THE SOLAR SYSTEM – Chapter 22
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Geocentric model of the solar system is the system early scientists believed. This had the
Earth as the center of the solar system, with the Sun, moon, stars and planets revolving
around the Earth. This model included only 5 planets – Mercury, Venus, Mars, Jupiter and
Saturn.
Heliocentric model of the solar system is the system first proposed by Polish astronomer
Nicholas Copernicus in 1543. This model has the Sun as the center of the solar system.
Planets revolve around the Sun and only the moon revolves around the Earth. Italian
astronomer Galileo Gallilei confirmed this model with the use of a telescope he constructed a
few years later. He concluded that because Venus has phases like the moon, it must also
revolve around the Sun.
Because the Sun’s mass is 99.86 percent of the mass of the whole solar system, it has a
tremendous gravitational pull which causes all the other objects of the solar system to revolve
around it.
Scientists believe our solar system formed in this way:
o a cloud of gas, ice and dust about 4.6 billion years ago was in the universe
o a nearby star exploded and caused the cloud to start contracting
o as the cloud contracted the temperature began to increase and nuclear fusion began –
a star, the Sun, was born.
o the remaining gas, ice and dust collided and stuck together, forming larger objects
(planets)
o the temperature of the planets close to the sun was hot enough to vaporize elements
which could not condense. These inner planets are rocky and solid with iron cores.
o the temperature of the planets farther from the sun was not as hot, so these elements
could condense, so these planets are mostly gaseous.
Orbits of planets are elliptical and revolve around the Sun at different speeds (Kepler).
Inner Planets
o Mercury (closest to the Sun)
 closest planet to the Sun and the smallest
 cratered surface with large cliffs
 weak magnetic field, indicated a large iron core
 thin atmosphere and nearness to the Sun causes extreme temperatures (450°C
in day; ̶ 170°C at night)
o Venus (2nd from the Sun)
 called Earth’s twin because it is similar in size and mass
 dense atmosphere with 96 times the pressure of Earth at sea level
 atmosphere mostly CO2 with droplets of sulfuric acid
 dense clouds – only 2% of the sunlight reaches Venus’ surface
 trapped sunlight casues temperatures of 470°C
 huge craters, faultlike cracks, volcanoes with visible lava flows
o Earth (3rd from the Sun)
 150 million km (1 AU – astronomical unit) from the Sun
 only planet where water exists as a solid, liquid and a gas
 only planet with life
 atmosphere causes most meteors to burn up before reaching surface
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 atmosphere protects life from Sun’s radiation
 one moon
o Mars (4th from the Sun)
 iron oxide in rocks give surface a reddish color
 polar ice caps which get larger during Martian winter and shrink during summer
(made up of frozen CO2 and H2O)
 long channels may have been carved by flowing water
 large volcano called Olympus Mons is extinct
 two moons – Deimos and Phobos
Outer Planets
o Jupiter (5th from the Sun)
 largest planet
 faint dust rings around it
 planet mostly composed of hydrogen and helium with some ammonia, methane
and water vapor.
 scientists theorize that the hydrogen/helium atmosphere changes to a
planetwide ocean of liquid hydrogen and helium toward the middle of the planet
 below liquid layer there may be a solid core
 storms of high-pressure gas are apparent – Great Red Spot is the largest of
these
 lightning has also been observed
 16 moons – four largest are Io, Europa, Ganymede and Callisto
o Saturn (6th from the Sun)
 second largest planet
 lowest density (so low that the planet would float on water).
 several wide rings composed of ice and rock particles
 thick outer atmosphere of mostly hydrogen and helium with some ammonia,
methane and water vapor
 deeper into the atmosphere, the gases change to liquid hydrogen and helium
 may have a small rocky core beneath
 at least 20 moons (most of any planet) – largest is Titan, which is larger than
Mercury with an atmosphere so thick, surface is not visible
o Uranus (7th from the Sun)
 thin, dark rings
 magnetic field is tilted 55 degrees from its rotational axis (Earth tilted only 11.5
degrees)
 atmosphere is hydrogen, helium and methane
 methane gives it a blue-green color
 under its atmosphere, Uranus may have liquid water, methane and ammonia
surrounding a rocky core
 rotational axis is on its side when compared to other planets which may have
been caused by a collision.
 17 moons
o Neptune (8th from the Sun)
 similar atmosphere to Uranus
 methane gives it a blue-green color
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dark-colored stormlike features similar to Jupiter largest called Great Dark Spot
may have liquid water under its atmosphere
probably has a rocky core
magnetic field is tilted 47 degrees from its rotational axis (Earth tilted only 11.5
degrees)
 8 moons – largest is Triton
o Pluto (formerly a planet 1939 – 2009)
 rocky
 smaller than Mercury
 large captured comet now considered part of the Kuiper belt (disk of icy comets
near Neptune’s orbit)
 orbit crosses Neptune’s
 one moon – Charon – half the size of Pluto - sometimes called the double
planet
Comets
o composed of dust and rock particles mixed with frozen water, methane and ammonia
o Oort Cloud located beyond Pluto’s orbit is a large collection of comets that lie in a cloud
completely surrounding the solar system.
o comet is a large mass of frozen ice and rock
o as the comet approaches the sun, water, methane and ammonia vaporize from their
frozen states because of the sun’s heat and bits of rock and dust are released
o vaporized gases and released dust form a bright cloud called a coma around the
nucleus (solid part) of the comet
o solar wind pushes on the gases which forms a tail. The tail always points away from
the sun
Meteoroid
o small pieces of a comet’s nucleus moving through space
Meteor
o a meteoroid that burns up in Earth’s atmosphere (sometimes called shooting stars)
Meteorite
o a meteor that doesn’t completely burn up in the atmosphere and strikes Earth.
Asteroid
o a piece of rock similar to material that formed into planets
o asteroid belt located between the orbits of Mars and Jupiter
o largest asteroid is Ceres
o asteroids may have been in the process of becoming planets when the solar system
formed, but Jupiter’s massive gravity may have prevented this.