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The Solar SystemOverview and origins

The whole story of comets and planets and the production of the world is founded upon such poor and trifling grounds, that I have

often wondered how ingenious men could spend all pains in making such fancies hang together. —Christian Huygens

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! The Sun (99.85% by mass)

! Planets (0.14%)

! Moons (satellites)

! Comets (0.01%)

! Kuiper Belt Objects (KPOs)

! Asteroids

! Meteoroids & dust

The Solar System

Theories of the Origin of the Solar System

• Close encounter theory (Chamberlin/Moulton Theory)

• Another star passed by the Sun.

• Solar Nebula Theory (Condensation Theory)

• The Sun and planets formed together from a spinning cloud of gas—the Solar Nebula

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Are We Alone?

• Close Encounter Theory:

• Only few stars have planets.

• The Solar Nebula Theory

• Most if not all stars have planets.

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2 Kinds of Origin Theories

• Catastrophic theories:

• unusual, rare, violent processes

• Example: floods

• Evolutionary theories:

• slow, gradual, commonplace processes

• Example: erosion

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Catastrophic or Evolutionary?

! Classify the following theories as catastropic or evolutionary.

1. The Universe began with a Big Bang. 2. The Moon formed after another planet collided

with the Earth. 3. Life developed by gradual change of species. 4. The Moon formed alongside the Earth as a sister

planet. 5. The Universe has always been in existence and

has always looked the same.6

The Scientific Method

Essential idea: the interplay of theory and observation

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! motions of planets

! angular momentum (spin) of the planets & sun

! composition of planets and meteoroids

! age of planets and meteoroids and the Sun

! observations of other solar systems

Clues to the Origin of the Solar System

Predictions of Motions• Close Encounter Theory:

• The planets move in the same direction.

• Most of the angular momentum (motion) will end up in the planets as the planet's orbital motion.

• The Solar Nebula Theory

• The planets move in the same direction.

• Most of the angular momentum will end up in the Sun as the Sun's rotation.

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Motions of the Planets

• Students are discussing the motions of the planets. Which students do you agree with?

John: All the planets move in the same direction.

Gina: The orbits of the planets are tilted in different ways.

Jorge: No, most of the orbits aren't tilted.

Helen: No, it's the axes that are tilted randomly.

Pedro: Actually, most of the axes are tilted only a small amount, but there are a couple of exceptions.

Beatriz: The orbits of the planets are all circles.

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! The planets move in the same plane: the ecliptic plane

! Their orbits are nearly circular.

! The planets orbit counter-clockwise, looking down on the north pole (eastward).

! Most planets rotate (spin) counter-clockwise.

Motions of the planets

No Good Theory?

"The nebular hypothesis had to be discarded...If the Sun had ever been spinning sufficiently fast in nebular form to eject the matter now in the planets, its present velocity of rotation would be at least 50 times its observed value...

All variations of the encounter theory assume that matter was torn out of the sun by a passing star...[but] it has recently been demonstrated that...matter drawn forcibly out of a star cannot condense directly into planets...

Perhaps the origin of the solar system is one of the ultimates that we shall never know."

— Lyman Spitzer Jr.12

A New Idea

• The Sun's magnetic field slowed down its rotation.

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Magnetic Braking

Composition Predictions

• Close Encounter Theory

• the planets are made of the same material as the Sun.

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• Solar Nebula Theory

• the planets are made of the same material as the Sun.

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! Inner planets are rocky.

Composition of the Planets

! Outer planets are icy or gaseous.

Trivia Question

The three states of matter are:

A) solid, liquid, gas

B) hydrogen, helium, iron

C) earth, metal, water

D)California, Oregon, Washington

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Condensation Sequence• Solar nebula contains gas and dust (rocky

bits and icy bits and iron filings)

• Young star vaporizes the dust.

• As the solar nebula cools, the dust returns to the solid state (condensation).

• Sequence: ironrockice

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4 Kinds of Worlds

• Terrestrial planets• iron core/rocky mantle

• Mass: 0.01 — 1 Earth

• Gas giants• liquid hydrogen

• Mass: 100-300 Earths

• Ice giants• liquid water

• Mass: 15 Earths

• "Icy moons"• rock core/ ice mantle 18

Age Predictions

• Close Encounter Theory:

• The Sun is older than the planets.

• The Solar Nebula Theory

• The Sun and planets are the same age.

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Radioactive Dating

• Uranium→lead at a predictable rate

• Half-life: the time it takes 1/2 of the uranium to turn into lead

• Half-life of uranium = 4.5 billion years

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Age of the Planets

• Earth: oldest rock fragments: 4.40 billion yr

• zircon crystals from Jack Hills, Western Australia

• Moon: 4.51 billion yr

• Meteorites: 4.567 billion yr

• Ca-Al inclusions in chondritic meteorites

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The Age of the Sun

• The Sun’s lifetime is 10 billion years.

• Stars get brighter with age.

• The Sun is halfway through its life.

• It is 4.5 ± 0.1 billion years old. (neutrino production rate + Uranium dating)

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Practice Question

! The Earth is about _______ as old as the Universe.

A. 1/10 B. 1/3 C. 2/3 D. About as old

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Other Solar Systems Dust Disks

• Stars <10 million years old are surrounded by disks of dust

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Predictions of Number of Planets in Galaxy

• Close Encounter Theory:

• The Sun is the one of few stars with planets.

• The Solar Nebula Theory

• Most if not all stars have planets.

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Detection of Planets around other stars

• Doppler shift method

• Eclipse method

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Other Solar Systems extrasolar planets

• Over 2000 planets around Sun-like stars are known.

• Most are between Earth & Neptune in size.

• Some are “hot Jupiters” — close to the star.

• Reason: Planet migration

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Earth Neptune Jupiter

Mass Distribution

• Planets more massive than 5 Jupiters are rare.29

Planet Migration in the Solar System

! J & S migrate inward to 1.5AU

! Mars moves out, kicking iron asteroids

! Saturn halted J's migration

! J & S kicked stony asteroids & carbon asteroids into belt

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What is a planet?

• Stars: >80 jupiters

• Brown dwarf stars: 15-80 jupiters

• “Brown dwarf desert” 25-50 jupiters

2Mass1207bmass: 5 jupiters

2Mass1207amass: 25 jupiters

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Practice Question

! The planets were formed from… A. rocks created in the Big Bang. B. particles condensed out of a cloud of gas. C. comets travelling thru space. D. stuff that came out of the Sun.

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The Nearest Exoplanet?

Star — Proxima• red dwarf in Alpha Centauri system • 4 LY away from SunPlanet

• 1/20 AU from star• 11 day period• Mass: 1.3 earth (minimum)

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Tau Ceti

• K star 11 LY away

• evidence of 5 planets, 2–6 Earth masses

• one is in the habitable zone

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Kepler 62

• 1200 LY away

• 62e & 62f: 1.5 x Earth size35

A Galaxy of Planets

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