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Chapter 7

Astronomy & Astrophysics
Course Code
Clifford Orwin

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Chapter 7: Our Planetary System
7.1 Studying the Solar System
Comparative Planetology: The study of the solar system by examining and
understanding the similarities and differences among worlds.
The essence of comparative Planetology lies in idea that we can learn more about
individual world, including Earth, by studying it in context of other objects in solar
Four Major Clues As To How Solar System Formed: (1) Large bodies in the solar
system have orderly motions (2) Planets fall into two major categories [1] small, rocky,
terrestrial planets [2] large, hydrogen-rich jovian planets i.e. Uranus, Saturn (3)
Swarms of asteroids and comets populate the solar system in the asteroid belt [btw. Mars
and Jupiter], the Kuiper Belt [beyond Neptune’s orbit], and Oort Cloud [spherical
region in distant solar system encompassing Kuiper and Asteroid Belts.] (4) Several
notable exceptions to the above stated trends stand out Ex. Uranus’ unusual axis tilts.
The Sun
Radius: 696,000 km = 108REarth
Mass: 333,000MEarth
Composition (by mass): 98% hydrogen and helium, 2% other elements.
More than a thousand times as massive as everything else in solar system combined.
Roiling sea of hot (apprx. 5800K, or 5500 degrees Celsius) hydrogen and helium gas.
Surface is speckled with sunspots that are slightly cooler than their surroundings.
Fusion transforms 600 million tons of Sun’s hydrogen into 596 million tons o’ He. 
Missing” 4 million is converted into energy according to E = mc2.
Gravity governs orbits of other planets.
Heat is primary influence on temps o’ planetary surfaces and atmosphere.

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Charged particles flowing outward from Sun (Solar wind) help shape planetary magnetic
field and can influence planetary atmospheres.
Average distance from the sun: 0.39 AU
Radius: 2440 km = 0.38REarth
Mass: 0.055MEarth
Composition: Rocks, metals
Average Surface Temperatures: 700 K (day), 100 K (night)
Moons: 0
Smallest of eight official planets
Desolate, cratered world with no active volcanoes, no wind, no rain, and no life.
Rotates once every 58.6 days Rotates exactly three times for every two of its 87.9 day
orbits of the Sun.
Shows evidence of past geological activity Plains created by ancient lava flows and
tall, steep cliffs that run hundreds of km in length. These cliffs, though, may be
wrinkles from an episode of “planetary shrinking.
Mercury’s high density indicates that it has an iron core.
Av dist from sun: 0.72 AU
Radius: 6051 km = 0.95REarth
Mass: 0.82MEarth
Composition: Rocks, metals
Average Surface Temp: 740 K
Moons: 0

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Rotates very slowly and opposite direction of Earth --? Days and nights very long and
Sun rises in west and sets in east instead of rising in east and setting in west.
Surface completely hidden from view by dense CO2 clouds.
Extreme greenhouse effect bakes Venus’s surface to 470 degrees Celsius trapping heat so
effectively that it offers no relief.
Thick atmosphere bears down on surface with pressure equivalent to that nearly a km
beneath ocean’s surface on Earth.
Venus has mountains, valleys, and craters, and shows many signs of past of present
volcanic activity.
Average distance from sun: 1.00 AU
Radius: 6378 km = 1REarth
Mass: 1.00MEarth
Average Density: 5.52 g/cm3
Composition: rocks, metals
Average surface temperature: 290K
Moons: 1
Only planet in solar system with oxygen to breath, ozone to shield surface from deadly
solar radiation, abundant surface water to nurture life.
Temperature pleasant b/c Earth’s atmosphere contains just enough carbon dioxide and
water vapour to maintain moderate greenhouse effect.
Oceans cover 3/4ths of it.
White clouds scattered above surface
Polar caps white with snow and ice.
Moon is largest relative to size of planet it orbits Hypothesis hold moon’s creation b/c
giant impact early in Earth’s history.
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