Neptune initially accreted in the Jupiter-Saturn region (where
more material was available), and through their gravitational interaction with a large
number of remaining planetesimals, migrated outward to their current positions over
hundreds of millions of years. In addition, Uranus and Neptune are thought to have
formed after Jupiter and Saturn did, when the strong solar wind had blown away
much of the disc material. As a result, they accumulated little hydrogen and helium.
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Hot Jupiters – Planetary migration is the most likely explanation for hot Jupiters –
exoplanets (see below) with Jovian masses, but with orbits very close to their
parent stars. A hot Jupiter is thought to form at a distance from its parent star
beyond the frost line, where the planet can accrete from rock, ice and gases. The
planet then migrates inward to the star where they eventually forms a stable orbit.
Over time, its atmosphere and outer layers are stripped away 12, and its remaining
core may become a classification of planet called chthonian.
Stage Eight – In roughly 5 billion years, the Sun will have fused almost all of the
hydrogen fuel in its core into helium, beginning its evolution into the red giant phase.
Though the Sun’s core will contract, the outer envelope will expand outward many
times its current diameter (engulfing Mercury, Venus, and most likely the Earth 13),
before casting off its outer layers as a planetary nebula and leaving behind a stellar
remnant known as a white dwarf. In time, the gravity of passing, nearby stars can
disrupt the orbits of the remaining planets, potentially ejecting them into interstellar
space. Ultimately, the Sun may be left with none of the original objects in orbit
around it.
The division of the origin and evolution of the solar system into eight stages is my
interpretation; that is, it does not represent an actual number of phases in the
development of the solar system.
Other Solar Systems
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T Tauri stars – The first indirect evidence for planet-forming disks came from studies
of T Tauri stars (named after the prototype star in the constellation Taurus), which are
similar in mass to the Sun, but very young (about one million years old). In the
1980s, astronomers realized that about a third of T Tauri stars emit an excess of
infrared radiation. This can be explained if these particular stars are surrounded by
dust heated by short-wavelength radiation from the stars.
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Protoplanetary Disks (proplyds) 14 – In the 1990s, astronomers using the Hubble
Space Telescope discovered many examples of disks around newly formed stars in
the great nebula in Orion. As the name suggests, protoplanetary disks (proplyds) are
thought to contain the material from which planets form around stars.
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Exoplanets – An exoplanet (or extrasolar planet) is a planet that orbits a star other
than the Sun. A few thousand candidate planets have been detected. There is at least
one planet on average per star 15.
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Extraterrestrial Life – The discovery of exoplanets has intensified interest in the
search for extraterrestrial life, particularly for those that orbit in the host star’s
habitable zone where it is possible for liquid water (and therefore life) to exist on
the surface. However, the study of planet habitability also considers a wide range
of other factors in determining the suitability of a planet for hosting life.
Approximately 1 in 5 Sun-like stars have an Earth-sized planet in the habitable
zone, with the nearest expected to be within 12 light-years distance from Earth.
Assuming 200 billion stars in the Milky Way, that would be 11 billion potentially
habitable Earth-sized planets in the Milky Way, rising to 40 billion if red dwarfs
(small and relatively cool stars) are included.
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Kepler 452b 16 – Kepler 452b is the first near-Earth-size planet discovered
orbiting within the habitable zone of a star (about 1,400 light-years away)
very similar to the Sun. It is 60% larger than the Earth, and is considered a
“super-Earth” size planet. While Kepler 452b’s mass and composition are not
yet determined, research suggests that planets its size have a good chance of
being rocky. Kepler 452b is older than the Earth (6 billion years), giving a
substantial opportunity for life to arise, should all the necessary ingredients
and conditions for life exist on this planet.
Detection Methods – Planets are extremely faint compared to their parent stars.
At visible wavelengths, they usually have less than a millionth of their host star’s
brightness. It is difficult to detect such a faint light source, and furthermore the
parent star causes a glare that tends to wash it out.
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Direct Imaging – All exoplanets that have been directly imaged are both large
(more massive than Jupiter) and widely separated from their parent star. One
of the clearest images of an exoplanet with its parent star (similar in mass to
our Sun, but much younger) is about eight times the mass of Jupiter, and over
30 billion miles out 17. Current models of how planets form around stars have
a very tough time putting a planet this massive that far out. It’s possible an
encounter with a more massive planet too close to the star to see
gravitationally flung the planet to its current distance.

Indirect Methods – The vast majority of exoplanets were detected through
various indirect methods rather than actual imaging. Most confirmed
exoplanets have been found using space-based telescopes. Many of the
detection methods can work more effectively with space-based telescopes that
avoid atmospheric haze and turbulence.
 Radial Velocity or Doppler Method – As a planet orbits a star, the star also
moves in its own small orbit around the system’s center of mass 18.
Variations in the star’s radial velocity – that is, the speed with which it
moves towards or away from Earth – can be detected from displacements
in the star’s spectral lines due to the Doppler Effect 19.
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Astrometry 20 – Astrometry consists of precisely measuring a star’s
position in the sky and observing the ways in which that position changes
over time. If the star has a planet, then the gravitational influence of the
planet will cause the star itself to move in a tiny circular or elliptical orbit
about their common center of mass. The Gaia space telescope uses this
method.
Transit Method 21 – If a planet crosses (or transits) in front of its parent
star’s disk, then the observed brightness of the star drops by a small
amount. The amount by which the star dims depends on its size and on
the size of the planet. The transit method reveals the radius of a planet,
and it has the benefit that it sometimes allows a planet’s atmosphere to be
investigated through spectroscopy. A number of space telescopes (Kepler
telescope) use this method.
Gravitational Microlensing 22 – Microlensing occurs when the
gravitational field of a star acts like a lens, magnifying the light of a
distant background star. Possible planets orbiting the foreground star can
cause detectable anomalies in the lensing event light curve. This method
is most sensitive to detecting planets around 1to10 AU away from stars
like the Sun.
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