AST 301 Exam #6 Study Guide 100% Solved
How far out does our Solar System extend to? - ANSWER Reaches about 1/3 of the way
to the nearest star
How far out does the Oort cloud extend from? - ANSWER It extends out from the Sun to
about 10,000 to 100,000 AU
What dominates the Solar System? - ANSWER The Sun
Why does the Sun dominate the Solar System? - ANSWER It is 9.7 times the diameter of
Jupiter. Its diameter is 3.6 times the distance from the Earth to the Moon. It is 1050 times
the mass of Jupiter. But it has only 0.3% of the angular momentum of the Solar System.
How to get rid of "excess rotation" when the Sun was grown out of a spinning debris is a
major puzzle.
What is there a complete continuity between? - ANSWER The sizes of planets and the
sizes of satellites. Smaller objects are more numerous.
Orbits of the Planets - ANSWER The Solar System is (mostly) a disk
- Planets orbit in nearly the same plane and all in the same direction
- Planets revolve and (mostly) rotate in the same direction (counterclockwise as seen
from north).
The Inner Planets - ANSWER "terrestrial"
made of rock, iron,...
The Outer Planets - ANSWER "Jovian"
Made of gases with compositions like that of the Sun.
What types of debris are there? - ANSWER Asteroids, Meteorites, Comets, Kuiper Belt
What do all Jovian Planets have? - ANSWER Rings
What are properties of the Solar System? - ANSWER 1. Disk shape: orbits in nearly the
same place; common direction of rotation and revolution
2. Two types of planets: Terrestrial= inner, small, high-density, rock + iron; Jovian=
outer, giant, low-density, Solar composition
3. Big planets have satellites; bigger planets generally have more satellites; Jovian
planets have rings
4. Debris fills the Solar System; mostly in the asteroid and Kuiper belts; inner debris
,tend to be rocky; outer debris is icy
5. There is a continuum of size from Jupiter to the smallest debris; smaller objects are
more common
6. Common age of about 4.6 billion years for Earth, Moon, meteorites, Sun
The Age of the Solar System - ANSWER The properties of the Solar System and our
understanding of star formation tell us that the Solar System formed with the Sun out of
its protostellar gas disk ---> 4.6 billion years old
How do we measure the ages of the Solar System? - ANSWER Radioactivity gives us a
way to measure ages of rocks and meteors= the time since they last solidified. Each kind
of radioactive atom and has a half-time. This is the time that it takes for 1/2 of the
radioactive atoms to decay. The half-life of common uranium (238U) is 4.5 billion years.
If we start with 1000 atoms of 238U, there will be ~ 500 atoms left after 4.5 billion years
and ~ 250 atoms left after 9 billion years. The rest turned into lead (206Pb).
We use this to measure the ages of rocks. By measuring the amount of uranium left and
the amount of lead formed, we can tell when the rock solidified. The oldest rocks on
Earth's surface are 4.4 billion years old. The oldest rocks from the Moon are 4.5 billion
years old. The oldest meteorites are 4.6 billion years old. Small rocks cool quickly in
space, so this should be the age of the Solar System. It agrees with theoretical
calculations for the age of the Sun.
What does uranium decay into? - ANSWER Lead
Comets and Stardust - ANSWER Comets have almost the same composition as the Sun
because small objects don't have enough gravity to hold onto light gases.
Comets and carbon-rich meteorites contain tiny dust grains. They are older than the
Solar System. Some grains are rich in carbon, others in metal. Different types of dust
formed in different environments. Carbon grains formed in the atmospheres of giant
stars. Metal-rich grains formed in supernova explosions. After solidifying, these dust
grains were mixed together with hydrogen and helium.
Some grains contain simple organic molecules. Such molecules cannot form in stars,
because the temperature is too high. They form in cold clouds of gas drifting between
stars.
The Solar System formed from such a cloud of dust and gas.
Planet Formation - ANSWER The proto-solar cloud was cold, so gas pressure inside it
was too small to resist gravity. Every atom in the cloud was gravitationally attracted to
every other atom. As a result, the cloud collapsed. At first, the cloud was irregular and
slowly rotating. But as the cloud got smaller, conservation of angular momentum
increased the rate of rotation in proportion to the amount of collapse. Eventually the
outer parts formed a disk in circular rotation around the center. Dust particles in the
disk collided, stuck together and grew into proto-planets. Protoplanets swept up much
,of the remaining disk, first by sticking together in collisions and later by gravitational
accretion.
Where are dust disks seen around? - ANSWER Many young stars
Gas and dust protoplanetary disk around star HL Tau (age ~ 1 million years) - ANSWER
We believe that planets are forming in the dark rings, where they clean out the material
that was in the ring by accreting it or by scattering it to other radii.
Planetesimal Collision - ANSWER Some collisions were violent. But most collisions were
gentle, because all material was revolving in the same direction. So particles tended to
stick together, not to smash each other.
Gradually the embryo Earth grew...
A violent collision with a Mars-sized protoplanet is believed to have knocked loose the
material that then collected together to form our Moon.
When the Moon assembled, it was much closer to the Earth than it is now.
Condensation - ANSWER The temperature of the protoplanetary disk decreased
outward from the Sun. So metals and silicates condensed near the Sun, forming the
rock-and-iron terrestrial planets. Farther out, it was cold enough for ices to condense.
The Jovian planets grew huge because of this. Then they could gravitationally capture
hydrogen and helium and get even bigger. This is also why debris in the outer Solar
System is mostly icy.
Condensation (Planet Formation) - ANSWER A particle adds atoms, one at a time (like
the growth of snowflakes).
Small grains grow rapidly
Accretion - ANSWER A particle grows because other particles hit it and stick to it
Gravitational Attraction - ANSWER helps accretion when a protoplanet gets big enough.
Big planetesimals grow fastest
Differentiation - ANSWER As planetesimals grow, they heat up (accretion, pressure,
radioactivity). Heavy stuff (like iron) melts and sinks to the center. Light stuff (like the
lightest rock) floats to the surface-->iron core + light rocky crust
Outgassing - ANSWER Trapped gases escape from the interior to form the atmosphere
Two Types of Planets - ANSWER Heat from the Sun boils off volatile gases like hydrogen
and helium. So:
There are 2 kinds of planets
(i) rocky-iron "terrestrial" planets near the Sun that lost their volatiles
, (ii) "Jovian" gas giant planets far from the Sun that kept their volatiles.
Radiation Pressure
Solar Wind
Sweeping up of small particles - ANSWER clears away most of the rest of the Solar
nebula
Planet Formation Summary - ANSWER Many orbits in the Solar System are unstable-
sometimes a little, sometimes a lot. Gravitational perturbations by (especially) Jupiter,..,
Neptune change orbits: eventually a close encounter ejects a planetesimal from the
Solar System unless (i) it gets swallowed, or (ii) it gets moved into a stable orbit. This
has cleared out some of the outer Solar System.
The Solar System is still evolving - ANSWER Orbits are getting rearranged. Small
objects are getting ejected. Debris fragments hit each other and hit planets. This can
cause planetary catastrophes like mass extinctions of life on Earth.
Where do we discover other planets? - ANSWER Around other stars by measureing the
reflex speed of the star and looking for periodic variations
How can hot Jupiters form? - ANSWER Their stars should have evaporated volatile
gases. Orbits much smaller than Earth's are common even for high-mass planets
Probably form far from their stars where it is cold-where protoplanets can accrete ices
and hydrogen + helium gas. Then they "sink" toward their star by flinging smaller
planets outward.--> hot Jupiter + colder Neptune
Formation of Hot Jupiters is Dangerous for Earth-like Planets - ANSWER It seems clear
that planet formation was chaotic and violent.
It is possible that there were more Earth-sized planets even in our Solar System that got
ejected by protoplanetary interactions.
Astronomers now believe that there are at least as many free-floating, interstellar
planets as there are planets that revolve around stars.
What do we learn about Earth-like planets? - ANSWER The discovery of extrasolar
planets confirms that Solar Systems are common. Most planets that have been found so
far live very close to their stars and are very hot. Many are hot Jupiters, but even more
are hot "super-Earths" At first, there was little direct evidence for Earth-like planets, but
the Kepler mission is now finding lots of them. Also, melted, formerly icy moons of hot
Jupiters-water worlds- could be very suitable for life.
If a star has lots of iron - ANSWER the probability that it has planets is large
Life Zones around Stars - ANSWER Is the region where a planet would have a moderate
temperature that allows water to be liquid. Red dwarf stars were thought to be
How far out does our Solar System extend to? - ANSWER Reaches about 1/3 of the way
to the nearest star
How far out does the Oort cloud extend from? - ANSWER It extends out from the Sun to
about 10,000 to 100,000 AU
What dominates the Solar System? - ANSWER The Sun
Why does the Sun dominate the Solar System? - ANSWER It is 9.7 times the diameter of
Jupiter. Its diameter is 3.6 times the distance from the Earth to the Moon. It is 1050 times
the mass of Jupiter. But it has only 0.3% of the angular momentum of the Solar System.
How to get rid of "excess rotation" when the Sun was grown out of a spinning debris is a
major puzzle.
What is there a complete continuity between? - ANSWER The sizes of planets and the
sizes of satellites. Smaller objects are more numerous.
Orbits of the Planets - ANSWER The Solar System is (mostly) a disk
- Planets orbit in nearly the same plane and all in the same direction
- Planets revolve and (mostly) rotate in the same direction (counterclockwise as seen
from north).
The Inner Planets - ANSWER "terrestrial"
made of rock, iron,...
The Outer Planets - ANSWER "Jovian"
Made of gases with compositions like that of the Sun.
What types of debris are there? - ANSWER Asteroids, Meteorites, Comets, Kuiper Belt
What do all Jovian Planets have? - ANSWER Rings
What are properties of the Solar System? - ANSWER 1. Disk shape: orbits in nearly the
same place; common direction of rotation and revolution
2. Two types of planets: Terrestrial= inner, small, high-density, rock + iron; Jovian=
outer, giant, low-density, Solar composition
3. Big planets have satellites; bigger planets generally have more satellites; Jovian
planets have rings
4. Debris fills the Solar System; mostly in the asteroid and Kuiper belts; inner debris
,tend to be rocky; outer debris is icy
5. There is a continuum of size from Jupiter to the smallest debris; smaller objects are
more common
6. Common age of about 4.6 billion years for Earth, Moon, meteorites, Sun
The Age of the Solar System - ANSWER The properties of the Solar System and our
understanding of star formation tell us that the Solar System formed with the Sun out of
its protostellar gas disk ---> 4.6 billion years old
How do we measure the ages of the Solar System? - ANSWER Radioactivity gives us a
way to measure ages of rocks and meteors= the time since they last solidified. Each kind
of radioactive atom and has a half-time. This is the time that it takes for 1/2 of the
radioactive atoms to decay. The half-life of common uranium (238U) is 4.5 billion years.
If we start with 1000 atoms of 238U, there will be ~ 500 atoms left after 4.5 billion years
and ~ 250 atoms left after 9 billion years. The rest turned into lead (206Pb).
We use this to measure the ages of rocks. By measuring the amount of uranium left and
the amount of lead formed, we can tell when the rock solidified. The oldest rocks on
Earth's surface are 4.4 billion years old. The oldest rocks from the Moon are 4.5 billion
years old. The oldest meteorites are 4.6 billion years old. Small rocks cool quickly in
space, so this should be the age of the Solar System. It agrees with theoretical
calculations for the age of the Sun.
What does uranium decay into? - ANSWER Lead
Comets and Stardust - ANSWER Comets have almost the same composition as the Sun
because small objects don't have enough gravity to hold onto light gases.
Comets and carbon-rich meteorites contain tiny dust grains. They are older than the
Solar System. Some grains are rich in carbon, others in metal. Different types of dust
formed in different environments. Carbon grains formed in the atmospheres of giant
stars. Metal-rich grains formed in supernova explosions. After solidifying, these dust
grains were mixed together with hydrogen and helium.
Some grains contain simple organic molecules. Such molecules cannot form in stars,
because the temperature is too high. They form in cold clouds of gas drifting between
stars.
The Solar System formed from such a cloud of dust and gas.
Planet Formation - ANSWER The proto-solar cloud was cold, so gas pressure inside it
was too small to resist gravity. Every atom in the cloud was gravitationally attracted to
every other atom. As a result, the cloud collapsed. At first, the cloud was irregular and
slowly rotating. But as the cloud got smaller, conservation of angular momentum
increased the rate of rotation in proportion to the amount of collapse. Eventually the
outer parts formed a disk in circular rotation around the center. Dust particles in the
disk collided, stuck together and grew into proto-planets. Protoplanets swept up much
,of the remaining disk, first by sticking together in collisions and later by gravitational
accretion.
Where are dust disks seen around? - ANSWER Many young stars
Gas and dust protoplanetary disk around star HL Tau (age ~ 1 million years) - ANSWER
We believe that planets are forming in the dark rings, where they clean out the material
that was in the ring by accreting it or by scattering it to other radii.
Planetesimal Collision - ANSWER Some collisions were violent. But most collisions were
gentle, because all material was revolving in the same direction. So particles tended to
stick together, not to smash each other.
Gradually the embryo Earth grew...
A violent collision with a Mars-sized protoplanet is believed to have knocked loose the
material that then collected together to form our Moon.
When the Moon assembled, it was much closer to the Earth than it is now.
Condensation - ANSWER The temperature of the protoplanetary disk decreased
outward from the Sun. So metals and silicates condensed near the Sun, forming the
rock-and-iron terrestrial planets. Farther out, it was cold enough for ices to condense.
The Jovian planets grew huge because of this. Then they could gravitationally capture
hydrogen and helium and get even bigger. This is also why debris in the outer Solar
System is mostly icy.
Condensation (Planet Formation) - ANSWER A particle adds atoms, one at a time (like
the growth of snowflakes).
Small grains grow rapidly
Accretion - ANSWER A particle grows because other particles hit it and stick to it
Gravitational Attraction - ANSWER helps accretion when a protoplanet gets big enough.
Big planetesimals grow fastest
Differentiation - ANSWER As planetesimals grow, they heat up (accretion, pressure,
radioactivity). Heavy stuff (like iron) melts and sinks to the center. Light stuff (like the
lightest rock) floats to the surface-->iron core + light rocky crust
Outgassing - ANSWER Trapped gases escape from the interior to form the atmosphere
Two Types of Planets - ANSWER Heat from the Sun boils off volatile gases like hydrogen
and helium. So:
There are 2 kinds of planets
(i) rocky-iron "terrestrial" planets near the Sun that lost their volatiles
, (ii) "Jovian" gas giant planets far from the Sun that kept their volatiles.
Radiation Pressure
Solar Wind
Sweeping up of small particles - ANSWER clears away most of the rest of the Solar
nebula
Planet Formation Summary - ANSWER Many orbits in the Solar System are unstable-
sometimes a little, sometimes a lot. Gravitational perturbations by (especially) Jupiter,..,
Neptune change orbits: eventually a close encounter ejects a planetesimal from the
Solar System unless (i) it gets swallowed, or (ii) it gets moved into a stable orbit. This
has cleared out some of the outer Solar System.
The Solar System is still evolving - ANSWER Orbits are getting rearranged. Small
objects are getting ejected. Debris fragments hit each other and hit planets. This can
cause planetary catastrophes like mass extinctions of life on Earth.
Where do we discover other planets? - ANSWER Around other stars by measureing the
reflex speed of the star and looking for periodic variations
How can hot Jupiters form? - ANSWER Their stars should have evaporated volatile
gases. Orbits much smaller than Earth's are common even for high-mass planets
Probably form far from their stars where it is cold-where protoplanets can accrete ices
and hydrogen + helium gas. Then they "sink" toward their star by flinging smaller
planets outward.--> hot Jupiter + colder Neptune
Formation of Hot Jupiters is Dangerous for Earth-like Planets - ANSWER It seems clear
that planet formation was chaotic and violent.
It is possible that there were more Earth-sized planets even in our Solar System that got
ejected by protoplanetary interactions.
Astronomers now believe that there are at least as many free-floating, interstellar
planets as there are planets that revolve around stars.
What do we learn about Earth-like planets? - ANSWER The discovery of extrasolar
planets confirms that Solar Systems are common. Most planets that have been found so
far live very close to their stars and are very hot. Many are hot Jupiters, but even more
are hot "super-Earths" At first, there was little direct evidence for Earth-like planets, but
the Kepler mission is now finding lots of them. Also, melted, formerly icy moons of hot
Jupiters-water worlds- could be very suitable for life.
If a star has lots of iron - ANSWER the probability that it has planets is large
Life Zones around Stars - ANSWER Is the region where a planet would have a moderate
temperature that allows water to be liquid. Red dwarf stars were thought to be
