copyright © 2012 pearson education, inc. chapter 6 formation of planetary systems: our solar system...

Copyright © 2012 Pearson Education, Inc.

Chapter 6Formation of Planetary Systems:Our Solar System and Beyond

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6.1 A Brief Tour of the Solar System

Our goals for learning:

• What does the solar system look like?

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The solar system exhibits clear patterns of composition and motion.

These patterns are far more important and interesting than numbers, names, and other trivia.

What does the solar system look like?

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Planets are very tiny compared to

distances between them.

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• Over 99.8% of solar system’s mass• Made mostly of H/He gas (plasma)• Converts 4 million tons of mass into energy each second

Sun

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• Made of metal and rock; large iron core • Desolate, cratered; long, tall, steep cliffs• Very hot and very cold: 425°C (day), –170°C (night)

Mercury

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• Nearly identical in size to Earth; surface hidden by clouds • Hellish conditions due to an extreme greenhouse effect• Even hotter than Mercury: 470°C, day and night

Venus

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• An oasis of life• The only surface liquid water in the solar system• A surprisingly large moon

Earth and Moon to scale

Earth

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• Looks almost Earth-like, but don’t go without a spacesuit!• Giant volcanoes, a huge canyon, polar caps, and more• Water flowed in the distant past; could there have been

life?

Mars

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• Much farther from Sun than inner planets

• Mostly H/He; no solid surface

• 300 times more massive than Earth

• Many moons, rings

Jupiter

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Jupiter’s moons can be as interesting as planets themselves, especially Jupiter’s four Galilean moons.

• Io (shown here): Active volcanoes all over• Europa: Possible subsurface ocean• Ganymede: Largest moon in solar system• Callisto: A large, cratered “ice ball” 15


Saturn

• Giant and gaseous like Jupiter• Spectacular rings• Many moons, including cloudy Titan• Cassini spacecraft currently studying it 16


Rings are NOT solid; they are made of countless small chunks of ice and rock, each orbiting like a tiny moon.

Artist’s conception of Saturn’s rings17


• Smaller than Jupiter/Saturn; much larger than Earth

• Made of H/He gas and hydrogen compounds (H2O, NH3, CH4)

• Extreme axis tilt• Moons and rings

Uranus

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• Similar to Uranus (except for axis tilt)

• Many moons (including Triton)

Neptune

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Pluto and Other Dwarf Planets

• Much smaller than other planets• Icy, comet-like composition• Pluto’s moon Charon is similar in size to Pluto

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What have we learned?

• What does the solar system look like?—Planets are tiny compared to the distances

between them.—Each world has its own character, but there

are many clear patterns.

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6.2 Clues to the Formation of Our Solar System


• What features of our solar system provide clues to how it formed?

• What theory best explains the features of our solar system?

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What features of our solar system provide clues to how it formed?

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Motion of Large Bodies

• All large bodies in the solar system orbit in the same direction and in nearly the same plane.

• Most also rotate in that direction.

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Two Major Planet Types

• Terrestrial planets are rocky, relatively small, and close to the Sun.

• Jovian planets are gaseous, larger, and farther from the Sun.

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Swarms of Smaller Bodies

• Many rocky asteroids and icy comets populate the solar system.

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Notable Exceptions

• Several exceptions to normal patterns need to be explained.

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What theory best explains the features of our solar system?

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According to the nebular theory, our solar system formed from a giant cloud of interstellar gas.

(nebula = cloud)

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• What features of our solar system provide clues to how it formed?—Motions of large bodies: All in same

direction and plane—Two major planet types: Terrestrial and

jovian—Swarms of small bodies: Asteroids and

comets—Notable exceptions: Rotation of Uranus,

Earth’s large moon, and so forth

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• What theory best explains the features of our solar system?—The nebular theory, which holds that our

solar system formed from a cloud of interstellar gas, explains the general features of our solar system.

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6.3 The Birth of the Solar System


• Where did the solar system come from?

• What caused the orderly patterns of motion in our solar system?

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Where did the solar system come from?

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Galactic Recycling

• Elements that formed planets were made in stars and then recycled through interstellar space.

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Evidence from Other Gas Clouds• We can see

stars forming in other interstellar gas clouds, lending support to the nebular theory.

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What caused the orderly patterns of motion in our solar system?

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Conservation of Angular Momentum

• The rotation speed of the cloud from which our solar system formed must have increased as the cloud contracted. 37


Rotation of a contracting cloud speeds up for the same reason a skater speeds up as she pulls in her arms.

Collapse of the Solar Nebula 38


• Collisions between particles in the cloud caused it to flatten into a disk.

Flattening

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Collisions between gas particles in a cloud gradually reduce random motions.

Formation of Circular Orbits

INSERT Formation_Circular_Orbits.jpg

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Collisions between gas particles also reduce up and down motions.

Why Does the Disk Flatten? 41


The spinning cloud flattens as it shrinks.

Formation of the Protoplanetary Disk 42


Disks Around Other Stars

• Observations of disks around other stars support the nebular hypothesis. 43


What have we learned?• Where did the solar system come from?

— Galactic recycling built the elements from which planets formed.

— We can observe stars forming in other gas clouds.

• What caused the orderly patterns of motion in our solar system?— The solar nebula spun faster as it contracted because

of conservation of angular momentum.— Collisions between gas particles then caused the

nebula to flatten into a disk.— We have observed such disks around newly forming

stars.44


6.4 The Formation of Planets

Our goals for learning:• Why are there two major types of planets?• Where did asteroids and comets come

from?• How do we explain the existence of our

Moon and other exceptions to the rules?• When did the planets form?

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Why are there two major types of planets?

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Inner parts of the disk are hotter than outer parts.

Rock can be solid at much higher temperatures than ice.

Temperature Distribution of the Disk and the Frost Line 47


Inside the frost line: Too hot for hydrogen compounds to form ices

Outside the frost line: Cold enough for ices to form 48


Formation of Terrestrial Planets

• Small particles of rock and metal were present inside the frost line.

• Planetesimals of rock and metal built up as these particles collided.

• Gravity eventually assembled these planetesimals into terrestrial planets.

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Tiny solid particles stick to form planetesimals.

Summary of the Condensates in the Protoplanetary Disk 50


Gravity draws planetesimals together to form planets.

This process of assembly is called accretion.

Summary of the Condensates in the Protoplanetary Disk 51


Accretion of Planetesimals

• Many smaller objects collected into just a few large ones.

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Formation of Jovian Planets

• Ice could also form small particles outside the frost line.

• Larger planetesimals and planets were able to form.

• The gravity of these larger planets was able to draw in surrounding H and He gases.

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The gravity of rock and ice in jovian planets draws in H and He gases.

Nebular Capture and the Formation of the Jovian Planets 54


Radiation and outflowing matter from the Sun—the solar wind— blew away the leftover gases.

The Solar Wind 55


Where did asteroids and comets come from?

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Asteroids and Comets

• Leftovers from the accretion process• Rocky asteroids inside frost line• Icy comets outside frost line

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Heavy Bombardment

• Leftover planetesimals bombarded other objects in the late stages of solar system formation.

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Origin of Earth’s Water• Water may

have come to Earth by way of icy planetesimals from the outer solar system.

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How do we explain the existence of our Moon and other exceptions to the rules?

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Captured Moons

• The unusual moons of some planets may be captured planetesimals.

Insert ECP6 Figure 6.22 with subcaptions

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Giant Impact

……then accreted into the Moon.then accreted into the Moon. 62


Odd Rotation

• Giant impacts might also explain the different rotation axes of some planets.

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Review of the nebular theory

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Thought Question

How would the solar system be different if the solar nebula had cooled with a temperature half its current value?

A. Jovian planets would have formed closer to the Sun.

B. There would be no asteroids.C. There would be no comets.D. Terrestrial planets would be larger.

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Thought Question

How would the solar system be different if the solar nebula had cooled with a temperature half its current value?

A. Jovian planets would have formed closer to the Sun.

B. There would be no asteroids.C. There would be no comets.D. Terrestrial planets would be larger.

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Thought Question

Which of these facts is NOT explained by the nebular theory?

A. There are two main types of planets: terrestrial and jovian.

B. Planets orbit in the same direction and plane.

C. Asteroids and comets exist.

D. There are four terrestrial and four jovian planets.

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Thought Question

Which of these facts is NOT explained by the nebular theory?

A. There are two main types of planets: terrestrial and jovian.

B. Planets orbit in the same direction and plane.C. Asteroids and comets exist.D. There are four terrestrial and four jovian

planets.

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When did the planets form?

• We cannot find the age of a planet, but we can find the ages of the rocks that make it up.

• We can determine the age of a rock through careful analysis of the proportions of various atoms and isotopes within it.

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Radioactive Decay

• Some isotopes decay into other nuclei.

• A half-life is the time for half the nuclei in a substance to decay.

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Thought Question

Suppose you find a rock originally made of potassium-40, half of which decays into argon-40 every 1.25 billion years. You open the rock and find 15 atoms of argon-40 for every atom of potassium-40. How long ago did the rock form?

A. 1.25 billion years agoB. 2.5 billion years agoC. 3.75 billion years agoD. 5 billion years ago

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Thought QuestionSuppose you find a rock originally made of potassium-40, half of which decays into argon-40 every 1.25 billion years. You open the rock and find 15 atoms of argon-40 for every atom of potassium-40. How long ago did the rock form?

A. 1.25 billion years agoB. 2.5 billion years agoC. 3.75 billion years agoD. 5 billion years ago

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Age dating of meteorites that are unchanged since they condensed and accreted tells us that the solar system is about 4.6 billion years old.

Dating the Solar System

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Dating the Solar System

• Radiometric dating tells us that the oldest moon rocks are 4.4 billion years old.

• The oldest meteorites are 4.55 billion years old.

• Planets probably formed 4.5 billion years ago.

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• Why are there two major types of planets?— Rock, metals, and ices condensed outside the

frost line, but only rock and metals condensed inside the frost line.

— Small solid particles collected into planetesimals that then accreted into planets.

— Planets inside the frost line were made of rock and metals.

— Additional ice particles outside the frost line made planets there more massive, and the gravity of these massive planets drew in H and He gases.

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What have we learned?• Where did asteroids and comets come from?

— They are leftover planetesimals, according to the nebular theory.

• How do we explain the existence of our Moon and other exceptions to the rules?— The bombardment of newly formed planets by

planetesimals may explain the exceptions.— Material torn from Earth’s crust by a giant impact

formed the Moon.• When did the planets form?

— Radiometric dating indicates that planets formed 4.5 billion years ago.

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6.5 Other Planetary Systems

Our goals for learning:• How do we detect planets around other

stars?• How do extrasolar planets compare with

those in our solar system?• Do we need to modify our theory of solar

system formation?

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How do we detect planets around other stars?

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Planet Detection

• Direct: Pictures or spectra of the planets themselves

• Indirect: Measurements of stellar properties revealing the effects of orbiting planets

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Gravitational Tugs

• The Sun and Jupiter orbit around their common center of mass.

• The Sun therefore wobbles around that center of mass with the same period as Jupiter.

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Gravitational Tugs

• Sun’s motion around solar system’s center of mass depends on tugs from all the planets.

• Astronomers who measured this motion around other stars could determine masses and orbits of all the planets.

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Astrometric Technique

• We can detect planets by measuring the change in a star’s position in the sky.

• However, these tiny motions are very difficult to measure (~0.001 arcsecond).

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Doppler Technique

• Measuring a star’s Doppler shift can tell us its motion toward and away from us.

• Current techniques can measure motions as small as 1 m/s (walking speed!).

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First Extrasolar Planet Detected

• Doppler shifts of star 51 Pegasi indirectly reveal planet with 4-day orbital period

• Short period means small orbital distance

• First extrasolar planet to be discovered (1995)

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First Extrasolar Planet Detected

• The planet around 51 Pegasi has a mass similar to Jupiter’s, despite its small orbital distance. 85


Thought Question

A. It has a planet orbiting at less than 1 AU.B. It has a planet orbiting at greater than 1 AU.C. It has a planet orbiting at exactly 1 AU.D. It has a planet, but we do not have enough

information to know its orbital distance.

Suppose you found a star with the same mass as the Sun moving back and forth with a period of 16 months. What could you conclude?

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Thought Question

A. It has a planet orbiting at less than 1 AU.B. It has a planet orbiting at greater than 1 AU.C. It has a planet orbiting at exactly 1 AU.D. It has a planet, but we do not have enough

information to know its orbital distance.

Suppose you found a star with the same mass as the Sun moving back and forth with a period of 16 months. What could you conclude?

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Transits and Eclipses

• A transit is when a planet crosses in front of a star.• The resulting eclipse reduces the star’s apparent brightness and

tells us the planet’s radius.• When there is no orbital tilt, an accurate measurement of planet

mass can be obtained.88


Direct Detection

• Special techniques for concentrating or eliminating bright starlight are enabling the direct detection of planets. 89


How do extrasolar planets compare with those in our solar system?

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Measurable Properties

• Orbital period, distance, and shape

• Planet mass, size, and density

• Composition

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Orbits of Extrasolar Planets

• Most of the detected planets have orbits smaller than Jupiter’s.

• Planets at greater distances are harder to detect with the Doppler technique.

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Orbits of Extrasolar Planets

• Most of the detected planets have greater mass than Jupiter.

• Planets with smaller masses are harder to detect with the Doppler technique.

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Planets: Common or Rare?

• More than one in ten stars examined so far have turned out to have planets.

• The others may still have smaller (Earth-sized) planets that cannot be detected using current techniques.

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Surprising Characteristics

• Some extrasolar planets have highly elliptical orbits.

• Some massive planets orbit very close to their stars: “hot Jupiters.”

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Hot Jupiters

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Do we need to modify our theory of solar system formation?

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Revisiting the Nebular Theory

• Nebular theory predicts that massive Jupiter-like planets should not form inside the frost line (at << 5 AU).

• The discovery of “hot Jupiters” has forced a reexamination of nebular theory.

• “Planetary migration” or gravitational encounters may explain “hot Jupiters.”

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Planetary Migration• A young planet’s

motion can create waves in a planet-forming disk.

• Models show that matter in these waves can tug on a planet, causing its orbit to migrate inward.

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Gravitational Encounters

• Close gravitational encounters between two massive planets can eject one planet while flinging the other into a highly elliptical orbit.

• Multiple close encounters with smaller planetesimals can also cause inward migration.

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Thought Question

What happens in a gravitational encounter that allows a planet’s orbit to move inward?

A. It transfers energy and angular momentum to another object.

B. The gravity of the other object forces the planet to move inward.

C. The planet gains mass from the other object, causing its gravitational pull to become stronger.

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Thought Question

What happens in a gravitational encounter that allows a planet’s orbit to move inward?A. It transfers energy and angular momentum to

another object.B. The gravity of the other object forces the planet

to move inward.C. The planet gains mass from the other object,

causing its gravitational pull to become stronger.

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Modifying the Nebular Theory

• Observations of extrasolar planets have shown that the nebular theory was incomplete.

• Effects such as planet migration and gravitational encounters might be more important than previously thought.

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What have we learned?• How do we detect planets around other

stars?— A star’s periodic motion (detected through

Doppler shifts) tells us about its planets.— Transiting planets periodically reduce a star’s

brightness.— Direct detection is possible if we can block the

star’s bright light.

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• How do extrasolar planets compare with those in our solar system?— Detected planets are all much more massive

than Earth.— Most have orbital distances smaller than

Jupiter’s, and have highly elliptical orbits.— “Hot Jupiters” have been found.

• Do we need to modify our theory of solar system formation?— Migration and encounters may play a larger

role than previously thought.

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copyright © 2012 pearson education, inc. chapter 6 formation of planetary systems: our solar system...

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