1. Formation of the Solar System
Courtesy: NASA

2. Formation of the Solar System The Formation of the Solar System
Evolutionary Theories
All evolutionary theories have their start with Descartes’s whirlpool or vortex theory proposed in 1644.
Using Newtonian mechanics, Kant (in 1755) and then Laplace (around 1795) modified Descartes’s vortex to a rotating cloud of gas contracting under gravity into a disk.
The Solar Nebula Hypothesis is an example of an evolutionary theory.
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3. Formation of the Solar System The Formation of the Solar System
Catastrophic Theories
Catastrophic theory is a theory of the formation of the solar system that involves an unusual incident such as the collision of the Sun with another star.
The first catastrophic theory - that a comet pulled material from the Sun to form the planets - was proposed by Buffon in 1745.
Other close encounter hypotheses have been proposed too.
Catastrophic origins for solar systems would be quite rare (relative to evolutionary origins) due to the unusual nature of the catastrophic incident.
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4. Formation of the Solar System Solar Nebula Hypothesis
Towards a Solar Nebula Hypothesis
The nebular cloud collapsed due the force of gravity on the cloud. But the cloud does not end up spherical (like the sun) because there are other processes going on:
Heating – The cloud increases in temperature, converting gravitational potential energy to kinetic energy. The sun would form in the center where temperatures and densities were the greatest
Spinning – as the cloud shrunk in size, the rotation of the disk increase (from the conservation of angular momentum).
Flattening – as cloud starting to spin, collisions flattened the shape of the disk in the plane perpendicular to the spin axis
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5. Formation of the Solar System Testing the Model
If the theory is correct, then we should see disks around young stars
Dust disks, such as discovered around beta-Pictoris or AU Microscopii, provide evidence that conditions for planet formation exist around many Sun-like stars.
Courtesy: NASA
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6. Formation of the Solar System Solar Nebula Hypothesis
The Formation of Planets
As the solar nebula cooled and flattened into a disk some 200 AU in diameter, materials began to “freeze” out in a process called condensation (changing from a gas to a solid or liquid).
The ingredients of the solar system consist of 4 categories (with % abundance):
Hydrogen and Helium gas (98%)
Hydrogen compounds, such as water, ammonia, and methane (1.4%)
Rock (0.4%)
Metals (0.2%)
Since it is too cool for H and He to condense, a vast majority of the solar nebula did not condense
Hydrogen compounds could only condense into ices beyond the frost line, which lay between the present-day orbits of Mars and Jupiter
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7. Formation of the Solar System Solar Nebula Hypothesis
Building the Terrestrial Planets
In the 1940s, Weizsächer showed that eddies would form in a rotating gas cloud and that the eddies nearer the center would be smaller.
Eddies condense to form particles that grow over time in a process called accretion. Materials such and rock and metal (categories #3 and #4).
These accreted materials became planetesimals, which in turn sweep up smaller particles through collision and gravitational attraction.
These planetesimals suffered gravitational encounters which altered their orbits caused them to both coalesce and fragment. Only the largest planetesimals grew to be full-fledged planets.
Verification of this models is difficult and comes in the form of theoretical evidence and computer simulations.
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8. Formation of the Solar System Solar Nebula Hypothesis
Building the Jovian Planets
Planetesimals should have also grown in the outer solar system, but would have been made of ice as well as metal and rock.
But Jovian planets are made mostly of H and He gas…
The gas presumably was captured by these ice/rock/metal planetesimals and grew into the Jovian planets of today.
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9. Formation of the Solar System Solar Nebula Hypothesis
Stellar wind is the flow of nuclear particles from a star.
Some young stars exhibit strong stellar winds. If the early Sun went through such a period, the resulting intense solar wind would have swept the inner solar system clear of volatile elements.
The giant planets of the outer solar system would then have collected these outflowing gases.
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10. Formation of the Solar System Solar Nebula Hypothesis
Explaining Other Clues
Over millions of years the remaining planetesimals fell onto the moons and planets causing the cratering we see today. This was the period of heavy bombardment.
Comets are thought to be material that coalesced in the outer solar system from the remnants of small eddies.
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11. Formation of the Solar System Solar Nebula Hypothesis
The formation of Jovian planets and its moons must have resembled the formation of the solar system. Jupiter specifically:
Moons close to Jupiter are denser and contain fewer light elements;
Moons farther out decrease in density and increase in heavier elements.
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12. Formation of the Solar System The Exceptions to the Rule
Captured Moons – satellites which go the opposite way were likely captured. Most of these moon are small are lie far away from the planet.
Giant impacts – may have helped form the Moon and explain the high density of Mercury and the Pluto-Charon system. Furthermore, the unusual tilts of Uranus and Venus can also be explained by giant impacts.
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Formation of the Solar System Radioactivity
Radioactivity
Certain isotopes (elements which contain differing number of neutrons) are not stable and will decay into two or more lighter elements
The time it takes for half of a given isotope to decay is called the half-life
By noting what percentage a rock (or human body) has left of a radioactive element can enable us to estimate the age of that object. This process is called radioactive dating. See Cosmic Calculations 6.1
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Formation of the Solar System Radioactivity
Earth rocks, Moon rocks, and meteorites
The oldest Earth rock date back to 4 billion years and some small grains go back to 4.4 billion years. Moon rock brought back from the Apollo mission date as far back as 4.4 billion years.
These tell us when the rock solidified, not when the planet formed
The oldest meteorites, which likely come form asteroids, are dated at 4.55 billion years, marking the time of the accretion of the solar system
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15. Formation of the Solar System Planetary Systems Around Other Stars?
Photographing planets around stars directly is very difficult since planet merely reflect (visible) light from the nearby stars. Using the infrared part of the spectrum, we can detect large objects known as brown dwarfs which are neither stars or planets
Stars exhibiting a discernable wobble from gravitation tugs can be evidence of an unseen companion - such as a large planet or group of planets. One can try to look for positional changes in the sky form this star – the astrometric technique, but this is difficult.
Since 1995, this Doppler Technique has found evidence of over 200 planets orbiting stars in the near vicinity of the Sun.
Some of the extrasolar planets can be detected when the transit the star. The star’s brightness dims just a bit during the transit. The Kepler mission has found over a 1000 planets using the transit technique.
Web link:
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16. Formation of the Solar System Planetary Systems Around Other Stars?
Comparisons to our Solar System
Many of these planets are more massive than Jupiter
Many of these planets are closer to their star than Mars is to the Sun
These discoveries are in part due to a selection effect – these are the easiest to detect
Jovian sized planets close to the star is not consistent with the standard solar nebular model. So how does one form a “hot Jupiter”?
Planetary migration – the gas giant form in the cooler, outer region of the nebular disk, but due to friction (and a loss of angular momentum) from the nebular disk, the planet in brought to a much closer distance.
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The End
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