Download Chapter-6 Lecture Spring Semester

Chapter 6
The Solar System
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6.1 An Inventory of the Solar System
Early astronomers knew
Moon, stars, Mercury,
Venus, Mars, Jupiter,
Saturn, comets, and
meteors
Galileo’s discovery of the
phases of Venus and the
moons of Jupiter
changed the view of the
universe
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6.1 An Inventory of the Solar System
Now known: The solar system has 169 moons,
one star, eight planets (added Uranus (1781)
and Neptune (1846)), eight asteroids (Ceres –
1801), more than 100 Kuiper belt objects more
than 300 km in diameter, and many smaller
asteroids, comets, and meteoroids.
© 2014 Pearson Education, Inc.
6.1 An Inventory of the Solar System
More than 3,472 extrasolar planets have
been found
Understanding planetary formation in our
own solar system helps understand its
formation as well as formation of other
systems
Comparative planetology – comparing and
contrasting the properties of the diverse
worlds we encounter – to understand the
conditions under which planets form and
evolve
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6.2 Measuring the Planets
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6.2 Measuring the Planets
• Distance – Kepler’s Laws (once the scale is known from radar
ranging Venus)
• Orbital Period – repeated observations of its location on the sky
• Radius – measuring the angular size of planet, and then
applying geometry
• Masses (planets with moons) – Newton’s laws of motion and
gravity
• Masses (Mercury and Venus) – observations of gravitational
influence on other planets or nearby bodies (now we can use
artificial satellite and space probes)
• Rotation – by watching surface features alternately appear and
disappear
• Density – mass (observations of a moon’s orbit) divided by
volume (4/3 πR3) Earth = 5.5 g/cm3 Jupiter = 1.3 g/cm3
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6.3 The Overall Layout of the Solar System
All orbits but
Mercury’s (off 7o) are
close to the same
plane
Orbits of the planets
are almost circular,
except Mercury’s
The distance between
adjacent orbits
increases farther
from the Sun
Spans 100 AU
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6.4 Terrestrial and Jovian Planets
In this picture of the eight planets and the Sun,
the differences between the four terrestrial and
four jovian planets are clear.
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6.4 Terrestrial and Jovian Planets
Terrestrial planets:
Mercury, Venus, Earth, Mars (within 1.5 AU of the Sun)
Jovian planets:
Jupiter, Saturn, Uranus, Neptune
Terrestrial planets are small and rocky, close to the Sun,
rotate slowly, have weak magnetic fields, few moons,
more dense, solid surfaces, refractory elements (Fe, Ni,
Silicates), and no rings
Jovian planets are large and gaseous, volatile elements
(H, He, CH4, NH3), far from the Sun, rotate quickly, have
strong magnetic fields, large dense cores, many moons,
less dense, no solid surface, and rings
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6.4 Terrestrial and Jovian Planets
Differences among the terrestrial planets:
• All have atmospheres, but they are very
different; surface conditions vary as well
• Only Earth has oxygen in its atmosphere and
liquid water on its surface
• Earth and Mars spin at about the same rate;
Mercury is much slower, Venus is slow and
retrograde
• Only Earth and Mars have moons
• Only Earth and Mercury have magnetic fields
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6.5 Interplanetary Matter
Asteroids and meteoroids have rocky
composition; asteroids are bigger (larger than
100 m); chemistry of the early solar system
Asteroid Vesta is 500 km across and orbits
between Mars and Jupiter (Asteroid Belt)
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6.5 Interplanetary Matter
Comets are icy, with some rocky parts, diameters
between 1 to 10 km, similar to the icy moons in
the outer solar system
Comet McNaught
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6.5 Interplanetary Matter
Dwarf Planet - Pluto (1930), once classified as
one of the major planets, is the closest large
Kuiper belt object to the Sun
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6.6 How Did the Solar System Form?
Solar System Properties:
1. Each planet is relatively isolated in space.
2. The orbits of the planets are nearly circular.
3. The orbits of the planets all lie in nearly the same plane.
4. The direction in which the planets orbit the Sun (counterclockwise
as viewed from above Earth’s North Pole) is the same as the
direction in which the Sun rotates on its axis.
5. Our planetary system is highly differentiated.
6. The asteroids are very old and exhibit a range of properties not
characteristic of either the inner or the outer planets or their
moons.
7. The Kuiper belt is a collection of asteroid-sized icy bodies orbiting
beyond Neptune.
8. The Oort cloud comets are primitive, icy fragments that do not
orbit in the plane of the ecliptic and reside primarily at large
distances from the Sun.
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6.6 How Did the Solar System Form?
Nebular contraction (Nebular Theory):
Cloud of gas and dust contracts due to gravity;
conservation of angular momentum means it spins
faster and faster as it contracts (Solar Nebula)
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6.6 How Did the Solar System Form?
Nebular contraction is followed by
condensation around dust grains
(condensation nuclei), known to exist
in interstellar clouds such as the one
shown here.
Accretion then leads to larger and
larger clumps (planetesimals); finally
gravitational attraction takes over
and first protoplanets and then
planets form.
High-speed collisions led to some
fragmentation.
A glowing protosun forms at the
center (not a true star).
Called Condensation Theory
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6.7 Jovian Planets and Planetary Debris
Terrestrial (rocky) planets formed near Sun, due to high
temperature—nothing else could condense there.
Near the Sun, the temperature was several thousand K.
At 10 AU, where Saturn now resides, temperature was 100 K
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6.7 Jovian Planets and Planetary Debris
T Tauri stars are in a highly active phase of their
evolution and have strong solar winds. These winds
sweep away the gas disk, leaving the planetesimals
and gas giants. Occurs a few million years after the
formation of the nebula.
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6.7 Jovian Planets and Planetary Debris
Jovian planets:
• Once they were large
enough, may have
captured gas from the
contracting nebula (coreaccretion theory)
•The size of the core may
prove which is correct
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6.7 Jovian Planets and Planetary Debris
Jovian planets:
•Or may not have formed
from accretion at all, but
directly from instabilities
in the outer, cool regions
of the nebula (gravitational
instability theory)
•The size of the core may
prove which is correct
•Also possible: The Jovian
planets may have formed
farther from the Sun and
“migrated” inward.
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6.7 Jovian Planets and Planetary Debris
Asteroid belt:
• Orbits mostly between Mars and Jupiter
• Jupiter’s gravity kept them from condensing into a
planet, or accreting onto an existing one
• Fragments left over from the initial formation of
the solar system
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6.7 Jovian Planets and Planetary Debris
General timeline of solar system formation
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6.7 Jovian Planets and Planetary Debris
Icy planetesimals far from the Sun were ejected into
distant orbits by gravitational interaction with the jovian
planets, into the Kuiper belt and the Oort cloud.
Some were left with extremely eccentric orbits and
appear in the inner solar system as comets.
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