Questions for Solar Nebula Theory Unit (Planet Building Part 1)
1. Why do astronomers think the solar system formed about 4.6 billion years ago?
Scientists have utilized radioactive dating techniques in order to determine the age of rocks from
various sources – including Earth, the moon, and meteorites - including a number of meteorites from
Mars. They are able to use the half-life of radioactive elements, such as Potassium 40, Uranium 238,
and other elements, in discerning the age of rocks.
Half-life is the time it takes for half of a radioactive substance, called the parent isotope, to decay
into daughter isotope atoms. The dating comes from the fact that over time the abundance of the
parent isotope decreases and it is replaced by the daughter isotope.
According to the SNT, the planets and other bodies of the solar system should be the same age as the
sun. This appears to be the case as the rocks that have been tested – along with an estimate of the
sun’s age – put the age of the solar system at about 4.6 billion years.
2. What is the sun’s elemental composition as reveled by spectrum analysis?
The sun’s elemental composition is mostly hydrogen, about 25% helium, and 2% heavier elements.
3. What can the composition of the sun, as well as other stars, tell us about the composition of the
solar nebula?
The chemical composition of the sun and other stars is indicative of what the chemical composition
of the original interstellar gas cloud would have been. The information from the sun and other stars
gives us a starting point when creating models of solar system formation.
4. What is the condensation sequence and how do we see it in our solar system (how has it affected
Earth)?
The condensation sequence is the sequence in which different materials condensed from the solar
nebula – it occurred based on temperature and thus distance from the sun. The condensation
sequence implies that different kinds of materials will condense in predictable ways – which is
exactly what we see. Temperature is what determined, and still determines, the condensation
sequence.
5. If low-density gases were allowed to escape from a body such as Jupiter (or the sun), what would
remain and how would it be like Earth?
Only the heavier elements would remain, which would make Jupiter (or the sun for that matter) a
terrestrial planet like Earth.
6. What is the solar system’s ice line and what is its significance?
The solar system’s ice line is between Mars and Jupiter. It is the line beyond which water vapor
freezes to form ice particles. Farther still, before reaching Jupiter, compounds such as methane,
ammonia, and other gases condense to form other kinds of ices.
We can think of the ice line as where the terrestrial planets end and the Jovian planets take over.
7. How was the solar nebula sorted?
The solar nebula was sorted by temperature, not by density (which is a common misconception). The
outer nebula was cold and the inner nebula was hot.