The Origin of the Solar System

In the beginning, we started out looking like this, just a huge cloud of gas in space….

A rotating cloud of gas contracts and flattens….

to form a thin disk of gas and dust around the forming sun at the center.

Planets grow from gas and dust in the disk and are left behind when the disk clears.

Solar Nebula Theory

Dust Disks Around Stars

Very cold, low density disks observed (in the infrared) around stars.• Debris left over from comets or collisions between small

bodies (like asteroids).• Evidence of planetary systems which have already formed.

Dust Disks Around Stars

Very cold, low density disks observed (infrared) around stars.• Debris left over from comets or collisions between small

bodies (like asteroids).• Evidence of planetary systems which have already formed.

Dense disks of gas and dust observed (visible & radio) orbiting young stars.• Stellar systems are too young for planets to have formed yet.• Probable sites of ongoing planetary formation.

Examples of the Dust Disks around stars

Planet Building:the Condensation of Solids

Different materials condense from the gas cloud onto grains of elements (atoms of different gasses) at different temperatures.

The temperature due to the Sun varied with distance, so different materials condensed at different distances from the Sun.

Close to the Sun (1200-1500K): metal oxides and pure metals.

Farther out (~700-1200K): silicates and rocky material. Outer regions (~50-200K): ices (water, methane &

ammonia).

First Important step in Planet formation

Planet Building:the Formation of Planetesimals

Planetesimals – small bodies on the order of kilometers in size.

Condensation – atoms of gas hit dust grains and stick, adding mass to the particle.

Accretion – solid particles collide and stick to one another.

Once particles were massive enough, the settled down into a disk rotating around the protosun (its not quite a star yet).

Second Important step in Planet formation

Accretion Taking Place

Planet Building:the Growth of Protoplanets

As planetesimals grew, they became more massive, and therefore had stronger gravitational fields.

At a certain point, they were able to gravitationally hold an atmosphere.

Planet Building Planetesimals

contain both rock and metal.

A planet grows slowly from the uniform particles.

The resulting planet is of uniform composition.

Heat from radioactive decay causes differentiation.

The resulting planet has a metal core and low-density crust.

The first planetesimals contain mostly metals.

Later the planetesimals contain mostly rock.

A rock mantle forms around the iron core.

Heat from rapid formation can melt the planet.

The resulting planet has a metal core and low-density crust.

Planet-building processes

Dust grains stick together planetesimals Planetesimals stick together protoplanets

• Terrestrial: metallic / rocky but small – not much material

• Jovian: LOTS OF ICES, so quickly grew more massive When ~15 x Earth’s mass, gravity strong enough to attract lots of H/He

from solar nebula got really really big – but not dense

The planets eventually formed and differentiated into: Terrestrial vs.

Jovian Planets

Planetary ringsNo ring system

Farther away (from 5.2 to 30 AU)Close to the Sun (within 1.5 AU)

Many moons (over 60)Few satellites (3)

Faster rotators, differential rotationSlow rotators

Lighter elements, H and HeHeavy gas atmospheres (N2, O2, CO2)

Low density, huge gaseous atmospheresDense, rocky solid surfaces

Large and massiveSmall size, low mass

Jovian PlanetsTerrestrial Planets

Four stages of terrestrial planetary development

1. Differentiation• early planet was molten• heavy elements sunk, light elements rose• On Earth:

Dense metal core Less dense rocky mantle Low-density rocky crust

• (outgassing made primitive• atmosphere – more on that later)

2. Cratering• “heavy bombardment” period (first 0.5 billion years)• many impacts with rogue planetesimals• craters made (some huge)• On Earth:

many craters later covered by ocean or erased by erosion)

Four stages of terrestrial planetary development

Four stages of terrestrial planetary development

3. Flooding• lava from below• rain from atmosphere• On Earth:

made oceans

Four stages of terrestrial planetary development

4. Slow surface evolution• On Earth:

wind / water erosion plate tectonics: moving sections of crust

Clearing of solar nebula

Sun pushed away remaining debris• radiation pressure (light)• solar wind (particles)

Planets• swept up debris (craters)• ejected debris

Clearing the Solar Nebula

Around 4.6 billion years ago, the cloud of gas (the solar nebula) vanished due to four effects:• Radiation Pressure – light from the Sun exerted pressure on

the particles, pushing them out of the solar system.• The Solar Wind – a flow of atoms from the Sun’s upper

atmosphere also helped push particles out of the solar system.• As planets moved through their orbits, they swept up any

material in their paths.• Gravitational effects due to massive planets ejected particles

out of the solar system.

Stellar Debris Asteroids – rocky objects, mostly found between Mars

and Jupiter (in the Astreroid Belt ~ 2.8 AU).• Range in size up to 100 km in diameter.• Irregularly shaped, and cratered.• Remnants of planet formation.

Comets – small icy bodies (dirty snowballs).• Large elliptical orbits can bring comets in close to the Sun.• Recent studies suggest they are at least 50% rock and dust.

Meteoroids – specks of dust and rock which encounter Earth’s atmosphere and either burn up or fall to the ground. (Most only about 1g in mass).• Meteors – Flash across the sky as the meteoroid burns up.• Meteorite – remnant of a meteoroid that reaches the ground.

Up close and personal with an asteroid

A Comet

Stellar Motions Due to Planets

Technically, planets don’t orbit around a star, but around the common center of mass.

If planets are massive enough, the center of mass is not located at the center of the star, and the star orbits around this point as well.

This motion can be detected through Doppler shifts in the star’s spectrum.

Approximately the same age:

• Earth rocks• Moon rocks• Martian meteorites• asteroidal meteorites

~ 4.6 billion years Determined by radioactive

dating:• compare original amount of

radioactive element with an amount present now

“half-life”: time it takes for ½ of radioact. elem. to decay into non-radioact. elem.

Using Radioactive Dating, We’ve Discovered:

Explaining the Solar System

Terrestrial: small, dense, low mass Jovian: large, low density, high mass

• Condensation sequence and accretion Terrestrial: heavy gas atmospheres Jovian: lighter elements

• Jovian planets can gravitationally hold onto lighter gas Terrestrial: few satellites, no ring system Jovian: many satellites, planetary rings

• Jovian planets gravitationally stronger Existence of comets and asteroids

• Leftover material from the formation of the solar system.

Evidence of Extrasolar Planets

Two methods which suggest the existence of extrasolar planets:• Detection of dust which accompanies planets around stars.• Detection of stellar motions due to the presence of orbiting

planets.

Known Extrasolar Planets

Most known extrasolar planets are high-mass and low-period planets. (Selection effect)• High-mass: the greater the mass, the greater the wobble

produced in the star’s motion.• Low-period: the lower the period, the shorter the period over

which the wobble occurs. How can high-mass, low-period planets form?

• In dense disks, friction may slow the planet’s down, causing them to spiral inward.