Our Solar System

1. 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

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 • 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

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

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

10. “Never memorize something that you can look up.” Albert Einstein