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Lecture 12

AS 101 Lecture 12: Module 3 Notes


Department
Astronomy
Course Code
AS101
Professor
All
Lecture
12

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Module 3 Notes
Comparave Planetology – seeking to understand the similaries and the dierences between and
among the planets
Solar Nebular Theory – main theory of formaon of our solar system
- imagines that some cataclysmic event iniated the collapse of a nebula that caused material falling
inward to some centre converng gravitaonal potenal energy into kinec energy making the centre, or
core, hoer and hoer
Terrestrial Planets – four inner planets - Mercury, Venus, Earth and Mars
- small, dense, rocky worlds with lile or no atmosphere
Jovian Planets – four outer planets – Jupiter, Saturn, Uranus, and Neptune
- large, low-density worlds with thick atmospheres and liquid or ice interiors
Planetary Characteriscs:
all planets orbit the Sun in the same direcon – counter clockwise (ccw)(as viewed from above
North Pole)
all orbits lie in nearly the same plane
almost all planets have nearly circular orbits (Mercury is a minor excepon)
most planets rotate ccw (Venus and Uranus are excepons) including the Sun
most moons orbit their planet in same direcon as the planet's rotaon and orbit in their
planet's equatorial plane
Mercury and Venus have no moons; Earth has one and Mars has two very small asteroid-like moons. The
Jovian planets, by contrast, have many. Jupiter is listed as having 6 but it actually has over 60; Saturn has
almost as many and Uranus and Neptune have 40 between them.
All Jovian planets have ring systems. Saturn’s rings are made of ice parcles. The rings of Jupiter, Uranus
and Neptune are made of dark rocky parcles. Terrestrial planets have no rings.
Asteroids lie primarily between Mars and Jupiter and a fairly broad belt in the same plane as planetary
orbits.
Comets follow either ellipcal orbits or parabolic/hyperbolic orbits passing close to the Sun once. They
are made largely of ices mixed with rocky dust, no bigger than a few km across. They come from two
major sources – the Kuiper belt (a doughnut shaped region starng around Neptune and extending out
into space) and the Oort Cloud (a spherical region completely surrounding the solar system and
extending out some 50,000 AU). As a comet gets close to the Sun it generates a coma (an atmosphere of
escaping gases and dust) around its nucleus and two tails: a plasma tail of ionized gas swept away by the
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