Download Chapter 7: A Planetary Overview

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, defined to be an object that passes rule (a)
and (b) but not (c). Pluto is an example of an object in the Kuiper belt (a disk-shaped
region beyond Neptune’s orbit, 30 to 1000 AU from the Sun).
3. All other objects except satellites orbiting the Sun shall be referred to collectively as
small solar-system bodies
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. We send radar signals to a planet and measure the time required for the signal to reach
the planet and bounce back, and because radar signals travel at the speed of light, we can
then calculate the distance to the planet.
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.
Advancing the Model: The Discovery of the Asteroids
1. The Titius-Bode law as well as observations by others such as Kepler, suggested there
might be an undiscovered planet between Mars and Jupiter.
2. In 1801, Giuseppe Piazzi, a Sicilian astronomer and monk, discovered an object
orbiting the Sun between Mars and Jupiter. Called Ceres and at first thought to be a
new planet, soon other objects were found in similar orbits.
3. These new objects were named asteroids, and several thousand of them are known
today.
Advancing the Model: The Voyage Spacecraft
1. Voyager 1 and Voyager 2 sent back unprecedented views of Jupiter, Saturn, Uranus,
and Neptune, and their moons and rings, during fly-bys from 1979 to 1989.
2. The spacecraft are now on an extended mission, searching for the outer limits of the
Sun’s magnetic field and outward flow of the solar wind (the heliopause boundary).
3. Electrical power on these spacecraft is produced from the heat generated by the
natural decay of plutonium; they have enough power to operate at least until 2025.
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
from orbits around the Sun.
4. Kepler’s third law applies to any system of orbiting objects, and we can calculate a
planet’s mass by using the orbits of its 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 almost 99.9% of the mass of the solar system. The planets and their
satellites account for only about 0.1% of the total solar system mass.
Calculating Average Density
1. In calculating average density, we assume that the object is approximately spherical,
and then apply the formula: 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. Most planets 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 four inner (terrestrial) planets and the four
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. A 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.
6. All planets—except Venus, Uranus, and Pluto—rotate in a counterclockwise direction,
as viewed from far above the Earth’s North Pole.
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. Some 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.
(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
and Uranus.
(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’ 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 Louis de Buffon in 1745. We now know comets are
not massive enough to cause such an event.
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. The outer layers of the Sun have different
concentrations of certain elements and isotope, such as deuterium, compared to the
planets.
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 GM Aurigae 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.
8. Spectroscopy can be used to detect elements expected to be present in the atmospheres
of planets such as water vapor and sodium, and complex molecules in the dust clouds that
might form planetary systems, and the composition of dust and gases in disks around
stars.
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 at least 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.