Exam Grades Due to the rush to get exams back, I did not record credit for those who wrote out their own answers. If you did write out your own answer to a question, please bring me your exam so I can update your score.

Announcements Note: the exam questions were mostly taken from the quiz and assignments. It is therefore important to understand the questions, not just memorize the correct answer, since they will be asked differently. Assignments will have TWO parts: (1) an application or practice of what you have learned, (2) questions about the reading for the next lecture. Planet Assignment 3, due Wednesday Feb 25, –Part 1: Astronomy Place tutorial “Formation of the Solar System” –Part 2: Angel Planet Assignment 3

Confusions & Questions? Solar Nebula Theory - collapse & heating –What triggers the collapse –How does it collapse –What heats it up Asteroids - why are they between Mars & Jupiter Dating rocks, radioactive decay Doppler Effect

What is a Planet? An object that has a mass between that of Pluto and the nuclear reaction threshold of 13 M J and that forms in orbit around a star that generates energy by nuclear reactions.

The Solar System

Solar System Properties Patterns of Motion –All planets orbit the Sun in the same direction (as Sun spins), in nearly the same plane, & in nearly circular orbit Two types of planets –Terrestrial: small, rocky, inner –Jovian: large, liquid H, He & ices, outer Astroids & Comets –Comets dirty ice, Astroids rocks & iron Ubiquitous impact craters –All old solid surfaces Exceptions –Uranus rotates sideways, Venus rotates backwards

Patterns of Motion

Two Types of Planets Terrestrial: small rocky, iron inner Jovian: large H, He, ices outer

Asteroid & Comets Rocks & iron Dirty ice

Ubiquitous Craters Mars Callisto Moon

Solar System - How did it Form? What produced the organized motions? Planets formed as part of the formation of the Sun. Begin with region of higher density composed of H & He + traces of heavier elements in the space between the stars (called “interstellar cloud”) Gravity makes the “cloud” collapse –Triggered possibly by explosion of a nearby star

Collapse of Interstellar Cloud Angular Momentum Is conserved. As “cloud” collapses it spins faster.

Why are orbits in nearly same plane? Collapses more along poles than in equator. Forms a disk.

Why are orbits nearly circular?

Solar System - How did it Form? Why are there two types of planets? Planets formed as part of the formation of the Sun. As “cloud” collapses gets hotter (falling converts gravitational potential energy to kinetic energy). Inner regions hotter than outer (fell farther). Rocks & iron solid at high temperatures Ices solid only at low temperatures

Solidification Temperatures

Condensation Regions

Terrestrial & Jovian Planets Close to Sun - rocks and iron are solid Make rocky terrestrial planets Far from Sun - Ices (water, carbon dioxide, methane, ammonia) are solid Make larger proto-planets with Gravity large enough to attract H & He gas Make Jovian planets

Terrestrial & Jovian Planets

Gravity captures H & He gas

Humans (& Earth) are made of the flotsam and jetsam of the universe

Leftover planetesimals. Solar System - How did it Form? What are Comets & Asteroids?

Asteroids Left over rocky & iron planetesimals from formation of planets

Comets Left over icy planetesimals from formation of the Jovian planets

Solar System - How did it Form? Why ubiquitous impact craters? Remains of accretion of planetesimals.

Accretion of Planetesimals

Review of nebular theory

When did the Solar System Form? Measurements of decaying elements in meteorites indicate age of solar system: 4.6 billion years

Thought Question Suppose you bought a carton of special eggs -- each day half of the eggs in the carton hatch. You open the carton and find 9 chicks and 3 eggs. How long ago was it packed? A. Yesterday B. Two days ago C. Three days ago D. Last week

Example: Half of all potassium-40 nuclei decay into argon-40 every 1.25 billion years

What about Moons?

Moons of Jovian Planets form in miniature disks

Earth’s moon was probably created when a big planetesimal tore a chunk out of the newly forming Earth.

Collision simulation

Other Evidence? Other Planetary Systems Activity –1. Predict what you expect to be found about the properties of other planetary systems –2. We will then see what was actually found, and you will compare that with your predictions You will not be graded on the accuracy of your predictions, but on their relevance & completeness.

How are Extrasolar Planets Detected? By the Wobble they produce in their parent star. Detection limit: > 0.3 M Jupiter

Doppler Effect Analogy Suppose photons are busses on Grand River Ave. Busses come once every 10 min on average If you are walking East towards Okemos, will the busses heading west from Okemos pass you every 10 min, or less or more? If you are walking West towards Lansing, will the busses heading west from Okemos pass you every 10 min, or less or more?

What has been found? 120 Planets around Main Sequence (solar like stars) In 105 planetary systems 15 multiple planet systems

What has been found - Masses

What has been found - Eccentricity Solar System Eccentricities < 0.1 Except Mercury & Pluto

What has been found - Composition

What we expected What we found Nearly circular orbits Jovian planets far from star Highly eccentric orbits Jovian planets close to star

Implications for Life Eccentric Jovian planets will eject small terrestrial planets from the system. Migrating Jovian planets will eject small terrestrial planets as they pass. Young stars with more heavy elements are more likely to form planets than older stars with less heavy elements.

Questions? How are circular orbits produced? Are they typical or atypical? How are hot, close Jovian planets produced? Do they form as outer planets and migrate inward? What makes them migrate?