Download Question 1 (7-5 thru 7-7 PPT Questions)

Chapter 7
7-5 thru 7-7
A Planetary Overview
Courtesy of The International Astronomical Union/Martin Kornmesser
7-5 Planetary Atmospheres
2. The escape velocity from the Earth’s
surface is 11 km/s (24,600 mi/hr).
The escape velocity from the Moon’s
surface is only 2.4 km/s (5370 mi/hr).
3. Phobos (a moon of Mars) is so small
that its escape velocity is about 50
km/hr (30 mi/hr).
© Photodisc
1. Escape velocity is the minimum
velocity an object must have in order
to escape the gravitational attraction
of an object such as a planet.
Gases and Escape Velocity
1. There are three states of matter in our normal
experience: solid, liquid, gas. The fourth state of
matter is the plasma state.
2. Properties of a gas:
(a) As gas molecules interact, different molecules have
different speeds.
(b) The average speed of the molecules depends on the
temperature of the gas.
(c) At the same temperature, less massive molecules
have greater speed.
3. The temperature of a substance is defined by the
average energy of its molecules.
4. There is little free hydrogen in Earth’s
atmosphere because low-mass hydrogen
molecules can achieve escape velocity at the
temperatures of the upper atmosphere.
5. On the sunlit side of the Moon even molecules of
oxygen and nitrogen—so prevalent in Earth’s
atmosphere—can achieve escape velocity in the
Moon’s low gravity.
The Atmospheres of the Planets
1. Ten times the average speed of molecules at a
particular temperature provides a good measure
of whether a planetary body will retain a gas for
billions of years.
2. Because of their size (and mass) the Jovian
planets have retained almost all of their gases.
3. Using spectroscopy we can accurately find the
composition of an object’s atmosphere.
Figure 7.08: The speed of gases
depends on their temperature.
• The dashed line represents 10
times the average speeds.
• All of the planets, Pluto, and
some planetary satellits are
indicated at their
corresponding temperatures
and escape velocities.
Question 1 (7-5 thru 7-7 PPT Questions)
Why doesn’t Earth have much hydrogen in it’s
atmosphere?
7-6 The Formation of the Solar System
1. There are two main categories of competing
theories to explain the origin of our solar system:
the evolutionary theories
the catastrophe theories.
Evidential Clues from the Data
A successful theory must be able to explain the following data:
(a) All the planets revolve around the Sun in the same direction,
and all planetary orbits are nearly circular (except for Pluto).
(b) All of the planets lie in nearly the same plane of revolution.
(c) Most of the planets rotate in the same direction as they orbit the
Sun, except for Venus, Uranus, and Pluto.
(d) The majority of planetary satellites revolve around their parent
planet in the same direction as the planets revolve around the
Sun.
(e) There is a pattern in the spacing of the planets as one moves
out from the Sun.
(f) Similarities of chemical composition exist among the
planets, but there are also differences.
The outer planets contain more volatile elements and are
less dense than the inner.
(g) All planets and moons that have a solid surface show
evidence of craters.
(h) All Jovian planets have ring systems.
(i) Asteroids, comets, and meteoroids populate the solar
system along with the planets, and each category of
objects has its own pattern of motion and location.
(j) The planets have more total angular momentum than does
the Sun, even though the Sun has most of the mass.
(k) Recent evidence indicates that planetary systems in
various stages of development exist around other stars.
Evolutionary Theories
1. All evolutionary theories have their start with Descartes’s
whirlpool or vortex theory proposed in 1644.
2. Using Newtonian mechanics, Kant (1755) introduced the idea
of a rotating cloud of gas contracting under gravity and
forming a disk. Laplace (1796) showed that such a disk will
break up into rings.
3. Such a rotating, contracting disk of gas should speed up
according to the law of conservation of angular momentum.
Figure 7.09
4. Angular momentum is a measure of the
tendency of a rotating or revolving object to
continue its motion.
5. Conservation of angular momentum is a law that
states:
the angular momentum of a system will not
change unless a net outside influence is exerted
on the system, producing a twist around some
axis.
6. The Sun—the center of the former rotating
cloud—should be rotating much faster than it is
observed to be.
The total angular momentum of the planets is
known to be greater than that of the Sun, which
should not occur according to Newton’s laws.
This contradiction caused the evolutionary
theory to lose favor early in the 20th century.
Catastrophe Theories
1. A catastrophe theory is a theory of the formation of the
solar system that involves an unusual incident such as the
collision of the Sun with another star.
2. The first catastrophe theory—that a comet pulled material
from the Sun to form the planets—was proposed by
Georges de Buffon in 1745.
3. More recently, it was proposed that the Sun was a part of a
triple star system that gave birth to the solar system
through tidal disruption.
4. Such theories were discredited in the 1930s when it was
shown that material pulled from the Sun would have been
too hot to condense to form planets and would have
subsequently dissipated into space.
5. Recent discoveries of planetary systems orbiting other
nearby stars further discredit catastrophe theories,
because catastrophic origins for such systems should be
quite rare due to the unusual nature of the incident.
6. Finally, a solution for the angular momentum problem has
been found, so catastrophe theories have been
abandoned.
Present Evolutionary Theories
1. In the 1940s Weizsäcker showed that eddies
would form in a disk-shaped rotating gas cloud
and that the eddies nearer the center would be
smaller.
2. Eddies condense to form small collections of
particles that over time grow to become
planetesimals, which in turn sweep up smaller
particles through collision and gravitational
attraction.
Figure 7.11: Eddies in a gas cloud
3. An object shrinking under the force of gravity
heats up.
High temperatures near the newly formed Sun
(protosun) will prevent the condensation of more
volatile elements.
Planets forming there will thus be made of
nonvolatile, dense material.
4. Farther out, the eddies are larger and the
temperatures cooler so large planets can form
that are composed of volatile elements (light
gases).
Figure 7.13: Development of a solar system—planetesimals have formed
and large eddies of gas and dust remain
Courtesy of Doctors Claude and Francois Roddier
5. As the young Sun heated up, it ionized the gas of
the inner solar system.
The Sun’s magnetic field exerted a force on the
ions in the inner solar system sweeping them
around with it, causing the ions to speed up.
As per Newton’s third law, this transfer of energy
to the ions caused the Sun to slow its rate of
rotation.
6. Stellar wind is the flow of particles from a star.
7. Some young stars exhibit strong stellar winds.
If the early Sun went through such a period, the
resulting intense solar wind would have swept
the inner solar system clear of volatile elements.
The giant planets of the outer solar system would
then have collected these outflowing gases.
Explaining Other Clues
1. Over millions of years the remaining planetesimals
fell onto the moons and planets causing the
cratering we see today.
2. Comets are thought to be material that coalesced in
the outer solar system from the remnants of small
eddies.
3. The formation of Jupiter and its moons must have
resembled the formation of the solar system.
As we move outward from Jupiter, its moons
decrease in density and increase in volatile
elements.
4. Catastrophes probably played a minor, more
localized role in the formation of the solar system,
but the overall origin of our solar system was
evolutionary in nature.
Question 2 (7-5 thru 7-7 PPT Questions)
Explain the difference between the catastrophe and
evolutionary theories of solar system formation.
Why do we believe the evolutionary system is the
correct theory?
7-7 Planetary Systems Around Other Stars
Is it common for stars to have planets?
Different categories of evidence can help answer
this question.
1. Direct observation/Infrared companions, such as is
seen with the brown dwarf 2M1207 and the star T
Tauri, can be evidence of possible planetary bodies.
Figure 7.14a
Figure 7.14b
Courtesy of Doctors Claude and Francois Roddier
2. Dust disks such as discovered around  Pictoris
and AU Microscopii provide evidence that
conditions for planet formation exist around many
Sun-like stars.
Figure 7.15a: An infrared photo of
Beta Pictoris
Courtesy of Mark McCaughrean and Mark Morten
Andersen of the Astrophysical Institute Potsdam (AIP) and
ESO
Figure 7.15b: AU Microscopii dust disk
Courtesy of Michael Liu, IFA-Hawaii/W.M. Keck Observatory
3. Pulsar companions discovered around pulsars as a result of
the variations in the rate of the received signals from the
pulsar.
4. A binary system is a pair of objects that are gravitationally
linked so that they orbit one another.
A discernable visual wobble exhibited by a star would
suggest the existence of an unseen companion— such as a
large planet or group of planets.
Figure 7.16a
5. In the Doppler (radial-velocity) method the wobble
observed in the Doppler shift of a star’s spectrum
suggests the existence of one or more planets
around the star.
Since 1995 this method resulted in the discovery of most of
the 100 or so exoplanets known to date.
Figure 7.16b
6. When a stellar occultation occurs (i.e., one
celestial object passes in front of another), the
total amount of light received decreases.
During a transit (when a planet passes in front of
its star), the star will dim.
Such dimming can confirm the existence of an
exoplanet or lead to an exoplanet’s detection.
Figure 7.16c
7. When a planet passes between the observer and a
distant star, the planet’s gravity acts like a lens and
produces a brief enhancement to the star’s brightness.
It is possible to detect exoplanets using this
gravitational microlensing method.
Figure 7.16d
The Formation of Planetary Systems
1. According to the core-accretion model of planetary
formation:
planets start as small chunks of rock, dust, and
debris and grow through accretion and collisions.
however, planets like Jupiter would take longer to
form than the lifespan of the accretion disk around
the star.
2. According to the disk-instability model, dense
regions forming in the disk accrete more material
and suddenly collapse to form one or more planets.
However, such instabilities require massive disks,
which are not commonly observed.
3. Observations also suggest that the size of the
largest planet formed around a star is directly
related to the star’s size.
4. Theoretical work supports observations suggesting
that 25% of Sun-like stars have planetary systems.
5. It is too early for us to reach conclusions on the
possibility of life existing on one or more
exoplanets.
Future missions might be able to detect Earth-like
planets and use spectroscopy to determine the
chemical composition of their atmospheres and
surfaces.
Question 3 (7-5 thru 7-7 PPT Questions)
Why is it important for us that other stars should have
planets?