Chapter 6Formation of Planetary SystemsOur Solar System and Beyond

The solar system exhibits clear patterns of composition andmotion.

• Over 99.9% of solar system’s mass• Made mostly of H/He gas (plasma)• Converts 4 million tons of mass into energy each second

Sun

• 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

• 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

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

Earth andMoon to scale

Earth

• Looks almost Earth-like• Giant volcanoes, a huge canyon, polar caps, and more• Water flowed in the distant past

Mars

• Much fartherfrom Sun thaninner planets

• Mostly H/He;no solid surface

• 300 times moremassive thanEarth

• Many moons,rings

Jupiter

Jupiter’s moonscan be asinteresting asplanetsthemselves,especiallyJupiter’s fourGalilean moons

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

Saturn

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

Rings areNOT solid;they are madeof countlesssmall chunksof ice androck, eachorbiting like atiny moon.

Artist’s conception

The Rings of Saturn

• Smaller thanJupiter/Saturn;much larger thanEarth

• Made of H/He gasand hydrogencompounds (H2O,NH3, CH4)

• Extreme axis tilt• Moons and rings

Uranus

• Similar to Uranus(except for axistilt)

• Many moons(including Triton)

Neptune

Pluto

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

Which planets have a rocky,relatively dense composition?

1. Jupiter, Saturn, Earth,and Mars

2. Uranus, Neptune,Earth, and Mars

3. Jupiter, Saturn,Uranus, and Neptune

4. Mercury, Venus,Earth, and Mars

What features of our solar systemprovide clues to how it formed?

Motion of Large Bodies• All large bodies

in the solarsystem orbit inthe samedirection and innearly the sameplane.

• Most also rotatein that direction.

Two Major Planet Types

Swarms of Smaller Bodies

Notable Exceptions

What theory best explains thefeatures of our solar system?

According to thenebular theory, oursolar system formedfrom a giant cloud ofinterstellar gas.

(nebula = cloud)

Galactic Recycling• Elements that

formedplanets weremade in starsand thenrecycledthroughinterstellarspace.

Evidence from Other Gas Clouds• We can see

stars formingin otherinterstellar gasclouds, lendingsupport to thenebular theory.

The Orion Nebula with Proplyds

Most of the solar system’s planets:

1. Are made of rocksand minerals

2. Are made of gas3. Revolve (orbit)

around the Sun in thesame direction

4. Rotate in the samedirection as they orbitthe Sun

5. 3 and 4

Why do we think the inner (terrestrial) planetsbecame more dense than the outer planets?

1. In the collapsing solarnebula, denser materialssank toward the center

2. The sun’s gravity pulleddenser materials toward thecenter

3. The inner nebula was sohot that only metals androcks were able tocondense

4. The rotating disk in whichthe planets formed spunlighter elements outwardby centrifugal force

What do we think the composition ofthe solar nebula was?

1. About half hydrogen andhelium, half heavierelements (iron, carbon,silicon, etc.)

2. About 98% hydrogenand helium, and 2%heavier elements

3. A less hydrogen andhelium, more heavierelements

Orbital and Rotational Properties of the Planets

Conservation ofAngular Momentum

Rotation of acontractingcloud speedsup for thesame reason askater speedsup as she pullsin her arms.

Collapse of the Solar Nebula

• Collisions betweenparticles in thecloud caused it toflatten into a disk.

Flattening

Collisionsbetween gasparticles in acloudgraduallyreduce randommotions.

Formation of Circular Orbits

Collisionsbetween gasparticles alsoreduce upand downmotions.

Why does the Disk Flatten?

The spinningcloudflattens as itshrinks.

Formation of the Protoplanetary Disk

Disks Around Other Stars

• Observations of disks around other starssupport the nebular hypothesis.

Why are there two major types ofplanets?

As gravitycauses thecloud tocontract, itheats up.

Conservationof Energy

Collapse of the Solar Nebula

Inner parts ofthe disk arehotter thanouter parts.

Rock can besolid at muchhighertemperaturesthan ice.

Temperature Distribution of the Disk and the Frost Line

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

Outside the frost line: Cold enough for ices to form

Fig 9.5

Tiny solidparticles stick toformplanetesimals.

Summary of the Condensates in the Protoplanetary Disk

Gravity drawsplanetesimalstogether to formplanets.

This process ofassemblyis calledaccretion.

Summary of the Condensates in the Protoplanetary Disk

Accretion of Planetesimals

• Many smaller objects collected into just afew large ones.

The gravity ofrock and ice injovian planetsdraws in H andHe gases.

Nebular Capture and the Formation of the Jovian Planets

Moons of jovian planets form in miniature disks.

Radiation andoutflowingmatter fromthe Sun —the solarwind — blewaway theleftovergases.

The Solar Wind

Asteroids and Comets

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

Heavy Bombardment• Leftover

planetesimalsbombardedother objectsin the latestages of solarsystemformation.

When the first solid bits in the solar nebula becamelarge enough to be called planetesimals, what began

to increase their growth rate?1. Gravity2. They were mostly

made of sticky stuff3. Electrical forces

(“static electricity”)4. Pressure5. Ice

Why could the Jovian planets growto be much larger than the terrestrial

planets?1. They were further from the

Sun and gravity was weaker2. They formed beyond the

frost line where ices cancondense so they includedhydrogen compounds

3. They were far enough fromthe Sun to escape the heavybombardment that batteredthe early solar system

Origin of Earth’s Water• Water may

have come toEarth by wayof icyplanetesimalsfrom the outersolar system.

How do we explain the existenceof our Moon and other exceptions

to the rules?

Captured Moons

• The unusual moons of some planets maybe captured planetesimals.

Giant Impact

Giant impact stripped matter from EarthGiant impact stripped matter from Earth’’s crusts crust

Stripped matter began to orbitStripped matter began to orbit

Then accreted into MoonThen accreted into Moon

Odd Rotation• Giant impacts

might alsoexplain thedifferentrotation axesof someplanets.

What is the “solar wind?”

1. Strong radiation thatcomes from the Sun

2. Similar to winds on earthbut faster and stronger

3. Similar to winds on earthbut less dense andweaker

4. Atoms and parts ofatoms ejected from theSun at high speed

What do we think happened to thesolar nebula after the planets

formed?1. The gas was all used up2. The rest of the gas

gradually drifted away3. The solar wind helped

blow the gas away4. The gas is still there–we

just can’t see it.

Where do asteroids come from?

1. A planet between Marsand Jupiter that broke up

2. They are escaped smallmoons

3. Leftover planetesimalsfrom the inner solarsystem

4. Leftover planetesimalsfrom the outer solarsystem

Where do comets come from?

1. A planet betweenMars and Jupiter thatbroke up

2. They are escapedsmall moons

3. Leftoverplanetesimals fromthe inner solar system

4. Leftoverplanetesimals fromthe outer solar system

How would the solar system be different ifthe solar nebula had cooled with a temperature

half its current value?1. Jovian planets would

have formed closer tothe Sun.

2. There would be noasteroids.

3. There would be nocomets.

4. Terrestrial planetswould be larger.

Which of these facts is NOTexplained by the nebular theory?

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

2. Planets orbit in the same direction andplane.

3. Asteroids and comets exist.4. There are four terrestrial and four jovian

planets.

When did the planets form?

• We cannot find the age of a planet, but wecan find the ages of the rocks that make itup.

• We can determine the age of a rockthrough careful analysis of the proportionsof various atoms and isotopes within it.

Radioactive Decay• Some isotopes

decay intoother nuclei.

• A half-life isthe time forhalf the nucleiin a substanceto decay.

Suppose you find a rock originally made of potassium-40,half of which decays into argon-40 every 1.25 billion years. Youopen the rock and find 15 atoms of argon-40 for every atom of

potassium-40. How long ago did the rock form?

1. 1.25 billion years ago2. 2.5 billion years ago3. 3.75 billion years ago4. 5 billion years ago

Age dating of meteoritesthat are unchanged sincethey condensed andaccreted tells us that thesolar system is about 4.6billion years old.

Dating the Solar System

Planet Detection

• Direct: Pictures or spectra of the planetsthemselves

• Indirect: Measurements of stellarproperties revealing the effects of orbitingplanets

Gravitational Tugs• The Sun and Jupiter

orbit around theircommon center ofmass.

• The Sun thereforewobbles around thatcenter of mass withthe same period asJupiter.

Stellar Motion due to Planetary Orbits

Gravitational Tugs• Sun’s motion around

solar system’s centerof mass depends ontugs from all theplanets.

• Astronomers whomeasured this motionaround other starscould determinemasses and orbits ofall the planets.

Astrometric Technique• We can detect planets

by measuring thechange in a star’sposition in the sky.

• However, these tinymotions are verydifficult to measure(~0.001 arcsecond).

Doppler Technique

Oscillation of a Star's Absorption Line

First Extrasolar Planet Detected• Doppler shifts of star

51 Pegasi indirectlyreveal planet with 4-day orbital period

• Short period meanssmall orbital distance

• First extrasolar planetto be discovered(1995)

First Extrasolar Planet Detected

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

How can the Doppler shift be used tosearch for planets around other stars?

1. Astronomers look for a periodic, red-then-blueshift in the spectrum of the planet

2. Astronomers look for a periodic, red-then-blueshift in the spectrum of the star

3. Astronomers look to see if the star “wobbles” inthe sky, shifting slightly back and forth

How do astronomers look for planetswhose orbits might cause them to pass infront of a star outside our solar system?

1. They look for a small black dot passing in frontof the star

2. The look to see if the star’s position shifts or“wobbles” slightly in the sky

3. The measure the star’s brightness, and look forperiodic dimming (“transits”)

Suppose you found a star with the same mass asthe Sun moving back and forth with a period of

16 months. What could you conclude?1. It has a planet orbiting at less than 1 AU.2. It has a planet orbiting at greater than 1 AU.3. It has a planet orbiting at exactly 1 AU.4. It has a planet, but we do not have enough

information to know its orbital distance.

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.

Planetary Transits

Direct Detection

• Special techniques for concentrating or eliminatingbright starlight are enabling the direct detection ofplanets.

Measurable Properties

• Orbital period, distance, and shape• Planet mass, size, and density• Composition

Orbits of Extrasolar Planets• Most of the detected

planets have greatermass than Jupiter.

• Planets with smallermasses are harder todetect with theDoppler technique.

Planets: Common or Rare?

• One in ten stars examined so far have turnedout to have planets.

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

Surprising Characteristics

• Some extrasolar planets have highlyelliptical orbits.

• Some massive planets orbit very close totheir stars: “Hot Jupiters.”

Hot Jupiters

Revisiting the Nebular Theory

• Nebular theory predicts that massiveJupiter-like planets should not form insidethe frost line (at << 5 AU).

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

• “Planetary migration” or gravitationalencounters may explain “hot Jupiters.”

Planetary Migration• A young planet’s

motion can createwaves in a planet-forming disk.

• Models show thatmatter in these wavescan tug on a planet,causing its orbit tomigrate inward.

Gravitational Encounters

• Close gravitational encounters between twomassive planets can eject one planet whileflinging the other into a highly ellipticalorbit.

• Multiple close encounters with smallerplanetesimals can also cause inwardmigration.

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

1. It transfers energy and angular momentum toanother object.

2. The gravity of the other object forces the planetto move inward.

3. It gains mass from the other object, causing itsgravitational pull to become stronger.

Modifying the Nebular Theory

• Observations of extrasolar planets haveshown that the nebular theory wasincomplete.

• Effects like planet migration andgravitational encounters might be moreimportant than previously thought.