Our Solar SystemComparative Planetology

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

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.

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

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.

Mass and Density

• 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

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

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?

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 End!

Some things to consider…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?