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CHAPTER 7
A Planetary Overview
CHAPTER OUTLINE
What’s In a Name?
1. We finally have a definition for what a planet is. The IAU defined in 2006 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 diskshaped region beyond Neptune’s orbit, 30 to 1000 AU from the Sun).
7-1 Sizes and Distances in the Solar System
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.
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.
Historical Note: The Titius-Bode Law
1`. 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.
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/P2 = K mSun
3. Newton’s reformulation of Kepler’s third law allows us to calculate the Sun’s mass.
4. The masses of 7 of the 9 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 nine 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 dwarf planet 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.
7-4 Classifying the Planets
1. 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.
7-5 Planetary Atmospheres
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.
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).
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.
4. The main characteristics of the terrestrial planets are: near the Sun, small, mostly solid, low
mass, slow rotation, no ring system, high density, thin atmosphere, and few moons.
5. The main characteristics of the Jovian planets are: far from the Sun, large, mostly liquid and
gas, great mass, fast rotation, ring system, low density, dense atmosphere, many moons.
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:
evolutionary and catastrophe theories.
Evidential Clues from the Data
1. 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.
2. Volatile element is a chemical element that exists in a gaseous state at a relatively low
temperature. Nonvolatile element is an element that is gaseous only at a high temperature and
condenses to a liquid or solid when the temperature decreases.
Evolutionary Theories
1. All evolutionary theories have their start with Descartes’ 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.
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 that 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. 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.
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).
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.
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 star T Tauri and the brown
dwarf 2M1207, can be evidence of possible planetary bodies.
2. Dust disks such as discovered around  Pictoris and AU Microscopii provide evidence that
conditions for planet formation exist around many Sun-like stars.
3. Pulsar companions such as discovered around the pulsar PSR 1257+12 as a result of the
variations in the rate of the received signals from the pulsar. (Pulsars are stars that result from
supernova explosions and emit beams of radio waves.)
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.
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.
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 (as in the case of
HD209458) or lead to the detection of an exoplanet (as in the case of OGLE-TR-56b).
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.
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 suggest that it is possible for planets to form at large distances from their star and
migrate inward until they reach stable orbits.
4. Observations also suggest that the size of the largest planet formed around a star is directly
related to the star’s size.
5. Theoretical work supports observations suggesting that 25% of Sun-like stars have planetary
systems.
6. 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.