Solar System Formation

And the Stuff that was Left Over

Things we need to explain

• The similarities in the motions and orbits of

the objects in the solar system

• Dichotomy in planetary morphology and

composition

• Asteroids and comets

• Peculiarities

Orbital Patterns

• All planets orbit the sun in the same direction

• Orbits nearly circular and in same plane

• Nearly all rotate the same direction as sun

• Satellites have properties similar to their host planets

Morphology Dichotomy

• Located in inner solar

system

• Small and dense

• Rocks and metals

• Few satellites

• No rings

• Located in outer solar

system

• Large and low density

• Lots of H compounds

• Lots of satellites

• Rings

Terrestrial Planets Jovian Planets

Peculiarities

• Uranus rotates on its side

• Venus rotates opposite to

its orbital motion

• Earth has one of the

largest satellites in inner

solar system

– Other inner planets don’t

have satellites or very small

ones

What theory can explain all

this?

The Nebular Theory

Orbital Motions

• Heating – because of conservation of energy,

as nebula contracts, it heats up

• Spinning – because of conservation of angular

momentum, as nebula contracts, it spins faster

and faster

• Flattening – due to collisions

Making Planets

• Start with some seed

• Accretion – the process of more and more material being added to the seed

• Gravity attracts more stuff

– Inside frost line: all heavy elements

– Outside frost line: Hydrogen compounds can condense

Clearing out the Nebula

• Young sun lights up and starts pouring out

radiation and particles

• Radiation and solar wind blow away the

rest of the H and He

• Clearing out the gas stops the planet from

accreting more material

• Planets are now stuck with what they’ve

got

Peculiarities

When did the Solar System Form?

• Radiometric dating shows

us the solar system is

about 4.5 billion years old

• Look at ratios of

radioactive isotopes

• Half life – the length of

time required for half of

the material to decay

Asteroids, Meteors, and

Comets

Oh My!

What’s the Difference?

• Asteroid – Rocky leftover planetesimals that never made it into a

planet

• Meteor – Flash of light in Earth’s atmosphere

– (The weather person is a meteorologist)

• Comet – Similar to asteroids but made of ice

– Where were they formed

• Dwarf Planet – Big enough to be round

– Not big enough to clear its orbit

Asteroids

• Small rocky

planetesimals

• Not big enough to

have enough gravity

to make them round

• Craters show us that

they have histories

similar to planets

Asteroid Belt

• Belt between Mars

and Jupiter where

most asteroids live

• Why is it there?

Asteroid Belt

• Orbital Resonances

with Jupiter

– Creates Kirkwood

gaps in belt

– Prevented asteroids

from ever coming

together and forming a

planet

Meteors

• Meteor – flash of light caused by

something entering Earth’s atmosphere

• Meteoroid – the object in Earth’s

atmosphere

• Meteorite – such objects once they have

reached the ground

Meteorites

• Primitive Meteorites

– Unchanged since the

formation of the solar

system

• Processed Meteorites

– From the core of a

shattered asteroid

– Often mostly Fe

Comets

Structure

Interior

• Nucleus

– Main chunk of ice from

which the comet is

made

• Coma

– Cloud of gas around

nucleus caused by

sun’s heating of the

ice

A Tale of Two Tails

• Plasma Tail – UV light from sun

ionizes gas in coma

– Solar wind pushes ions away from sun

• Dust Tail – Dust-sized particles

unaffected by solar wind

– Pushed away by radiation pressure

Origins

• Oort Cloud – Goes out to 50,000 AU

– Contains trillions of comets

• Kuiper Belt – 30-50 AU

• Heavy bombardment

• Comets get flung way out by big things like Jupiter

Kuiper Belt Objects

Fate

• Ice binds comet

together

• When all the ice is

melted away, the

comet might

disintegrate

Fate

Tunguska Video

Exoplanets

Detection

• Direct Detection – Images of the planet

itself

• Indirect Detection – precise

measurements of the host star that tell us

a planet must be orbiting it

Direct Detection

• Extremely difficult

– Star is usually too

bright

– Need outstanding

angular resolution

• Fomalhaut and its

planet seen by HST

Indirect Methods

Gravitational Wobble:

Astrometric Technique

Doppler Technique

Transits

• Planet travels in front of star, blocking

some of stars light, which we can measure

• Movie