James T. ShipmanJerry D. WilsonCharles A. Higgins, Jr.

The Solar System

Chapter 16

Introduction

• Astronomy – the scientific study of the universe beyond Earth’s atmosphere

• Universe – everything, all energy, matter, and space

• The Milky Way– one of 50 billion galaxies scattered throughout the universe

• Solar System – contains our Sun and 9 planets• Sun – supplies the energy for nearly all life on

the planet earth

Intro

Electromagnetic Spectrum

Astronomers are interested in studying the full range of electromagnetic spectrum coming from space

Intro

Astronomy

• Much of the incoming solar radiation does not make it to the Earth’s surface – due to atmospheric absorption

• Electromagnetic radiation that will pass through the Earth’s atmosphere can be studied using ground-based detectors

• Other regions of the electromagnetic spectrum must be detected by space-based instruments• The Hubble Space Telescope is a good example of

an instrument outside Earth’s atmosphere

Intro

The Solar System

• The solar system - complex system of moving masses held together by gravitational forces

• Sun is center • Sun is the dominant mass• Revolving around the sun – 8 major planets with

over 170 moons, 4 dwarf planets, and 1000’s of other objects (asteroids, comets, meteoroids, etc.)

Section 16.1

The Solar System

• Geocentric Model – early belief that the Earth was motionless and everything revolved around it• Claudius Ptolemy (A.D. 140)

• Heliocentric Model – a Sun-centered model• Nicolaus Copernicus (1473-1543)

Section 16.1

Johannes Kepler (1571-1630)

• German mathematician and astronomer• Kepler’s 1st Law – Law of Elliptical Orbits – All

planets move in elliptical orbits around the Sun with the Sun as one focus of the ellipse

• An ellipse is a figure that is symmetric about two unequal axes

Section 16.1

Drawing an Ellipse

• An ellipse has two foci, a major axis, and a semimajor axis

• In discussing the Earth’s elliptical orbit, the semimajor axis is the average distance between the Earth and the Sun = Astronomical Unit (AU) = 1.5 x 108 km

Section 16.1

Kepler’s Second Law

• Law of Equal Areas – An imaginary line (radial vector) joining a planet to the Sun sweeps out equal areas in equal periods of time

Section 16.1

The speed of a revolving planet varies

• Perihelion – the closest point in a planet’s orbit around the Sun, speed is the fastest• Perihelion occurs for Earth about January 4

• Aphelion – the farthest point in a planet’s orbit around the Sun, speed is the slowest• Aphelion occurs for

Earth about July 5

Section 16.1

Kepler’s Third Law

• Harmonic Law – the square of the sidereal period of a planet is proportional to the cube of its semimajor axis

• T2 = k R3

• T = period (time of one revolution)• R = length of semimajor axis• k = constant (same for all planets) = 1y2/AU3

Section 16.1

Major Planet Classification and Orbits in Our Solar System

• Sun – 99.87% of the mass of solar system• Of the remaining 0.13%, Jupiter is > 50%• Planets with orbits smaller than Earth are

classified as “inferior”• Planets with orbits larger than Earth are

classified as “superior”

Section 16.2

Revolution/Rotation

• All planets revolve (orbit) counterclockwise (prograde motion) around the Sun as observed from the north pole. Each planet also rotates counterclockwise on its axis except Venus and Uranus (retrograde motion).

Section 16.2

Planet Classification

• Terrestrial planets – Mercury, Venus, Earth, Mars• High percent of more massive (non-gaseous)

elements

• Jovian planets - Jupiter, Saturn, Uranus, and Neptune• High percent of less massive gaseous elements

Section 16.2

The Solar System -- drawn to scale with the eight major planets

Section 16.2

A Planet’s Period

• Sidereal Period – the time interval between two successive conjunctions as observed from the sun• Mercury’s sidereal period is 88 Earth days

• Synodic Period – the time interval between two successive conjunctions of the planet with the Sun as observed from Earth• Mercury’s synodic period is 116 Earth days

Section 16.2

Mercury shown in inferior and superior conjunctions with Earth

Section 16.2

Planet Earth

• The Earth is the third planet from the sun, and is a solid, spherical, rocky body with oceans and an atmosphere

• Large amounts of surface water in all three phases – solid, liquid, and gas – exist on Earth

• An oxygen-containing atmosphere, temperate climate, and living organisms all make Earth a unique planet

Section 16.3

Composition of the Earth

• Atmosphere – 21% oxygen• Earth’s crust – over 90%, by volume, of the

rocks/minerals are oxygen!• We live in an oxidized environment• Examples of very common minerals at the

Earth’s surface include: • Quartz – SiO2, Calcite – CaCO3, Feldspar – KAlSi3O8

• Note that most common minerals have oxygen (O) in their formula

Section 16.3

Earth’s Shape

• The planet Earth is not a perfect sphere, but rather an oblate spheriod• Flattened at the poles• Bulging at the equator• Due to rotation about its axis

• Pole Diameter is about 43 km less than the Equatorial Diameter• Since the Earth has an average diameter of 12,900

km this difference is only a small fraction

Section 16.3

Albedo

• Albedo – the fraction of the incident sunlight reflected by an object

• Earth’s albedo is 33%• Moon’s albedo is 7% (from Earth the moon is

the 2nd brightest object in the night sky)• Venus’ albedo is 76% (3rd brightest is sky)• Since the Moon is so close to Earth it is brighter

than Venus

Section 16.3

Earth Motions

• Daily Rotation on its axis (daily cycle)• Rotation – spin on an internal axis

• Annual revolution around the sun (annual cycle)• Revolution – movement of one mass around another

• Precession – the slow change of the earth’s rotational axis (now at 23.5o) – see chapter 15

Section 16.3

Earth’s Rotation on its Axis

• Not generally accepted until 19th century• Very difficult to prove???• 1851, experiment designed by French engineer,

Jean Foucault• The Foucault Pendulum – very long pendulum

with a heavy weight at the end• Basically, the Foucault pendulum will swing back

and forth as the Earth moves under it

Section 16.3

Foucault Pendulum

The pendulum does not rotate with reference to the fixed stars. Experimental proof of the Earth’s rotation

Section 16.3

Parallax

• Parallax – the apparent motion, or shift, that occurs between two fixed objects when the observer changes position

• Parallax can be seen with outstretched hand• The motion of Earth as it revolves around the

Sun leads to an apparent shift in the positions of the nearby stars with respect to more distant stars

Section 16.3

Stellar Parallax

• The observation of parallax is indisputable proof that the Earth revolves around the Sun.

• In addition, the measurement of the parallax angle is the best method we have of determining the distance to nearby stars

Section 16.3

Aberration of Starlight

• A 2nd proof of Earth’s orbital motion• Telescopic observations of a systematic change

in the position of all stars annually• Due to the motion of the Earth around the Sun

• Angular discrepancy between the apparent position of a star and its true position, arising from the motion of an observer relative to the path of the beam of light observed• This is similar to what you see when driving in the rain

Section 16.3

Aberration of Starlight

• This discrepancy is very small and is measured in a -- parsec

• Parsec parallax + second• Recall that a circle contains 360o. Each degree

is divided into 60 minutes, and each minute into 60 seconds

• Therefore 1 second = 1/3600 degree• Parsec = the distance to a star when the star

exhibits a parallax of 1 second.

Section 16.3

Terrestrial Planets

• The terrestrial planets include: Mercury, Venus, Earth, Mars

• Due to physical/chemical characteristics they resemble Earth

• All four terrestrial planets are • Relatively small in size and composed of rocky

material and metals• Relatively close together and close to Sun• Have no rings• Only Earth and Mars have moons• Only Earth has surface water and oxygen

Section 16.4

Mercury

• Mercury is the closest planet to the Sun• Mercury has the shortest period of revolution (88

days), and is the fastest moving• Mercury was named by the early Greeks after

the swift messenger of the gods• Temperatures on Mercury range from about

473oC on the side facing the Sun to about -173oC on the dark side

• Due to its small size and closeness to the Sun, Mercury has practically no atmosphere

• It has a density close to that of Earth.

Section 16.4

Mercury, a Terrestrial Planet

Rotates 3 times while circling the Sun twice

Section 16.4

The Messenger spacecraft mapped the surface of Mercury. It was the first spacecraft to orbit the planet.

Venus

• Venus is the closest planet to Earth• Venus is the third brightest object in the sky• Due to its brightness it was named after Venus

the goddess of Beauty• The surface of Venus cannot be seen from

Earth, due to dense, thick clouds that cover the planet

• Magellan radar images indicate that the surface of Venus is composed of black, hot rock• Most surface rocks appear to be volcanic

Section 16.4

The Atmosphere of Venus

• Venus’ atmosphere is composed of 96% CO2

• It is so dense that the surface of Venus has a pressure of 90 atm

• The large percent of CO2 in the atmosphere results in high surface temperatures (477o C) due the “greenhouse effect”

• Radar images have revealed relatively few impact craters• Most of these craters are fairly large, because the

smaller incoming objects are consumed by Venus’s thick atmosphere

Section 16.4

Venus, a Terrestrial Planet

Atmosphere rotates faster than solid planet.Retrograde rotation of planet

Section 16.4

A radar map of Venus. The inset is from the Russian spacecraft Venera 13.

Mars

• Mars has a red color, as viewed from the Earth, and was named for the Roman god of war

• The surface of Mars has two outstanding features that have intrigued scientists for decades; polar ice caps and extinct volcanoes

• The ice caps are composed of frozen CO2 in the winter and CO2 vapor with frozen water in the summer

• The red color is thought to be due to fine grain iron oxide minerals.

Section 16.4

Martian Volcano – Mt. Olympus

• The largest known volcano in the solar system, at 24 km in height, it is about three times that of Mauna Loa

Section 16.4

Mars at closest approach

Section 16.4

Section 16.4

Mars – Valles Marineris

This canyon on Mars is 4000 km in length and 6 km deep

Geologists think that it is a crustal fracture caused by internal forces

Section 16.4

In 2006 the Opportunity rover reached the edge of Victoria crater

Section 16.4

The Jovian Planets

• Jupiter, Saturn, Uranus, Neptune • Much larger than the terrestrial planets• Composed mainly of hydrogen and helium

• The four Jovian planets have a very low average density (approximately 1.2 g/cm3)

• All four are thought to have a rocky core composed of iron and silicates

• Thick layers of frozen methane, ammonia, and water are found above the core

Section 16.5

Formation of the Terrestrial Planets

• The two least massive elements – H & He – were the most abundant when the planets started to coalesce about 5 billion years ago

• Due to the heat from the Sun most of these less massive elements escaped the gravitational pull of the inner planets• Leaving behind more of the massive elements and

resulting in thick rocky cores and higher densities for the inner planets

Section 16.5

Formation of the Jovian Planets

• The four large outer planets were much farther from the Sun and therefore much colder

• The Jovian planets retained most of their H and He which now surround their ice layers and innermost rocky cores

• As a consequence the Jovian planets have a much lower average density

Section 16.5

Jupiter

• Largest planet of the solar system, in both volume and total mass

• Named after the supreme Roman god of heaven because of its brightness and giant size

• Diameter is 11 times Earth’s -- 318 times more mass than Earth

• The average density of Jupiter approximately 1.3 g/cm3 • Jupiter is covered with a thin layer of clouds composed

of hydrogen, helium, methane, ammonia, and several other substances

• It is a fast rotator – taking about 10 hours to rotate.

Section 16.5

Jupiter

Section 16.5

Jupiter’s Great Red Spot (“eye”)

• The Great Red Spot has erratic movement, shape, color, and size – sometimes even disappearing

• Likely a huge counterclockwise “hurricane-like” storm, lasting hundreds of years

Section 16.5

Saturn

• Distinctive system of three prominent rings• Rings are inclined 27o to orbital plane

• The rings are thought to be composed of ice and ice-coated rocks (micrometers 10 m)

• Most spectacular sight that can be viewed from Earth with a small telescope

• Diameter is 9 times Earth’s -- 95 times more mass than Earth

• Average density of only 0.7 g/cm3

Section 16.5

Saturn and its rings

Section 16.5

Uranus

• Discovered in 1781 by William Herschel (1738-1822), an English Astronomer

• Named after Uranus, the father of the Titans and the grandfather of Jupiter

• Thin ring system composed of boulder-size particles (>1m), with very little dust-size

• Average density of only 1.3 g/cm3

Section 16.5

Uranus – unlike the other planets Uranus revolves around the Sun on its side and rotates

in a retrograde fashion

Neptune

• Discovered in 1846 by Johann Galle, a German astronomer

• Englishman John Couch Adams and Frenchman U.J.J. Leverrier were mathematicians using Newton’s law of gravitation

• They noted that Uranus’ motion was disturbed and predicted the location of another planet – this is how Galle eventually discovered Neptune

Section 16.5

Neptune

• Neptune also has a large dark spot similar to Jupiter’s and thought to be the result of large wind systems

• Neptune and Uranus are similar in size and in the composition of their atmospheres

• In many respects these two planets can be considered twins

Section 16.5

Neptune

Section 16.5

Designations of Celestial BodiesInternational Astronomical Union (IAU)

• In 2006 the IAU adopted the following criteria for a solar system body to be a planet:• (1) It must be in orbit about the Sun.• (2) It must have sufficient mass for self-gravity to

form a nearly round shape.• (3) It must be the dominant body within its orbit.

• The last statement disqualifies Pluto

Section 16.6

The Dwarf Planets

• Pluto’s orbit takes it inside that of Neptune’s, Pluto is not the dominant body of its orbit.

• The IAU established two new categories for objects that orbit the Sun.

• Dwarf planets is one of the categories• Pluto, Ceres and Eris are now designated dwarf

planets

Section 16.6

Ceres – First Dwarf Planet from the Sun Between Mars and Jupiter

• Lies in the asteroid belt between Mars and Jupiter• Discovered in 1801 by Italian Giuseppe Piazzi and

named after the Roman goddess of agriculture• Has a diameter of only 940 km and is the smallest dwarf

planet

Section 16.6

Pluto – Second Dwarf Planet from the Sun Beyond Neptune

• Names for the god of outer darkness• Average density of only 1.65 g/cm3

• Discovered in 1930, by C.W. Tombaugh• Investigating discrepancies in the orbital path of

Neptune and Uranus

• Does not resemble either the terrestrial or Jovian Planets

• Pluto has not been visited by a space probe – flyby planned in 2015.

Section 16.6

Pluto

• There are similarities between Pluto and Triton, of Neptune’s moons

• Some scientists think that both are large asteroids captured from interplanetary space

• If this is the case, Pluto has maintained its own orbital path, and Triton was captured by Neptune

Section 16.6

Pluto and its Satellites

Section 16.6

The Solar SystemPluto’s orbit is greatly inclined to

the orbital plane of the major planets

Section 16.6

Eris – Third Dwarf Planet from the Sun Far Beyond Neptune and Pluto

• Previously known as 2003 UB313 (or Xena)• Officially now named Eris, after the Greek

goddess of chaos and strife.• Slightly larger and about 3 times farther away

form the Sun than Pluto• Highly elliptical orbit that takes 560 Earth years

for one revolution.

Note on Xena: Dr. Mike Brown- CalTech scientist- used data from the Palomar Observatory to identify it.

Section 16.6

Dwarf planets Haumea and Makemake

• Haumea mass 30% of Pluto and orbits at 43 AU.• Makemake mass 75% of Pluto and orbits at 75

AU

Section 16.6

Outermost reaches of the Solar System

• The Kuiper Belt extends just beyond the orbit of Neptune and into the space of Eris.• Consists of comet and cometary material and other

small objects – Trans Neptunian Objects

• Many astronomers put the edge of the solar system to be at about 100 AU.

• Voyager 1, launched in 1977, and in 2004 reached 100 AU. In 2010 it crosses the boundary of zero solar wind velocity.

Section 16.6

Origin of the Solar System

• Any theory that purports to explain the origin and development of the solar system must account for its present form

• According to our best measurements, our solar system has been in its present state for about 4.5 billion years

• A valid theory for solar system formation – must be able to explain a number of major properties of our solar system

Section 16.7

Major Questions Concerning Solar System

• Origin of material?• Forces that formed the solar system?• Isolated planets, circular orbits, in same plane?• Revolution (orbit) in the same direction?• Most Rotate in same direction (except two)?• Terrestrial versus Jovian planets?• Origin of the asteroids?• Origin of comets and meteoroids?

Section 16.7

Formation of the Solar System

• Began with a large, swirling volume of cold gases and dust – a rotating solar nebula

• Contracted under the influence of its own gravity – into a flattened, rotating disk

• Further contraction produced the protosun and eventually accreted the planets

• As particles moved inward, the rotation of the mass had to increase to conserve angular momentum (like an ice skater bringing in her arms)

Section 16.7

The Formation of the Solar SystemCondensation Theory

Section 16.7

Other Planetary Systems

• Are there other planetary systems in the universe?

• If so, we would expect to find some of these systems in different stages of formation• In other words, we should be able to find clouds of

gas and dust, primordial nebula, and protosuns, etc.

• We should also be able to use gravitational effects to detect small wobbles due to rotational objects in space

• These are called exoplanets or extra-solar planets

Section 16.8

Gravitational Effects

• A star with a large planet orbiting about it will have a small wobble superimposed on its motion as a result of gravitational effects

• This change in motion (the wobble) is likely to be very slight, but in some cases may be detected as a Doppler shift of the star’s spectrum• As the star approaches the observer, the

wavelengths are compressed (‘blue shift’)• As the star move away from the observer, the

wavelengths are lengthened (‘redshift’)

Section 16.8

Gravitational Effects

• The amount of wobble can be used to determine the planet’s mass (related to gravitational pull)

• The wobble’s cycle time can be used to determine the orbital period

• Once the orbital period is known, Kepler’s third law (T2= kR3) can be used to determine the planet’s average distance from the star

Section 16.8

Star WobbleDue to the gravitational pull of an orbiting planet.

Section 16.6

The wobbling in this illustration is greatly exaggerated!Section 16.8

Transit method

• A planet passing in front of its star as seen from Earth

• The star’s light will temporarily dim• The Kepler mission uses the transit method.

Section 16.8

First Planets Discovered Beyond our Solar System

• In 1992, using the Arecibo Observatory in Puerto Rico, astronomers reported the discovery of two objects revolving about a pulsar• Pulsars are very dense, rapidly rotating stars• Pulsars have a very precise rotation period• If the rotation period is disrupted, this would indicate

the presence of an object rotating about the pulsar

• These two objects are the first planets detected beyond our solar system

Section 16.8

Planets Beyond our Solar System

• There have now been approximately 500 planets detected around other stars and Kepler is finding more and more.

• These findings strongly indicate the existence of many other planetary systems in the universe

• Scientists are also searching for signals from extraterrestrial intelligence (SETI)

• Equipment today is being used to scan wide frequency ranges over vast areas of the sky

Section 16.8