## ACTIVITY 1: DISTANCE VS. SIZE

Concepts:

• graphing

• scientific notation

• ratios

• different ways of showing size

• analyzing and interpreting data

• composition of planets

• formation of solar system

Activities: Using the attached table of values, make three graphs:

• mass vs. orbital distance (distance from sun)

• radius of planet vs. orbital distance

• mean density vs. orbital distance

• extra: calculate mean density using mass and radius, convert to scientific notation, or convert to ratios: (mass of Jupiter / mass of Earth)

Sample plot:

Interpretation:
• What can you tell from looking at your graphs?

Q: What general trends can you identify?

A:

• mass of planet increases with distance from sun

• radius of planet increases with distance from sun

• density generally decreases with distance from sun

• The planets can be generally grouped into two sets: the terrestrial planets (Mercury, Venus, Earth, Mars) are small and high density, and the gas giants (Jupiter, Saturn, Uranus, Neptune) are large and low density.

Q: Which category does Pluto fit into?

A: Pluto is certainly more like the terrestrial planets than the gas giants, but it's most like the moons of the outer solar system.

• To see small differences better, graph just the terrestrial planets on one set of graphs, and just the gas giants on another set.

Q: Which planet is most like the Earth?

A: Venus is most like the Earth in size and density.

• Venus is often called Earth's sister planet, but there are also some major differences between the two (Venus is hot, thick atmosphere (activity 3)).

Q: Compare the densities of metal, rock, ice, and gas to the average planetary densities: what can you guess about the compositions of the planets?

A: The planets in the inner solar system are made of mostly metal and rock, and the outer planets are mostly rock, ice, and gases.

Q: Which planet would float in water?

A: Saturn's density is less than the density of water, so it would float (if you could find an ocean big enough!).

Scientific context: Scientists use graphs like the ones you just made to try to determine how the solar system formed. The terrestrial planets are all small and rocky, and the outer planets (except for Pluto) are large and gaseous. Why? Scientists think that the planets all formed out of leftover material that was in a huge disk of gas and dust around the sun in the early days of the solar system. As material got farther from the sun (see exercise 3), the temperature got cooler, and some of the components began to condense and rain out. Heavy materials like iron and other metals were the first to condense out, and scientists think that this is why the terrestrial planets have high densities and are made up mostly of rock and metal. It wasn't until out near the orbit of Jupiter that the temperature got cool enough to allow volatiles like water and other ices to condense out of the disk, and this could be why the outer planets are made up mostly of gases and ices. This theory is still being developed, and scientists still aren't sure of all the details. It seems, however, that formation of planets is a natural by-product of star formation. We can see billions of stars in the sky, so it seems likely that at least some must have planets around them. Since planets are so much smaller (and therefore dimmer) than stars, they're hard to detect, but recently we've had the first confirmed discoveries of planets around other stars than the sun. As our telescopes and detectors get better and better, we seem sure to detect even more. So maybe someday we'll be able to see how our theories of planetary formation apply to other solar systems!

Back to Introduction or Forward to Activity 2

This module was written by Cynthia Phillips, Dept. of Planetary Sciences, University of Arizona, Tucson AZ, and funded in part by the NASA Spacegrant program.

Galileo Solid State Imaging Team Leader: Dr. Michael J. S. Belton

The SSI Education and Public Outreach webpages were originally created and managed by Matthew Fishburn and Elizabeth Alvarez with significant assistance from Kelly Bender, Ross Beyer, Detrick Branston, Stephanie Lyons, Eileen Ryan, and Nalin Samarasinha.

Last updated: September 17, 1999, by Matthew Fishburn