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Chapter 8 Formation of the Agenda • Announce: – Transit – Part 2 of Projects due next Thursday • Ch. 8—Formation of the Solar System • Philip on The Physics of Trek • Radiometric Dating Lab

What properties of our solar 8.1 The Search for Origins system must a formation theory explain? • What properties of our solar system must a 1. Patterns of motion of the large bodies formation theory explain? • Orbit in same direction and plane • What theory best explains the features of 2. Existence of two types of our solar system? • Terrestrial and Jovian 3. Existence of smaller bodies • and 4. Notable exceptions to usual patterns • of , ’s , etc.

What theory best explains the Close Encounter Hypothesis features of our solar system?

• The nebular theory states that our solar system • A rival idea proposed that the planets formed from the of a giant formed from debris torn off the by a interstellar gas cloud—the solar (Nebula is the Latin word for cloud) close encounter with another star. • Kant and Laplace proposed the nebular • That hypothesis could not explain hypothesis over two centuries ago observed motions and types of planets. • A large amount of evidence now supports this idea

1 Where did the solar system come 8.2 The Birth of the Solar System from?

• Where did the solar system come from? • What caused the orderly patterns of motion in our solar system?

Galactic Recycling Evidence from Other Gas Clouds

• Elements that • We can see formed forming planets were in other made in stars interstellar and then gas clouds, recycled lending through support to the interstellar nebular theory space

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

• Angular momentum = r * m * v

• Rotation speed increased as the cloud contracted…v gets big when r gets small

2 Flattening Disks around Other Stars

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

• Observations of disks around other stars support the

8.3 The Formation of Planets Why are there two types of ?

• Why are there two types of planets? • How did terrestrial planets form? • How did jovian planets form? • What ended the era of planet formation?

Conservation of Energy Inner parts of disk are hotter than outer parts. As causes cloud can be to contract, it solid at much heats up higher than ice.

3 How did terrestrial planets form?

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

Fig 9.5 • of rock and metal built up as these particles collided Inside the frost line : Too hot for compounds to form ices. • Gravity eventually assembled these planetesimals into terrestrial planets Outside the frost line : Cold enough for ices to form.

Tiny solid particles stick to form planetesimals. Gravity draws planetesimals together to form planets

This process of assembly is called

Accretion of Planetesimals How did jovian planets form?

• Ice could also form small particles outside the frost line. • Larger planetesimals and planets were able to form. • Gravity of these larger planets was able to draw in surrounding H and He gases. • Many smaller objects collected into just a few large ones

4 Gravity of rock and ice in jovian planets draws in H and He gases

Moons of jovian planets form in miniature disks

What ended the era of planet formation? Outflowing matter from the Sun -- the solar wind -- blew away the leftover gases

What have we learned? Solar Rotation • Why are there two types of planets? – Only rock and condensed inside the frost line • In nebular theory, – Rock, metals, and ices condensed outside the frost line young Sun was • How did the terrestrial planets form? spinning much faster than now – Rock and metals collected into Planetesimals – Planetesimals then accreted into planets • Friction between solar magnetic • How did the jovian planets form? field and solar – Additional ice particles outside frost line made planets there nebular probably more massive slowed the – Gravity of these massive planets drew in H, He gases rotation over • What ended the era of planet formation? time.. magnetic – Solar wind blew away remaining gases braking – Magnetic fields in early solar wind helped reduce Sun’s rotation rate

5 Where did asteroids and comets 8.4 The Aftermath of Planet Formation come from?

• Where did asteroids and comets come from? • How do we explain “exceptions to the rules”? • How do we explain the existence of Earth’s moon? • Was our solar system destined to be?

Asteroids and Comets How do we explain “exceptions to the rules”?

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

Heavy Bombardment Origin of Earth’s Water

• Leftover • Water may planetesimals have come to bombarded Earth by way other objects of icy in the late planetesimals stages of solar from outer system solar system formation

6 Captured How do we explain the existence of Earth’s moon?

• Unusual moons of some planets may be captured planetesimals

Giant Impact Odd Rotation

Giant impact stripped matter from Earth ’s crust • Giant impacts might also Stripped matter began to orbit explain the different rotation axes of some planets Then accreted into Moon

Thought Question Thought Question

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

a) Jovian planets would have formed closer to Sun a) Jovian planets would have formed closer to Sun b) There would be no asteroids b) There would be no asteroids c) There would be no comets c) There would be no comets d) Terrestrial planets would be larger d) Terrestrial planets would be larger

7 Was our solar system destined to be? Thought Question Which of these facts is NOT • Formation of explained by the nebular theory? planets in the solar nebula a) There are two main types of planets: seems inevitable terrestrial and jovian. • But details of b) Planets orbit in same direction and plane. individual c) Existence of asteroids and comets. planets could have been d) Number of planets of each type (4 different terrestrial and 4 jovian).

Thought Question Which of these facts is NOT 8.5 The Age of the Solar System explained by the nebular theory? • How does radioactivity reveal an object’s a) There are two main types of planets: age? terrestrial and jovian. • When did the planets form? b) Planets orbit in same direction and plane. c) Existence of asteroids and comets. d) Number of planets of each type (4 terrestrial and 4 jovian).

How does radioactivity reveal an Radioactive Decay object’s age? • Some isotopes decay into other nuclei • A half- is the time for half the nuclei in a substance to decay

8 Thought Question Thought Question

Suppose you find a rock originally made of Suppose you find a rock originally made of potassium-40, half of which decays into argon-40 potassium-40, half of which decays into argon-40 every 1.25 billion years. You open the rock and find every 1.25 billion years. You open the rock and find 15 atoms of argon-40 for every atom of potassium-40. 15 atoms of argon-40 for every atom of potassium-40. How long ago did the rock form? How long ago did the rock form? a) 1.25 billion years ago a) 1.25 billion years ago b) 2.5 billion years ago b) 2.5 billion years ago c) 3.75 billion years ago c) 3.75 billion years ago d) 5 billion years ago d) 5 billion years ago

When did the planets form?

• Radiometric dating tells us that oldest moon rocks are 4.4 billion years old • Oldest are 4.55 billion years old Final Thoughts • Planets probably formed 4.5 billion years ago

Physics in the Theory Physical Theories

• Conservation of energy —collapse heats up • Here we had a choice of two competing theories: • Conservation of ang. Momentum —regular – One relied less on improbable events – One supported by evidence (other similar systems motion of large bodies observed) • Theory of gravity —clumping, formation of – One accommodated all the features planets • Started with simple hypothesis: nebula • Temperature/Phase Transition —difference – Worked out consequences to explain features between Terrestrial and Jovian planets – Added/modified: impacts create our moon, tilt Uranus • Still open to other theories

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