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Saturn from Cassini Astronomy 161:
An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 2:30

Lecture 31:
The Family of the Sun

Key Ideas:

The Solar System contains:

The planets all lie in nearly the same plane and orbit in the same general direction.

The Golden Age of Planetary Exploration

The Solar System has been explored using robotic spacecraft & astronauts:

Much of what we know about the Solar System has been learned in the last 35 years of planetary exploration.

The next few years will see a number of new missions to visit Mars, Saturn, Comets and Asteroids.

The Family of the Sun

The Sun is a middle-aged, average sized star surrounded by a system of orbiting objects:

The Terrestrial Planets:

The Jovian Planets:

Dwarf Planets:

Small Solar System Bodies:

Solar System Mass vs. Semi-Major Axis Plot
Contents of the Solar System by Mass
Plot of the Mass (in Earth Masses) vs. orbital Semi-Major Axis (in AU) for all of the major constituents of the Solar System other than the Sun. Click on the image to view full size.

The Eight Planets, in order from the Sun:

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, & Neptune

First 6 were known from antiquity, and are all visible to the naked eye as viewed from the Earth.

The last two were discovered using telescopes:

Discovered by William Herschel (UK) in 1781.

Predicted mathematically using orbital deviations of Uranus and Newtonian Gravity by Urbain LeVerrier (France) and John Couch Adams (UK)
Found at predicted location by Johann Galle (Germany) in 1846.

Basic Properties of the Planets


All orbit in the same direction & plane:

These facts provide us with important clues to the formation of the Solar System.

The Sun

The Sun is a middle-aged, average-sized star.

The Sun shines because it is hot:

Kept hot by nuclear fusion in its core

Terrestrial Planets

Mercury, Venus, Earth & Mars

"Earth-Like" Rocky Planets:

Rocky Planets:

The Jovian Planets

Jupiter, Saturn, Uranus & Neptune

Gas Giants (Jupiter & Saturn):

Ice Giants (Uranus & Neptune):

Dwarf Planets

New class of objects defined by the IAU in 2006. They have the following properties:
  1. They must orbit the Sun, and not be satellites of another, larger body

  2. They are shaped by self-gravity: which means that they have sufficiently large masses that their self-gravity overcomes internal rigid-body forces and shapes them into spheroids in "hydrostatic equilibrium". Usually means it is larger than about 800km in diameter, but it depends on the material it is made of (rock, ice, or a mix).

  3. Their gravity is insufficient to have cleared the neighborhood around their orbit.
The third property is what distinguishes dwarf planets from the other, larger 8 planets in our Solar System. It is a statement about gravitational dominance in their immediate vicinity - dwarf planets are too small to have altered their immediate surroundings, unlike planets.

There are currently four (4) recognized Dwarf Planets as of July 2008:

There are currently another 40 or so candidate Dwarf Planets, most in the outer solar system, that are subjects of ongoing study to determine if they satisfy the "shaped by gravity" criterion to become offical dwarf planets.
In June 2008, the IAU formally defined the term plutoid to designate the subclass of trans-Neptunian dwarf planets. At this writing (July 2008) there are 3 "plutoids" so-defied: Pluto, Eris, and Makemake.

The Giant Moons

A Moon is any natural satellite orbiting around another, larger object (planets or dwarf planets).

The largest of these are the "Giant Moons":

Many smaller moons, both rocky & icy, are found throughout the solar system orbiting all planets except for Mercury & Venus. Only these two planets have no moons.

Trans-Neptunian Objects

Dwarf Planets Pluto and Eris are the largest of a class of icy bodies found orbiting beyond Neptune.


The Leftovers: Small Solar System Bodies

The remaining inhabitants of the Solar System are a huge number of small objects:




All of these are the leftover material from the original formation of the Solar System.

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Updated: 2008 July 22
Copyright Richard W. Pogge, All Rights Reserved.