Download Question 2 (7-1 thru 7-4 PPT Questions)

Chapter 7
7-1 thru 7-4
A Planetary Overview
Courtesy of The International Astronomical Union/Martin Kornmesser
1. In 2006, the IAU defined a planet to be a
celestial body that
(a) orbits the Sun,
(b) has sufficient mass for its self-gravity to
overcome rigid body forces so that it
assumes a nearly-round shape, and
(c) has cleared the neighborhood around its
orbit.
2. Pluto has been demoted to a dwarf
planet, an example of an object in the
Kuiper belt (a disk-shaped region
beyond Neptune’s orbit, 30 to 1000 AU
from the Sun).
Courtesy of NASA, ESA, and A. Feild (STScI)
7-1 Sizes and Distances in the Solar System
Astronomical Unit: A unit of distance equal to the
average distance between the Earth and the Sun.
1. Diameter of Sun (1.39  106 km) is about 110 times
that of Earth (1.3  104 km).
2. Jupiter’s diameter is about 11 times that of Earth.
3. Pluto’s diameter is about 1/5 that of Earth.
Figure 7.01: The Sun, planets, and a few of the large moons drawn to scale.
Measuring Distances in the Solar System
1. Copernicus used geometry to determine relative distances
to the planets, while today we measure them using radar.
2. An outgoing radar signal is usually a burst of 400 kilowatts
(4  105 watts), but the returning signal is only 10-21 watt.
3. Though Mars is about 1.5 AU from the Sun, the distance
from Earth to Mars varies from about 0.5 AU to 2.5 AU.
Advancing the Model: The Titius-Bode Law
The Titius-Bode “law” is an empirical relationship that allows us to
approximate the distances to the planets. It is not based on any
theoretical framework.
Question 1 (7-1 thru 7-4 PPT Questions)
What do you notice about the layout of the solar
system?
7-2 Measuring Mass and Average Density
1. Newton reformulated Kepler’s third law to include
masses:
a3/P2 = K  (m1 + m2), where K = G/(42)
2. If one of the objects is the Sun and the other is a
planet, the sum of their two masses is essentially
equal to the mass of the Sun: therefore,
a3/P 2 = K  MSun
3. Newton’s reformulation of Kepler’s third law allows
us to calculate the Sun’s mass.
4. The masses of 6 of the 8 known planets can be calculated
based on the distances and periods of revolution of these
planets’ natural satellites.
5. For Mercury and Venus, which do not possess any natural
satellites, accurate determinations of their respective
masses had to await orbiting or flyby space probes.
6. The Sun contains 99.85% of the mass of the solar system.
The eight planets and their satellites account for about
0.135% of the total solar system mass.
Calculating Average Density
1. In calculating average density, we assume that
the object approximately spherical: average
density = mass/volume.
2. If we know the average density of an object we
can gain reasonable insights into its makeup.
7-3 Planetary Motions
1. All planetary orbits are ellipses, but all (except
Pluto’s) are nearly circular.
2. Each of the planets revolves around the Sun in a
counterclockwise direction as viewed from far
above the Earth’s North Pole.
3. All planets—except Venus, Uranus, and Pluto—
rotate in a counterclockwise direction, as viewed
from far above the Earth’s North Pole.
4. Most of the satellites revolving around planets also
move in a counterclockwise direction as viewed from
far above the Earth’s North Pole, though there are
some exceptions.
5. The elliptical paths of all the planets are very nearly
in the same plane, though Mercury’s orbit is
inclined at 7° and Pluto’s at 17°.
6. The inclination of a planet’s orbit is the angle
between the plane of a planet’s orbit and the ecliptic
plane.
Figure 7.04
Question 2 (7-1 thru 7-4 PPT Questions)
What do you think is significant about all the planets
sitting roughly in the same plane and revolving the
Sun all in the same direction?
7-4 Classifying the Planets
The eight planets fit into two groups: the inner
(terrestrial) planets and the outer (Jovian) planets.
Size, Mass, and Density
1. The Jovian planets have much bigger diameters and
even larger masses than the terrestrial planets.
2. The terrestrial planets are denser than the Jovian
planets.
Satellites and Rings
1. The Jovian planets have more satellites than the terrestrial
planets. The four Jovian planets have a total of 150 satellites
compared to only 3 satellites for the four terrestrial planets.
2. Pluto has 3 satellites.
3. Each Jovian planet has a ring or ring system. None of the
terrestrial planets do.
Rotations
1. Solar day is the amount of time that elapses between successive
passages of the Sun across the meridian.
2. Meridian is an imaginary line that runs from north to south,
passing through the observer’s zenith.
3. Sidereal day is the amount of time that passes between
successive passages of a given star across the meridian.
4. The Earth’s solar day and sidereal day differ by about 4 minutes.
5. All the Jovian planets rotate faster than any of the terrestrial
planets.
Figure 7.05: The tilt between a planet’s axis of rotation and its orbital plane
varies among the planets in our solar system.
Question 3 (7-1 thru 7-4 PPT Questions)
What are some key differences between terrestrial and
jovian planets?