AST 111 Lecture 15

Formation of the Solar System

Proplyd

Proplyds

Proplyds

Past vs. Present

• A formation theory must be consistent with:– Motion of the planets– Two types of planets (terrestrial and jovian)– Many asteroids and comets

• Can test formation theories!– We can observe forming solar systems

First Theories

• Nebular hypothesis:– Solar System formed from collapse of interstellar

gas cloud called Solar Nebula– This theory is now accepted

• Close encounter hypothesis:– Near collision between the Sun and another star

pulled blobs of gas from the Sun that formed planets

Initial Solar System Formation

• Cloud starts as large and diffuse– Slowly rotating– Cold

• Gravity alone may or may not have pulled it together– Shock from nearby

supernova?

Initial Solar System Formation

• The cloud begins to collapse– Gravitational potential

energy converted to thermal energy

• Temperature and density highest at center– The sun forms there

Initial Solar System Formation

• Conservation of angular momentum causes it to spin faster– Like an ice skater pulling

her arms in

Initial Solar System Formation

• It flattens into a disk

Initial Solar System Formation

• Spinning, flattened disk (200 AU)• Mass and temperature greatest at center

Observations

• This is consistent with the planets:– Orbiting in the same direction– (Mostly) rotating in the same direction

• The heating caused by the cloud’s collapse should emit infrared radiation

– We have seen this!

Observations

• The star AU Microscopii has a debris disk:

Solar Nebula

• Spinning disk was about 200 AU in diameter

• Constant mixing of gas should ensure uniform composition of 98% H and He with 2% heavier elements

Early Formation of Planets

• Planets had to start as “seeds”

– Clumps of matter which attracted other clumps of matter, and grew

– One critical factor caused Jovian seeds to differ from terrestrial seeds

Condensation

• When something condenses, it turns from a gas to either a solid or liquid

– Do things tend to condense at higher or lower temperatures?

• An element has to condense in order to stick to a planet seed

Solar Nebula: Composition

• Three chemical components:

– Hydrogen and Helium: 98%

– Hydrogen Compounds (ICES): 1.4%

– Rock and metal: 0.6%

Condensation Temperatures

• In the Solar Nebula:

– One does not condense

– One condenses at low temperature

– One condenses at high temperature

• Which ones?

Early Formation of Planets

• Metals and rock condense at a higher temperature than ices

• Ices (hydrogen compounds) much more abundant than rock and metal

Early Formation of Planets

• Near (what is now) Mercury’s orbit:

– Temperatures dropped below the condensation point for rocks and metals

– But rocks and metals are the least abundant material in the solar system!

Early Formation of Planets

• The Frost Line is the point where temperatures allow ices to condense

• Located between (what is now) Mars’ and Jupiter’s orbit

Early Formation of Planets

Beyond the Frost Line, a whole newabundance of material can condense

onto the planet seed.

Early Formation of Planets

• Terrestrial planets started from SMALL seeds– Rock and metal

• Jovian planets started from LARGE seeds– Ice (H-compounds) – Also rock and metal

Terrestrial Planet Formation

• Very little rock and metal– Terrestrial planets are smaller

• Accretion:– Particles moving in mostly the same direction– They gently smushed together

• Once large enough, gravity could take over– Large “boulders” called planetesimals

Terrestrial Planet Formation

• As planetesimals grew larger, they hit more stuff

• Eventually, they hit all the stuff they could hit– They would sometimes hit each other– Only the largest could survive

Terrestrial Planet Formation

• Ends with a large molten mass

• Cools and solidifies

Jovian Planet Formation

• Given all the ice (from H-compounds), why aren’t the jovian planets large ice spheres?

– Jovian planets started as ice spheres.

– Moons of jovian planets are ice spheres.

Jovian Planet FormationRecall that 98% of the forming Solar System

was H and He.

The larger planet seeds could capture H and He. Due to their larger mass and colder temperatures,they could hold onto it. Terrestrial planets could

not.

This is why the Jovian planets are “gas giants” (and not ice spheres).

Moons

• Recall:

– Moons of terrestrial planets “out of place”

– Jovian planets have large moons that follow the rotation of their parent planets

Jovian Moon Formation

• Jovian planets formed their own accretion disks– Created moons,

captured smaller moons

• The moons aren’t gas giants– Too small to hang onto H

and He

Terrestrial Planet Moons

• Terrestrial planets didn’t have “mini-solar systems”

• They didn’t form moons.

• Mars stole them from the nearby asteroid belt

Terrestrial Planet Moons

• Earth’s moon formed via catastrophic collision

The End of Planet Formation

• So… what about the leftover material?

• When the Sun “turned on”, solar wind swept leftover material away

• Be thankful for the timing of this.– Too early: planetary material would have been swept

away– Too late: cooling could have turned terrestrial planets

into icy worlds (condensation of H-compounds)

The Aftermath of Planet Formation

• Some planetesimals remained after solar wind kicked in– These are the asteroids and comets

• Main asteroids (between Mars and Jupiter):– Why are they rocky?

• Kuiper belt (past Neptune):– Why are they icy?