1. Comparative Planetology
Our Solar System
Comparative Planetology

2. Terms Planet – a celestial body that is in orbit around the Sun
has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
has cleared the neighborhood around its orbit.
Asteroid – rocky object larger than a few hundred meters in diameter
Orbits the Sun
Not classified as a planet or moon
Meteoroid – a small rock in interplanetary space
Meteor and meteorite are not the same…
Comet – a small body of ice and dust that orbits the Sun
Meteor – light seen when space debris vaporizes in Earth’s atmosphere or “shooting star”
Meteorite – space debris that has survived passage through the Earth’s atmosphere

3. Comparative Planetology
Compare and contrast all features of similar objects one at a time
Orbits
Size
Mass
Density
Spectra
Albedo
Once we understand these features, we can look at each planet individually so we can connect all properties to each object.

4. Orbits
The path of an object that is moving around a second object; planets orbiting the Sun
Orbital eccentricity – a measure between 0 and 1 which indicates how close to a circular path a planet’s orbit is (circle = 0)
Venus eccentricity is ~0.0068
Mercury eccentricity is ~0.21
Orbital inclinations – angles of the orbital planes of the other planets with respect to the ecliptic
Uranus inclination ~0.77°
Mercury inclination ~7.01°
Orbital period – the time it takes for 1 object to completely orbit another

5. Size
The planets fall into 3 size groups: small, medium, and large…basically.
Terrestrial planets – 4 inner planets are the smallest; made of rock and metal
Gas giants – Jupiter and Saturn are the largest planets; mostly H and He
Outer planets – Uranus and Neptune are medium sized; mostly H and He but also much water.

6. Mass and Density The average density of liquid water is 1000kg/m³
Mass – the total amount of matter an object contains
Inner planets have smaller masses than outer planets
Average density – how much mass an object has in a unit of volume
Average density = total mass/total volume
The average density of liquid water is 1000kg/m³
Density of typical surface rock is 3000kg/m3…implication of iron and nickel core.

7. Spectra
Radiation that is primarily sunlight scattered off the surface or clouds that surround the object
Provides details about the object’s surface or atmospheric chemical composition
Giant planets are mostly H and He
Mars is rich in iron oxides
This shock wave plows through space at over 500,000 kilometers per hour. Moving right to left in the beautifully detailed color composite, the thin, braided filaments are actually long ripples in a sheet of glowing gas seen almost edge on. Cataloged as NGC 2736, its narrow appearance suggests its popular name, the Pencil Nebula. About 5 light-years long and a mere 800 light-years away, the Pencil Nebula is only a small part of the Vela supernova remnant. The Vela remnant itself is around 100 light-years in diameter, the expanding debris cloud of a star that was seen to explode about 11,000 years ago. Initially, the shock wave was moving at millions of kilometers per hour but has slowed considerably, sweeping up surrounding interstellar gas.

8. Albedo
The fraction of incoming light returning directly into space from a celestial body (0.0 - charcoal to 1.0 mirror); multiply by 100
The surfaces or upper cloud layers of the planets scatter different amounts of light
Mercury, Earth, and Mars have albedos of 37% or less because of a mixture of dark and light surfaces
Venus, Jupiter, Saturn, Uranus, and Neptune have albedos of 0.47 or greater because of bright materials and clouds
Earth’s average albedo is 30-35%. Why is Earth’s albedo continually changing?

9. Moons
Natural satellite; a celestial body that orbits another which in turn orbits a star
Mercury and Venus do not have moons
At least 166 known moons in our solar system
Most moons are potato shaped
Moons vary just as much as planets do
The strangest moon in the Solar System is bright yellow. This picture, an attempt to show how Io would appear in the "true colors" perceptible to the average human eye, was taken in 1999 July by the Galileo spacecraft that orbited Jupiter from 1995 to Io's colors derive from sulfur and molten silicate rock. The unusual surface of Io is kept very young by its system of active volcanoes. The intense tidal gravity of Jupiter stretches Io and damps wobbles caused by Jupiter's other Galilean moons. The resulting friction greatly heats Io's interior, causing molten rock to explode through the surface. Io's volcanoes are so active that they are effectively turning the whole moon inside out. Some of Io's volcanic lava is so hot it glows in the dark.

10. Some things to consider… The End!
What if our Moon was potato shaped?
What if our year was as long as Neptune’s year?
What if our planet was the size of Mercury?
What if nobody asked questions?
The End!