formation of solar system introduction origin of solar system formation of the planets planets and...

Formation of Solar System

• Introduction• Origin of Solar System• Formation of the planets• Planets and their orbit• Extrasolar planets

Introduction• Planetary astronomy is the study of our solar system• To understand our own solar system and thus explore the possibility of life in outer space• A few known approved theories of the formation and evolution of solar system

Origin of Solar System

There are a few theories about how the solar system evolved. These can generally be divided into 2 broad categories:• That the solar system is purely an accidental catastrophic event

• Eg. Near collision between the Sun and another star

• That the solar system evolved in a natural, non-catastrophic way

• Eg. Birth of a star


• First conceived in 1745 by French naturalist Georges Buffon (1707-1788)

• Proposed that material ripped off from the Sun by collision with a comet had condensed into the planets

• Giant eruptions were pulled off the Sun by the gravitational attraction of a passing star.

• Harold Jeffreys (b. 1891) and James Jeans (1877-1946), theorized that a cigar-shaped gaseous filament was pulled from the Sun by the sideswiping action of a passing star. The middle section condensed into the Jovian planets, and the ends condensed into the smaller planets.

Encounter Theories


• Solar System formed out of a huge rotating gaseous nebula slowly contracting and condensing

• 2 origins of nebula – nova / supernova• Nebula origin still under debate

• As the large, slowly rotating solar nebula of hot gaseous matter contracted, it rotated faster and faster, flattening into an equatorial ring

• Gravity shrinks the solar nebula• Given angular momentum must remain constant unless energy is taken out of rotation and put into some other form•Therefore rotational velocity increases as radius decreases

Nebular Hypothesis

Origin of Solar SystemNebular Hypothesis


• When centrifugal force on the outer rotating edge of the solar nebula exceeds inward gravitational force of the nebular mass, a ring of gaseous matter will split off, eventually coalescing into a planet

• Splitting process repeats itself, making concentric rings that formed into planets

• Central portion condensed to become the Sun

Nebular Hypothesis

Origin of Solar SystemNebular Hypothesis


• While 99 percent of the Solar System mass resides in the Sun, 99 percent of the angular momentum of the system resides in the planets' orbital and rotational motions

• A hot gaseous ring of the type postulated would disperse into space and not pull itself together gravitationally to form a planet.

Nebular Hypothesis - Inconsistencies

Origin of Solar SystemProtoplanet Theory, the Solar Nebula


• Improvement of nebular hypothesis

• Starts with a nebula fragmented from interstellar cloud

• Composed mainly of hydrogen and helium• Trace amounts of other elements

• The central region, being more dense, collapsed more rapidly than its outlying parts.

• Known as: Solar nebula

• The original interstellar cloud must have been rotating, and as it fragmented, rotation was imparted to each fragment

Protoplanet Theory, the Solar Nebula


• As the solar nebula contracted, it rotated more rapidly, conserving angular momentum

• The solar nebula grew by accretion as material continued to fall inward from its surroundings

• Large-scale turbulence from gravitational instabilities breaks the thin disk into rings, each containing many small particles. These particles gradually built up into larger bodies by some combination of adhesive forces



• As the central portion of the solar nebula contracted, the temperature rose to around 2000 K

• Vaporises all compounds in the dust except metallic and silicate minerals in the inner portion of the disk

• Planets that formed close to the young Sun, such as the Terrestrial planets, would be expected to contain less of the volatile icy and gaseous materials and thus be richer in the rocky materials


Formation of the planetsNearer to the Sun, temperatures are very high, so that they allow only rock and other minerals can condense. Thus, the formation of rocky planets: Mercury, Venus, Earth, Mars.

The asteroid belt originally was theorized to be a planet, which was hit by a large comet / other large bodies, and broke into the million pieces orbiting the Sun.

From about Jupiter outward, ammonia and methane will be formed due to very cold temperatures, giving those distant planets a mixed composition of water, ammonia, and methane ice impregnated with a small amount of rocky matter.

There is much debate about whether Pluto is a planet, even. As its composition is mostly ice, it could even be a comet captured by the Sun’s gravity.

Formation of the planets

Planets and their orbit

Planets and their orbit• All orbits have at least a small tilt in their orbit path

• Exception: Mercury, Pluto• All orbits are only nearly circular

• Exception: Mercury and Pluto have highly elliptic orbits

• All planets rotate west-east, same as the Sun’s rotation• Exception: Venus, Uranus, Pluto

• All planets revolve around the Sun west-east, same as the Sun’s rotation• Terrestrial planets have high mean densities and relatively thin or no atmospheres, rotate slowly, and possess few or no satellites•The giant planets have low mean densities, relatively thick atmospheres, and many satellites, and they rotate rapidly

Planets and their orbitTitus-Bode’s Law

• Suppose you write out the multiples of 3:•0, 3, 6, 9, 12, 15, 18

• Add 4 to each number:•4, 7, 10, 13, 16, 19, 22

• Divide each number by 10:•0.4, 0.7, 1, 1.3, 1.6, 1.9, 2.2

• This series of numbers give the distance (AU) of each planet from the Sun!• Likely due to the way the Solar System evolved, the numbers give a very close estimate to the distances of the planets and the Sun

Planets and their orbitTitus-Bode’s Law: Inconsistencies

• The inconsistency of Titus-Bode’s law occurs from Saturn, mostly after Uranus• Probably due to the fact that the outer planets were formed from rings broken away from the solar nebula• Another theory for the inconsistency: the warping of space due to the masses of the planets (Theory of Relativity)

Extrasolar Planets

• In just the last few years, astronomers have discovered large, cool, dust envelopes around infrared stars in the interstellar clouds of the Milky Way. They may be in the early stages of nebular condensation of the protoplanet theory

• Very recent findings: Vega (in Lyrae), Fomalhaut (in Piscis Austrinus), Epsilon Eridani (in Eridanus), and Beta Pictoris (in Pictor) and about eight other Sunlike stars

• Ground-based discoveries of circumstellar disks around three other stars• The disk surrounding Vega has a radius of some 80 to 90 AU (about twice Pluto's distance from the Sun)

Extrasolar Planets

• Although such observation is neither the same as finding evidence for planetlike bodies, nor that planets will eventually form around these stars, it does lend support to the contention that stellar nebulae, similar to the solar nebula in the protoplanet hypothesis, form around other stars in our Galaxy as a natural part of the processes responsible for the birth of stars

• Hence other ‘Solar Systems’ probably exist somewhere out in the distant reaches of space.

formation of solar system introduction origin of solar system formation of the planets planets and...

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