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.
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