ASTRONOMY 202 Spring 2007: Solar System Exploration

Instructor: Dr. David Alexander Web-site: www.ruf.rice.edu/~dalex/ASTR202_S07

Class 22: Formation of the Solar System [3/14/07]

� Announcements

� The Nebular Theory

� The Two types of Planets

� Age of the Solar System

� Extrasolar Planets

Chapters 7&8Chapters 7&8

Now Playing: Sun Hits the Sky - Supergrass

Announcements

Web Project

Window is open to choose web project – window closes March 26See class web-site for details and some news sites

- science.nasa.gov is a good one for solar system

Observing

This week is lousy but we will try to get a night next week and I will also try to get the solar telescopes working

Homework #5

Homework #5 is now available online

Happy Birthday Albert Einstein (1879 Happy Birthday Albert Einstein (1879 –– 1955)1955)

“Wisdom is not a product of schooling but of the lifelong

attempt to acquire it.""Reality is merely an illusion, albeit a very persistent one."

"Anyone who has never made a mistake has never tried anything

new."

"The most incomprehensible thing about the world is that

it is comprehensible."

The origin of the solar system

Several ‘theories’ have been promoted to explain the origin of the solar system.

None have been as successful as The Nebular Theory.

The basis of the Nebular theory can be traced to Immanuel Kant, who in 1755 suggested that the solar system formed from the collapse of an interstellar cloud of gas and dust. This was independently put forward by Laplace in 1795.

The Latin for cloud is nebula, hence The Nebular Theory.

1 – 5 light years~200 AU

The Solar Nebula

Modern observations show that stars, perhaps with planetary systems around them, are forming out of low density interstellar clouds.

Orion Nebula

NGC 3603

Protoplanetary disks in Orion

The Collapse of the Solar NebulaThe formation of the solar system as we know it is a result of the conservation of energy(gravitational to kinetic to thermal) and the conservation of angular momentum.

• Heating

• Spinning

• Flattening

• Emptying the Nebula

As the nebula collapses, the density and temperature rapidly increases.

The collapsing sphere rotated faster and faster as it collapsed.

Collisions in the spinning, collapsing nebula result in the sphere flattening to form a disk.

A plasma wind from the newly formed stars sweeps the bulk of the hydrogen and helium gas from the solar system.

The three process of heating, spinning, and flattening explain the tidy layout of the solar system. The clearing out of the gas early in the history

of the solar system was crucial to determining the final nature of the solar system.

The Two Types of Planets

To understand this aspect of the solar system we need to know the composition of the solar nebula and then how this tenuous loosely bound collection of gas and dust interacted to form the various planets.

Galactic Recycling

The production of elements in the Big Bang produced Hydrogen, Helium and small amounts of Lithium.

The heavier elements were produced in the cores of massive stars early in the history of the Universe.

The scattering of this material throughout the Universe provided the basic building blocks for other stars, planets and us.

The Solar System is 98% H and He, with the other 2% left over for everything else.

The Two Types of Planets

Condensation: The importance of the frost line.

In the outer parts of the collapsing nebula gravity needed a hand.

The cool temperatures in the outer reaches of the nebula allowed solid particles to condenseout of the gas.

• Hydrogen and Helium gas

• Hydrogen compounds

• Rock

• Metals

98% by mass, do not condense at nebular temperatures

Form ices of methane (CH4), ammonia (NH3) and water (H2O) below 150 K. Made up about 1.4% of mass.

Mostly silicon-based minerals making up about 0.4% nebular mass. Rock is gaseous above 500 – 1300 K.

E.g. iron, nickel, aluminium making up ~0.2% of the mass. Gaseous metals present above 1600 K.

The Two Types of Planets

Condensation: The importance of the frost line.

Different ‘seeds’ for condensation form at different parts of the collapsing nebula.

The Frost Line is defined by the temperature at which the H, He and H compounds could condense out, i.e ~150 K.

In the Solar System, the frost line lies between the orbit of Mars and Jupiter.

The Two Types of PlanetsAccretion

Once condensation had formed the ‘seeds’ of the clumping material, collisions between them resulted in the seeds growing to a point were gravity started to take over. The growth process is know as accretion.

The path from the production of the basic building blocks through galactic recycling through the processes of condensation and accretion results in a solar system which looks very much like our own.

The larger amounts of building materials in the outer nebula resulted in planetesimals beyond the frost line being much bigger than the metallic and rocky planets of within the frost line. This in turn allowed for the accumulation of the H and He gas resulting in the gas giant planets.

Forming the Solar System

The Age of the Solar System

We can determine the age of the solar system by measuring the ages of the rocks within it using a process known as radiometric dating.

Radioactive isotopes in rock undergo spontaneous change (radioactive decay) from one isotope to another or one element to another. By measuring the amounts of the different isotopes and elements we can determine how long it has been since the rock solidified.

The oldest Earth rocks solidified 4 billion years ago.

Lunar rocks have yielded an age of ~4.5 billion yrs.

Extra-Solar Planets

We are now able to investigate some rudimentary aspects of the star system formation process around other stars.