Formation of the Solar System

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• Formation of the Solar System

• Uncovering the origin of the Solar system
• Early days of the formation
• Building the planets and other stuff
• Other planetary systems

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Comparative Planetology

• Studying planets as worlds and compare them with
  each other is called comparative planetology

• Planetology is applied to any noticeably large
  object in the system (planets, moons, asteroids,
  comets)

To start we need to seek clues to the origin of
the Solar system
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Four Challenges

• Pattern of Motion
• All planets orbit the Sun in the same direction
  (counterclockwise as seen from the Earths North
  Pole)
• Planet orbits are nearly circular and co-planar
• Planets rotate in the same direction which they
  orbit
• Almost all moons orbit their planets in the
  direction of the planet rotation
• The Sun rotates in the direction planets orbit it

Explain Why is this order so good?
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Four Challenges

• Different types of planets
• Two distinct groups of planets
• Terrestrial planets (Mercury, Venus, Earth, Mars)
• Small, rocky, abundant in metals, few moons

Jovian planets (Jupiter, Saturn, Uranus,
Neptune) Large, gaseous (made of hydrogen and its
compounds), no solid surfaces, have rings, a lot
of moons (made of low-density ices and rocks)
Explain Why is the inner and outer Solar system
divided so neatly?
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Four Challenges
3. Asteroids and Comets
Asteroids are small, rocky bodies that orbit the
Sun mostly between Mars and Jupiter (the asteroid
belt) Almost 9,000 asteroids have been discovered
Comets are small and icy bodies that spend most
of their lives beyond the orbit of Pluto They
occupy 2 regions Kuiper belt and Oort cloud
Explain The existence and general properties of
the large number of these small bodies
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Four Challenges
4. Exception to the Rules
Mercury and Pluto have larger orbital
eccentricities Uranus and Pluto have tilted
rotational axes Venus rotates backwards
(clockwise) Earth has a large moon Pluto has a
moon almost as big as itself
Allow for these exceptions
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The Nebular Theory
The Solar system was formed from a giant,
swirling interstellar cloud of gas and dust
The hypothesis was originally suggested by
Immanuel Kant (1755) and Pierre-Simon Laplas
(1790)
A cloud is called nebula - nebular hypothesis
The collapsed piece of cloud that formed our own
solar system is called the solar nebula
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Collapse of the Solar Nebula

• Three important processes gave form to our
  system, when it collapsed to a diameter of 200
  A.U.
• The temperature increased as it collapsed
• The rotation rate increased
• The nebula flattened into a disk (protoplanetary
  disk)

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Evolution of the Solar System
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Building the Planets
Initial composition 98 hydrogen and helium, and
2 heavier elements (carbon, nitrogen, oxygen,
silicon, iron)
Condensation the formation of solid or liquid
particles from a cloud of gas Different kinds of
planets and satellites were formed out of
different condensates
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Ingredients of the Solar System
Metals iron, nickel, aluminum, etc. Condense
into solid form at 1000 1600 K 0.2 of the
solar nebulas mass
Rocks primarily silicon-based minerals Condense
at 500 1300 K, 0.4 of the mass
Hydrogen compounds methane (CH4), ammonia
(HN3), water (H2O) Condense into ices below 150
K, 1.4 of the mass
Light gases hydrogen and helium Never condense
in solar nebula 98 of the mass
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Condensation
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Accretion
Accretion is growing by colliding and
sticking The growing objects formed by accretion
planetesimals (pieces of planets)
Small planetesimals came in a variety of shapes,
reflected in many small asteroids Large
planetesimals (gt100 km across) became spherical
due to the force of gravity
Inner solar system only rocks and metals
condensed and only small bodies formed
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Nebular Capture
Nebular capture growth of icy planetesimals by
capturing larger amounts of hydrogen and
helium It led to the formation of the Jovian
planets
Numerous moons were formed by the same processes
that formed the protoplanetary disk Condensation
and accretion created mini solar systems around
each Jovian planet
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The Solar Wind
Solar wind is a flow of charged particles ejected
by the Sun in all directions It was stronger when
the Sun was young
The wind swept out a lot of remaining gas and
interrupted the cooling of the nebula
If the wind were weak, the ices could have
condensed in the inner solar system
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Leftover Planetesimals
Planetesimals remained from the clearing became
comets and asteroids They were tugged by the
strong gravity of the jovian planets and got more
elliptical orbits Rocky leftovers became
asteroids Icy leftovers became comets
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Planetary Evolution - Geological
Internal heating leads to geological activity
volcanism, tectonics
As core cools and solidifies, activity slows, and
eventually stops (Moon)
Earth and Venus are large enough to be active
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Planet Activity
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Planetary Evolution - Atmosphere

• Atmospheres are formed by
• gases escaping from interior
• - impacts of comets (volatile-rich debris)

Fate of water depends on temperature (distance
from the Sun)
Atmospheres changed chemically over time
Life on Earth substantially changed the atmosphere
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Other Planetary Systems
Over 100 extrasolar planets have been discovered
since 1995 The Extrasolar Planet Encyclopedia
Stars are too far away from the Sun, and direct
imaging cannot detect planets near them Current
strategy involves watching for the small
gravitational tag the planet exerts on its
star The tag can be detected using the Doppler
effect
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Extrasolar Planets in the Sky
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Planet Transits
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The Nature of Extrasolar Planets
The discovery of extrasolar planets gives us an
opportunity to test the solar system formation
theory
Most of the discovered planets are different from
those of our system They are mostly Jupiter-size
and located closer to their stars But possible
planet migration discovered planets are
exceptions
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Summary
All the planets were formed from the same cloud
of dust and gas Chance events may have played a
large role in the formation and evolution of
individual planets Planet-forming processes are
apparently universal