Solar System

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Solar System
Astronomers have always noticed planets, the
stars, and the moons. We will use the powerful
and still emerging perspective of comparative
planetology to understand better the conditions
under which planets form and evolve.
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Solar System
comparative planetology The systematic study of
the similarities and differences among the
planets, with the goal of obtaining deeper
insight into how the solar system formed and has
evolved in time.
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Solar System
solar system The Sun and all the bodies that
orbit itMercury, Venus, Earth, Mars, Jupiter,
Saturn, Uranus, Neptune, (and Pluto) their moons,
the asteroids, and the comets.
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Solar System
Comets appear as long, wispy strands of light in
the night sky that remain visible for periods of
up to several weeks, then slowly fade from view.
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Solar System
Meteors, or "shooting stars" are sudden bright
streaks of light that flash across the sky,
usually vanishing less than a second after they
first appear. Asteroids or "minor planets"
orbiting the Sun, mostly in a broad band (called
the asteroid belt) lying between Mars and
Jupiter.
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Meteor
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Solar System
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Solar System
Orbital Semi-Major Axis The major axis of an
ellipse Longest diameter, a line that runs
through the widest points of the shape. The
semi-major axis is one half of the major axis,
and thus runs from the centre, through a focus,
and to the edge of the ellipse. The distance of
each planet from the Sun is known from Kepler's
laws once the scale of the solar system is set by
radar-ranging on Venus.
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Solar System
Orbit Period Length of time the object takes to
orbit the sun. A planet's (sidereal) orbital
period is easily measurable from repeated
observations of its location on the sky, so long
as Earth's own motion around the Sun is properly
taken into account.
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Solar System
Planet Mass The masses of planets with moons may
be calculated by application of Newton's laws of
motion and gravity, just by observing the moons'
orbits around the planets. The masses of Mercury
and Venus (as well as those of our Moon) are a
harder to determine because these bodies have no
natural satellites
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Solar System
Planet Mass We observe their influence on other
planets or nearby bodies. Mercury and Venus
produce small but measurable effects on each
other's orbits, as well as that of Earth. The
Moon causes small "wobbles" in Earth's motion as
the two bodies orbit their common center of mass.

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Solar System
Rotation Period Length of time for an object to
rotate completely around its axis. A planet's
rotation period is determined simply by watching
surface features appear and disappear again as
the planet rotates. For some planets this is
difficult to do, as their surfaces are hard to
see or may even be nonexistent
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Solar System
The planets paths are all ellipses, with the Sun
at (or very near) one focus. Most planetary
orbits have low eccentricities. The exceptions
are the innermost and the outermost worlds,
Mercury and Pluto. High eccentricities indicate
more oval and less circular shapes. Accordingly,
we can think of most planets' orbits as circles
centered on the Sun.
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Solar System
Maybe future space voyagers travel far enough
from Earth to gain this perspective on our solar
system Except for Mercury and Pluto, the orbits
of the planets lie nearly in the same plane. As
we move out from the Sun, the distance between
the orbits of the planets increases. The entire
solar system spans nearly 80 A.U.
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Solar System
AnAstronomicalRuler.MOV
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Solar System
The Titius-Bode law seemed to "predict" the radii
of the planetary orbits remarkably well. Even
the asteroid belt between Mars and Jupiter
appeared to have a place in the scheme, which
excited great interest among astronomers and
numerologists alike. There is apparently no
simple explanation for this empirical "law."
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Solar System
On large scales, the solar system presents us
with a sense of orderly motion. The planets move
nearly in a plane, on almost concentric and
nearly circular paths They move in the same
direction around the Sun, at steadily increasing
orbital intervals. However, the individual
properties of the planets themselves are much
less regular.
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Solar System

• A clear distinction can be drawn between the
  inner and the outer members of our planetary
  system based on densities and other physical
  properties.
• The inner planetsMercury, Venus, Earth, and
  Marsare small, dense, and rocky in composition.
• The outer worldsJupiter, Saturn, Uranus, and
  Neptune (but not Pluto)are large, of low
  density, and gaseous.

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Solar System
Diagram, drawn to scale, of the relative sizes of
the planets and our Sun. Notice how much larger
the joviian planets are than Earth and the other
terrestrials and how much larger still is the
Sun.
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Solar System

• The terrestrial worlds lie close together, near
  the Sun
• the jovian worlds are widely spaced through the
  outer solar system.
• The terrestrial worlds are small, dense, and
  rocky
• the jovian worlds are large and gaseous, being
  made up predominantly of hydrogen and helium (the
  lightest elements), which are rare on the inner
  planets.
• The terrestrial worlds have solid surfaces
• the jovian worlds have none (their dense
  atmospheres thicken with depth, eventually
  merging with their liquid interiors).

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Solar System

• The terrestrial worlds have weak magnetic fields,
  if any
• the jovian worlds all have strong magnetic
  fields.
• The terrestrial worlds have only three moons
  among them
• the jovian worlds have many moons each, no two of
  them alike and none of them like our own.
• Furthermore, all the jovian planets have rings, a
  feature unknown on the terrestrial planets.
• Despite their greater size, the jovian worlds all
  rotate much faster than any terrestrial planet.

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Solar System
TheTerrestrialPlanetsI.MOV
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Solar System
The Terrestrial Planets
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Solar System
TheGasGiantsI.MOV
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Solar System
Gas Giants
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Solar System

• Beyond the outermost jovian planet, Neptune, lies
  one more small world, frozen and mysterious.
• Pluto doesn't fit well into either planetary
  category.
• Indeed, there is debate among planetary
  scientists as to whether it should be classified
  as a planet at all. In both mass and composition,
  it has much more in common with the icy jovian
  moons than with any terrestrial or jovian planet.
  Astronomers speculate that it may in fact be the
  largest member of a newly recognized class of
  solar system objects that reside beyond the
  jovian worlds.