AST lec 12: Planet Formation

What is The Nebular Hypothesis for planetary formation?

Gravitational pull on a molecular cloud → flattened disc of gas + dust → small bits start to stick tg → gravity pulls planetesimals tg.

How does The Nebular Hypothesis explain why the planets orbit the Sun in the same direction it rotates?

The Earth and Sun were formed out of the same flat, spinning gas cloud and angular momentum is conserved.

What is the Frost Line?

The zone of the flat, spinning disc of gas and dust within which only metals can condense into planets, and outside of which most solids can condense.

Which planets formed inside the Frost Line?

Terrestrial Planets

How did the moons of the Jovian Planets form, and why do they mostly orbit in the same direction as planetary rotation?

Each planet had its own disk of spinning gas and dust, the particles of which condensed into many moons.

Terrestrial planets are thought to form when…

Bits of dust stick together to form larger and larger rocks.

Why do Jovian planets form only in the outer parts of the Solar System?

Because the temperature is lower at greater distance from the Sun.

What becomes of planetesimals in the inner VS outer Solar System that do not become planets?

Inner Solar System: rocky asteroids.

Outer Solar System: icy comets

When was the Heavy Bombardment Period? What was the result?

Early on in Solar System formation, there were large numbers of planetesimals and collisions were frequent.

It left the surfaces of airless bodies heavily cratered.

How can we determine the age of the Solar System?

Some materials are radioactive (= decay into other elements).

We determine the half-life of the material (how long it takes for it to decay).

By measuring the ratio of the 2 elements, we can determine how old the object is.

What 2 elements do we measure in ratio to calculate the approximate age of the Earth?

Potassium-40 and Argon-40.

How much older is the Universe than the Earth?

3 times.

How did the Moon form?

Early in Earth’s formation, a Mars-sized planetesimal collided with it, liquifying the planet. Most of Earth recombined, but a chunk remained in space and was captured in orbit. Both bodies solidified.

Why do Earth, Saturn, and Uranus have tipped orbits?

Likely giant collisions like the one that formed Earth’s Moon.

Why does Neptune’s Triton orbit it out of the Ecliptic Plane and backwards?

It was likely captured into Neptune’s orbital gravity, rather than being formed by a collision.