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Home , Planetary migration, Tug of war (astronomy)

110: SURVEY OF ASTRONOMY

4. Formation of Solar Systems

1. A Survey of the

2. The Solar System’s Early History

3. Other Planetary Systems The solar system’s rich and varied structure points to its formation from a gas cloud collapsing due to self- gravity some 4.5 billion years ago. In this proto-solar system, small grains of solid matter clumped together, eventually producing the we see today. Other stars with planets have now been observed; a wide variety of planetary systems can arise when stars form. 1. A SURVEY OF THE SOLAR SYSTEM

a. Overview of the solar system

b. A brief tour of the premises

c. Clues to its formation Overview: Structure

100 AU

S

20,000 AU 5 AU

Inner system: Outer system: : terrestrial planets, giant planets and . . , “KBOs”. Overview: Motion

All planets move in the same direction in nearly circular orbits; most rotate in the same direction as they orbit. Overview: Planetary Types

• rocky & metallic terrestrial planets (inner system) • hydrogen-rich jovian planets (outer system) A Brief Tour: The

• Over 99.9% of solar system’s mass • Largely H and He A Brief Tour:

• Large iron core and desolate rock crust A Brief Tour:

• ’s “twisted sister” • Extreme greenhouse effect A Brief Tour: Earth

• Liquid surface water!

• Complex and dynamic atmosphere

• Remarkably large A Brief Tour:

• Cold & dry, but water flowed long ago • Complex and interesting topography A Brief Tour:

• Largest gas-giant — no solid surface!

• Many different moons, some as big as planets A Brief Tour:

• Spectacular • , a moon with atmosphere A Brief Tour:

• Water and other hydrogen compounds

• Extreme axis tilt A Brief Tour:

• Most distant major planet

• Large moon with “backwards” orbit A Brief Tour: and Other “Icy Dwarfs”

• Tiny compared to other planets

• Ices of H2O & other H compounds

• Many still to be discovered! XXX (g/cm3) Clues to Formation: Motion

All planets and most moons orbit in the same direction and stay close to the same plane.

The sun and most planets also rotate in that direction. Conservation Laws:

An object with mass m moving in a circle of radius r with velocity v has angular momentum mvr.

If there is no torque (roughly speaking, “twisting” force) on the object, its angular momentum is conserved.

Wikipedia: Kepler’s Laws Clues to Formation: Two Types of Planets

Terrestrial Planets Jovian Planets small size & mass large size & mass high density low density

rock & metal H, He, H2O, CH4, NH3, . . . solid surface no solid surface few moons, no rings many moons & rings close to sun, warm far from sun, cold Clues to Formation: Small Objects

Asteroids: • small (< 1000 km) • rock & metal (?) • inner system

Dwarf planets: Comets: • small (~ 1000 km) • small (~ 10 km) • ice & rock • ice & rock • • Kuiper belt & Oort cloud

Dwarf planets: • small (~ 1000 km) • ice & rock • Kuiper belt Clues to Formation: Exceptions to the Rules

Rotation of Venus: • Reverse direction • Extremely slow Rotation of Uranus: • Axis tipped 98° Earth’s Moon: • Very large for our planet • Iron deficiency Neptune’s moon : • Reverse orbit direction A SURVEY OF THE SOLAR SYSTEM: REVIEW

1. Ordered motion of Solar System Orbits & spins are “fossils” of motion in early Solar System. 2. Two types of planets Terrestrial planets near Sun, jovian planets further away.

3. Numerous small objects Asteroids in the inner system, icy dwarfs & comets further out.

4. Exceptions to the rules Earth’s big Moon, and unusual spins and orbits of some objects. 2. THE SOLAR SYSTEM’S EARLY HISTORY

a. Collapse of the solar nebula

b. Planet formation: the frost line

c. The age of the solar system The Nebular Theory Collapse: Galactic Recycling

Our solar system formed from gas which had already been cycled through many generations of stars.

Each cycle increased the amount of “heavy” elements. Collapse: Birthplace of the Solar System

Hubble’s sharpest image of the Orion Nebula Hubble’s sharpest image of the Orion Nebula Collapse: From Cloud to Disk

1. A gas cloud starts to collapse due to its own gravity.

2. It spins faster and heats up as it collapses.

3. Vertical motions die out, leaving a spinning disk.

4. The solar system still spins in the same direction. Collapse: Angular Momentum and Energy

1. Angular momentum conservation causes the cloud to spin faster as it contracts: (rotation speed) ∝ 1 (cloud diameter) Collapse stops when the cloud spins at orbital speed.

2. Energy conservation causes the cloud to heat up:

potential kinetic thermal energy energy energy gas shocks 1. What would have happened if the gas cloud had been rotating a little faster to begin with?

A. The cloud would collapse even more before reaching orbital speed. B. The cloud would collapse a little less before reaching orbital speed. C. The cloud would fly apart instead of collapsing. D. The cloud would fall straight inward and never form a disk. 2. If the cloud collapsed further before forming a disk, would it be hotter or colder?

A. Hotter, because more gravitational energy would be released. B. Colder, because less gravitational energy would be released. C. Neither hotter nor colder. Collapse: Disks Around Other Stars

We can see disks around other stars, as expected if these stars formed from collapsing gas clouds. Planet Formation

At the end of the collapse phase, the solar nebula was a uniform mixture of different materials.

Some of these materials began condensing out of the gas. Planet Formation: The Frost Line

The disk was hot at the center, and cool further out.

Inside the frost line, only Outside, hydrogen compounds rocks & metals can condense. can also condense.

The frost line was between the present orbits of Mars and Jupiter — roughly 4 AU from the Sun. Planet Formation: Terrestrial Planets

1. Within the frost line, bits of rock and metal clumped together to make . 2. As the planetesimals grew, they became large enough to attract each other. 3. Finally, only a few planets were left. Planet Formation: Jovian Planets

1. Outside the frost line, icy planetesimals were very common, forming planets about 10 times the mass of Earth.

2. These planets attracted nearby gas, building up giant planets composed mostly of H and He.

3. The disks around these planets produced moons. Planet Formation: Asteroids and Comets

• “Leftovers” from early stages of planet formation • Asteroids form inside frost line, comets outside • Scattered by jovian planets into present orbits Planet Formation: Explaining the Exceptions

1. Giant impacts in early solar system: — explain rotation of Venus, Uranus — form Moon from collision debris

2. Satellite capture after near-miss:

— captured from belt — Triton captured from Kuiper belt 3. How would the solar system be different if the nebula had been cooler?

A. Jovian planets would form closer to the Sun. B. There would be no asteroids. C. There would be no comets. D. Terrestrial planets would be larger. 4. Which of these facts is not explained by the nebular theory?

A. There are two kinds of planets: terrestrial and jovian. B. All planets orbit in the same direction. C. Asteroids and comets are common. D. There are four terrestrial and four jovian planets. The Age of the Solar System

Radioactive elements decay into stable ones; e.g., 40K → 40Ar + e+ (Potassium-40) (Argon-40) (positron)

The rate of decay is fixed by the element’s half-life, the time for 50% to decay; for 40K, this time is 1.25 Gyr (1 Gyr = 1 billion years). Rocks contain no 40Ar when they form; by measuring the ratio of 40Ar to 40K, the rock’s age can be found. 5. If a mineral has 15 40Ar atoms for every 40K atom, how old is it?

A. about 3 Gyr B. about 4 Gyr C. about 5 Gyr D. about 6 Gyr E. about 7 Gyr The Age of the Solar System: Dating Rocks

• The oldest Earth minerals are 4.4 Gyr old.

• The oldest Moon rocks are also 4.4 Gyr old.

The Cartoon History of the Universe • The oldest are 4.55 Gyr old; this is how long ago minerals started condensing in the disk. THE SOLAR SYSTEM’S EARLY HISTORY: SUMMARY

a. Collapse of the solar nebula

b. Planet formation: the frost line

c. The age of the solar system 3. OTHER PLANETARY SYSTEMS

a. How to find ‘em

b. What we find

c. What it means How To Find Other Planetary Systems

1. The Doppler Technique — measure effect of planet on motion of star — can detect systems with multiple planets 2. Transits and Eclipses — measure dimming of star’s light by planet — can be done using small telescopes 3. Direct Detection — seeing is believing, but hard to do — need for accurate analysis of planetary surfaces The Doppler Method: Gravitational Tug-Of-War

As a planet orbits, the star The combined effect of eight must move slightly in response planets (mostly J. & S. ) makes (Newton’s 3rd law). the Sun “dance around”.

These motions are small — how can we detect them? The Doppler Shift

Doppler Effect Doppler Effect

A stationary source sends If the source is moving, out waves of the same the waves bunch up wavelength in all ahead of its motion, and directions. spread out behind. The Doppler Shift: Light

We get a similar effect with light. The change in wavelength λ depends on the source’s velocity v red-shift blue-shift toward or away from us:

λ - λ v shift rest = λrest c where λshift is the observed (shifted) wavelength, λrest is the wavelength with the source at rest, and c is the speed of light. The Doppler Shift: Astronomy Applications

1. All lines in a spectrum shift by same amount:

Hydrogen lines in lab (no shift).

Shift to red — star receding.

Shift to blue — star approaching. 2. No shift from sideways motion. The Doppler Method

A planet orbiting a star induces alternating red and blue shifts in the star’s spectrum (unless orbit is face-on).

51 Pegasi

These tiny shifts can be used to find the planet’s mass and the properties of its orbit. Transits and Eclipses

A star with a planet in an edge-on orbit dims slightly every time the planet crosses (transits) its face.

HD209458

Pegasi

Half an orbit later, there’s a slight drop in the combined brightness as the star hides (eclipses) the planet. Direct Detection

Hubble Directly Observes Planet Orbiting Fomalhult What We Find: Orbits

Many planets found so far orbit very close to their central stars. Some orbit in only a few days! Many have very elliptical orbits.

These systems are very different from our solar system! What We Find: Masses and Orbital Periods

Most planets found Hot so far are even more 10 massive than Jupiter. 1 ) Transit and doppler J (M methods tend to find M planets near stars. 0.1

direct detection

0.01 • Systems like ours are • doppler method • transit method hard to detect. 0.01 0.1 1 10 100 1000 10000 P (yr) Wikipedia: Extrasolar planet What We Find: “Hot Jupiters”

Many known because they are easy to find — they may not be especially common. What it Means

1. Are Earth-like planets rare or common? — hot (or warm) jupiters would disrupt Earth’s orbit — many stars do not have hot (or warm) jupiters — not yet possible to detect Earth-like planets

2. Do Hot Jupiters imply the Nebular theory is wrong? — jovian planets cannot form inside frost line — planetary migration may explain this puzzle Planet Migration

A planet embedded in a disk around a star can excite spiral waves — this process robs the planet of angular momentum, causing it to spiral inward. Planet Migration

1. Can explain hot jupiters and eccentric orbits — migration can move planets very close to star — encounters between planets disturb orbits

2. Why didn’t this happen in our solar system? — disk cleared by Sun’s wind or external effects — some migration may be needed to form Oort cloud