An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 9:30
The Family of the Sun
The Solar System contains:
- The Sun
- Terrestrial Planets
- Jovian Planets
- Dwarf Planets
- Giant Moons
- Trans-Neptunian Objects
- Asteroids, Comets, & Meteoroids
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 &
- Landed on the Moon, Venus, Mars, Titan, & asteroid 433 Eros
- Returned rocks from the Moon (~382 kg)
- Probed the Atmospheres of Venus, Mars, & Jupiter.
- Flown spacecraft past all planets
- Mapped Venus and parts of Titan with Radar
- Flown by asteroids & comets, with one asteroid landing.
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:
- Rocky Planets: Mercury, Venus, Earth & Mars
The Jovian Planets:
- Gas Giants: Jupiter, Saturn, Uranus & Neptune
- Rocky & Icy Bodies: Pluto, Eris, & Ceres
Small Solar System Bodies:
- Rocky: Asteroids & Meteoroids
- Icy: Kuiper Belt Objects (KBOs) & Comets
|Contents of the Solar System by Mass|
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. Based on an original plot by
Scott Gaudi (OSU).
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
- Terrestrial Planets are found in the inner solar system: 0.4-1.5AU
- Jovian Planets are found in the outer solar system: 5-30 AU
All orbit in the same direction & plane:
- Orbit counterclockwise, in the same sense as the rotation
of the Sun.
- All orbit very near the Ecliptic plane.
These facts provide us with important clues to the formation of the
The Sun is a middle-aged, average-sized star.
- Mostly Hydrogen & Helium
- Contains 99.8% the mass of the Solar System
- About 4.6 Gyr old
The Sun shines because it is hot:
- Surface (photosphere) is ~6000 K
- Radiates mostly Visible light plus UV & IR
Kept hot by nuclear fusion in its core
- Builds Helium from Hydrogen nuclei, liberating nuclear binding
Mercury, Venus, Earth & Mars
"Earth-Like" Rocky Planets:
- Largest is the Earth
- Only found in the Inner Solar System (0.4 to 1.5 AU)
- Solid Surfaces
- Mostly silicates and iron
- High Density: 3.9-5.5 g/cc (rock & metal)
- Earth, Venus & Mars have atmospheres
The Jovian Planets
Jupiter, Saturn, Uranus & Neptune
- Largest Planets: at least 15 times mass of Earth.
- Jupiter is the largest (318 Earth Masses)
- Found only in the Outer Solar System (between 5 and 30 AU)
Gas Giants ("Jupiter-like"):
- No Solid Surfaces (mostly atmosphere)
- Mostly Hydrogen & Helium
- Rocky/icy inner cores
- Low density: 0.7 to 1.7 g/cc (water is 1 g/cc)
New class of objects defined by the IAU in 2006. They have the
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.
- They must orbit the Sun, and not be satellites of another, larger body
- 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).
- Their gravity is insufficient to have cleared the neighborhood
around their orbit.
There are currently three recognized Dwarf Planets:
There are currently about 10 or so candidate Dwarf Planets, subject to
future study to determine if they satisfy the "shaped by gravity"
- Ceres: first and largest of the Asteroids, discovered
in 1801 by Guiseppe Piazzi.
- Pluto: first large Trans-Neptunian Object, discovered
in 1930 by Clyde Tombaugh.
- Eris: largest known Trans-Neptunian Object, discovered
in 2005 by Mike Brown, Chad Trujillo, and David Rabinowitz.
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":
- Earth: The Moon
- Jupiter: Io, Europa, Ganymede, & Callisto (the
4 Galilean moons discovered by Galileo in 1610)
- Saturn: Titan
- Neptune: Triton
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.
Dwarf Planets Pluto and Eris are the largest of a class of icy bodies
found orbiting beyond Neptune.
- Found only in the outer Solar System beyond 30 AU
- Densities of 1.2 to 2 g/cc, indicating they are made
mostly of ices.
- Kuiper-Belt Objects (30-50 AU)
- Charon, Pluto's large moon
- Sedna & Quaor: distant, large icy bodies
The Leftovers: Small Solar System Bodies
The remaining inhabitants of the Solar System are a huge number of small
- Made of rock & metals (density 2-3 g/cc)
- Sizes: Range from few 100km down to the size of large boulders
- Largest are in the Dwarf Planet range (e.g., Ceres)
- Bits of rock and metal (iron)
- Sizes: grains of sand up to small boulders
- Composite rock & ice "dirty snowballs"
- Develop longs tails of gas & dust are swept off them
when they pass near the Sun.
All of these are the leftover material from the original formation of
the Solar System.
Readings in Universe:
Chapter 7, sections 7-1 thru 7-5
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Updated: 2006 November 5
Copyright © Richard W. Pogge,
All Rights Reserved.