AST101 - Lecture 13

Terrestrial Planets

• Small

• Rocky

• Thin or no atmosphere

• Few moons

• Made from heavy/ elements 

• Mercury, Venus, Earth, Mars

• All close to the sun

Jovian Planets

• Saturn, Jupiter, Uranus, Neptune

• Large

• Liquid + gas

• Small rocky core

• Many moons

• Light elements

• Far from the Sun

T/F: Venus moves in prograde motion

False, Venus moves in retrograde motion

T/F: Uranus moves in prograde motion

False, Uranus moves in retrograde motion

Prograde

• Planets all orbit in the same direction

• Counterclockwise from the sun

• Sun and most of the planets rotate in the same direction

• Most of the massive moons of the planets orbit their planets in this same direction

• Venus and Uranus moves in retrograde motion

Properties of Solar System

• Almost everything (including moons) is spinning and orbiting in the same direction and moving in a flat plane

• There are weird exceptions to this (Venus, Uranus)

• Most orbits are circular

• Terrestrial planets are all near the sun

• Jovian planets are all farther out

• There are many asteroids and comets mostly located in three regions (Asteroid belt, Kuiper belt, oort Cloud)

Nebular Hypothesis

1. Force of gravity pulls a molecular cloud (made of gas and dust) together

2. Begins to collapse

3. Initial cloud will be slowly rotating in some random direction

   1. Angular momentum is conserved

   2. As it collapse it spins faster

4. The cloud begins to spin rapidly

5. Rotation ensured not all the material collapses to the center

Who Proposed the Nebular Hypothesis and When

Kant and Laplace in the 1700s

Abandoned in the early in 20th century

Widely accepted now

Formation of Planets

Small objects/debris form together to form planetesimals

Asteroid

Rocky left-over inner planetesimals

Comets

icy left-over outer planetesimals are

Half-Life

Time taken for ½ of the radioactive atoms to decay

How Old is the Solar System?

Around 4.55 billion years old (32.9% of the Age of the Universe)

How old is the Universe?

13.82 billion years

Why do Uranus/Venus move in retrograde motion?

Their retrograde motion can be explained by giant collisions in the early solar system

Where can/did terrestrial planets form?

• Planets form only from rocks and metal inside the frost line

• Each planet has its own disk, which then forms moons

Where can/did Jovian planets form?

• Outside the frost line, planets form from rocks, metal and ice.

• Their larger cores use gravity to accrete large gas envelopes

• Each planet has its own disk, w

Frost Line

• Disk of a star is 98% hydrogen and helium and 2% other atoms/molecules

• Inner parts of the disk are warmer than the outer parts

• Inside the frost line there Rocks, metal, gas

• Outside the frost line there Rocks, metal, ice, and gas