SOLAR SYSTEM

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Solar System Formation Questions

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Q1.    What is chemical differentiation and what effect has it had on the planets?  Answer

Q2.    Why did the terrestrial planets chemically differentiate after they formed? What are the consequences of this event?  Answer

Q3.    Why does a collapsing cloud of gas become flattened? Be sure to describe completely the reasons why this happens.   Answer

Q4.    Why are planets that formed close to the sun made of rocks and iron while those that formed far from the sun mostly gaseous?  Answer

Q5.    What role does rotation play in explaining the properties of planets that form in a solar system?  Answer

Q6.    What role does temperature play in explaining why planets close to the sun are small and dense?  Answer

Q7.    In general, how do planets form from the material that surrounds a forming star?  Answer

Q8.    The giant planets undergo a second process that allows them to become even bigger. Describe this process and explain how it affects the nature of the planet.  Answer

Q9.    Why do planets melt soon after they form? What affect does this have on the planet?  Answer

Q10.    What is the role of temperature in planet formation? How did the temperature depend upon distance from the forming sun during planet formation?  Answer

Q11.    What role did gas play in the formation of giant planets?  Answer

Q12.    Why are all the planets differentiated? How did differentiation occur during the formation of the solar system; that is, what caused it?  Answer

Q13.    What event stops planet formation?  Answer

Q14.    Why is the biggest planet in the middle of the solar system?  Answer

Q15.    Why are the gas giant planets so much larger than the terrestrial planets? Why are Uranus and Neptune not as large as Jupiter and Saturn?  Answer

Q16.    Why did the planets chemically differentiate soon after their formation? What is the consequence of this event for planets today?  Answer

Q17.    Explain why temperature is so crucial to the formation of solids from a gas. How does this effect explain the difference in composition between Mercury and Earth?  Answer

Q18.    Describe the processes which occur to create planetesimals (small planets) in the disk surrounding a forming star.  Answer

Q19.    Why is Jupiter the biggest of the planets (Hint: There are two reasons)?  Answer

Q20.    Which is the first common material to form solid grains in a cooling cloud of gas? Why is the formation of ice so much more important?  Answer

Q21.    How do dust grains grow to form small planets?  Answer

Q22.    In what two ways does the existence of planets like Earth depend upon the occurrence of supernovae explosions?   Answer

Solar System Formation Answers

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A1.     Chemical differentiation occurs in a liquid or gas when heavy material sinks and less dense material rises. In the early history of the terrestrial planets, when they were entirely molten, this allow the heavy, dense materials (such as iron and nickel) to sink to the center to form dense cores while lighter materials rose to the surface to form a less dense mantle.

A2.    Once the terrestrial planets had formed, they quickly melted as result of large scale impacts and radioactive heating. When the entire planet was molten, heavier material sank toward the center and lighter material rose to the surface. As a result, most of Earth's iron is in the core and our surface is composed predominantly of silicate rocks.

A3.    If the cloud is rotating, the centrifugal force of rotation will impede the collapse of material near the equator more than it will for material near the axis of rotation. Centrifugal force is the force "created" because matter "wants" to move in a straight line instead of rotating. Since it always points away from the axis of rotation, it is exactly opposite gravity for matter near the equator, but in a different direction from gravity (and weaker also) for matter near the axis of rotation. Thus matter near the axis will collapse faster and the cloud will become flattened at the poles.

A4.     Planets form from tiny solid particles that collide and stick together. Close to the sun, the temperature was high and only a few materials like iron and rock could remain solid. Further from the sun, the temperature was much lower and various ices could also condense to participate in planet formation. Hence, the planets close to the sun are made of dense rock and iron while those further away contain large quantities of ices, most of which have melted and turned to gases.

A5.    As a collapsing cloud gets smaller, it will begin to spin faster and faster. The increasing rate of spin will cause the outward force of rotation to become stronger. Because this force is strongest at the equator (where the material spins fastest), the outward force there will work against gravity and slow down the collapse. Near the axis of rotation, the outward force will be less effective in slowing the collapse. These regions will collapse faster, and the cloud will become flattened. Eventually, material will collect in a thin disk surrounding the central concentration of matter. The central concentration becomes the star while planets form in the disk. This explains why the orbits of the planets all lie in about the same plane and move in the same direction.

A6.     Planets form as small solid particles stick together. Temperature determines what kind of material is solid. Close to the sun, where the temperature is high, only a few materials could be solid – mostly silicate rocks and iron. Further from the sun, where the temperature is much lower, water could also freeze. Thus, the inner planets are made of dense and relatively scarce material while the outer planets are both larger and less dense.

A7.    As described above, planets form as small solid dust particles gently bump into each other and stick together. Repeated collisions cause these particles to gradually become larger and larger, building up the planets once piece at a time.

A8.     Because the outer planets form from the very common ices, they become larger than the inner planets. Because they are bigger, they also have stronger gravities. This strong force of gravity is enough to pull in individual atoms from the surrounding gas in the disk. This makes them even bigger, and gives them an overall gaseous composition.

A9.    New planets become quite hot as a result of both the numerous collisions with other objects they experience and the relatively high level of radioactivity they possess. After a planet has melted, its heavier material will sink to the center while lighter material will float to the outside.

A10.     Temperature determines which materials can form solid particles in a gas. Since only solid particles participate in planet formation (see next question), the value of the temperature at a given location determines which material, and how much material, is available for planet formation at that location. Close to the forming sun, the temperature was very high and only a few materials could form solids. The temperature rapidly dropped with distance from the sun until at the distance of Jupiter the hydrides such as water could freeze.

A11.    The giant planets had much more raw material to work with because the hydrides are much more plentiful than are the silicates and metals. They were able to grow large enough for their gravity to be strong enough to begin to attract and hold individual atoms of gas from the surrounding cloud. This allowed them to become much larger, since there is so much more gas present than solids. This process was much more effective for Jupiter and Saturn, where the overall density of material was higher, and less important for Uranus and Neptune, which are further from the sun.

A12.    As the planets formed, a great deal of heat built up inside the planets. This heat came both from the violent collisions which were forming the planets and from the radioactive material which was incorporated into the planets. This internal heat was enough to melt the planet. In the liquid interior of the planet, heavy material (metals) sank to the center while lighter material (rocks) floated to the surface. This resulted in a differentiated planet.

A13.     When the sun has completed its formation, a wind of matter is blown away from the sun. This wind of matter strips away any remaining gas and dust from which planets might form. Without additional raw material to work with, planet formation is stopped.

A14.    The largest planet in the solar system had to form far enough from the sun for ices to be in solid form so that its solid core could grow to large size and close enough to the sun for the density of gas to be high so that a lot of gas could be attracted by the gravity of the large core.

A15.    The temperature in the gas cloud which surrounded the sun as it formed became progressively cooler at greater distances from the sun. Beyond the so-called frost line water could be solid, thereby dramatically increasing the amount of material in solid form. The gas giant planets formed in this region, while the terrestrial planets formed closer to the sun where only the rarer rocky materials could participate in the formation of the planets. Still further from the sun the overall density was much less, so the growth of the planets was slower. By the time Uranus and Neptune became big enough for their gravity to attract gas atoms, very little time was left before the forming sun blew the remaining gas away. Hence, they did not grow as large as Jupiter or Saturn.

A16.     After their formation, the planets were hot enough to completely melt. During this molten phase, the heavier materials sank to the center while the lighter, less dense materials floated to the surface. As a result the planets today have dense cores, usually rich in iron, and lighter mantles.

A17.    Any material changes from a liquid to a solid at some exactly specified temperature. For example, water freezes at exactly 32 ^oF. Planet formation begins as small solid particles stick together and gradually become larger and larger. Since Mercury formed closer to the sun than did Earth, where the temperature was higher, only iron and few of the rocky materials were solid there. At Earth’s distance from the sun, many more rocky compounds had solidified.

A18.    Planet formation begins when microscopic particles condense from the gas as the temperature cools. These small particles collide and stick together, gradually growing through a process known as accretion. Still later, still larger objects collide and aggregate as the planets begin to grow.

A19.     Jupiter was able to grow larger because it is the first planet (in distance from the sun) to form where water ice could condense into small particles. since there is much more ice than the metallic and rocky materials in the solar cloud, that provided much more material to make Jupiter than the inner planets had available. Even further from the sun, the overall density of the cloud was smaller, and thus the planets are smaller also.

Once the protoplanet Jupiter became large enough, its gravity began to attract gas atoms to the planet, making it much bigger still. This second process of growth resulted in the final giant planet we see today. Again, the drop in overall cloud density prevented more distant planets from growing equally large.

A20.     Iron is the common material with the highest freezing temperature. It condenses first in a cooling cloud. Because oxygen is a thousand times more abundant than iron or any other heavy material, when it reaches its freezing temperature, a great deal more material can freeze than for any other substance.

A21.     Small planets form as tiny microscopic dust grains stick together when they gently bump into each other as they orbit the still forming sun. These particles gradually grow through more and more collisions. This process continues on an ever increasing scale until a small planet has formed.

A22.    The incredible temperatures that occur during a supernova explosion allow nuclear reactions that consume energy to occur. These nuclear reactions produce heavy elements that would never be produced in normal stars. The explosion distributes these products into space, where they are much later incorporated into new generations of stars and planets.

The rapidly expanding cloud of debris from the explosion can also stimulate star and planet formation if it passes through a nearby giant molecular cloud. The interaction causes pieces of the giant molecular cloud to be compressed, which initiates the collapse that leads to the formation of a star, along with its attendant planets.