Welcome to the Solar System

“How vast those Orbs must be, and how inconsiderable this Earth, the Theater upon which all our mighty Designs, all our Navigations, and all our Wars are transacted, is when compared to them. A very fit consideration, and matter of Reflection, for those Kings and Princes who sacrifice the lives of so many People, only to flatter their Ambition in being Masters of some pitiful corner of this small Spot.”

Christiaan Huygens (1629 -- 1695) Dutch Astronomer and Scholar

Welcome to the Solar System

“How vast those Orbs must be, and how inconsiderable this Earth, the Theater upon which all our mighty Designs, all our Navigations, and all our Wars are transacted, is when compared to them. A very fit consideration, and matter of Reflection, for those Kings and Princes who sacrifice the lives of so many People, only to flatter their Ambition in being Masters of some pitiful corner of this small Spot.”

Christiaan Huygens (1629 -- 1695) Dutch Astronomer and Scholar

What is density?

density = mass/volume

typical units: [ kg/m3]

Density of water is defined as 1 g/cm3 or 1000 kg/m3.

Average speed of particles in a gas:

where k = 1.38 x 10-23 [J / K] T = temperature [K] m = mass [kg]

[m/s]8

m

kTv

��

Homework #5

1) What is the average speed of hydrogen atoms (m=1.67x10-27 kg) in the Sun’s photosphere (T =

5800 K)?

2) What is the escape velocity from Mercury?

The Layout of the Solar System• Large bodies in the Solar System have orderly motions

– planets orbit counterclockwise in same plane– orbits are almost circular– the Sun and most planets rotate counterclockwise– most moons orbit counterclockwise

The Layout of the Solar System

• Planets fall into two main categories– Terrestrial (i.e. Earth-like)– Jovian (i.e. Jupiter-like or gaseous)

Mars Neptune

Terrestrial Jovian

The Layout of the Solar System

• Swarms of asteroids and comets populate the Solar System

A Few Exceptions to the Rules…

• Uranus is tilted on its side.• Venus rotates “backwards” (i.e.

clockwise).• Triton orbits Neptune “backwards.” • Earth is the only terrestrial planet with a

relatively large moon.

The Sun – King of the Solar System

• How does the Sun influence the planets?– Its gravity regulates the orbits of the planets.– Its heat is the primary factor which determines

the temperature of the planets.– It provides practically all of the visible light in

the Solar System.– High-energy particles streaming out from the

Sun influence planetary atmospheres and magnetic fields.

A Brief Tour of the Solar System -- Motions

A Brief Tour of the Solar System – Composition

A Brief Tour of the Solar System – Summary

The planets are tiny compared to the distances between them (a million times smaller than shown here), but theyexhibit clear patterns of composition and motion.

The patterns are far more important and interesting than numbers, names, and other trivia

Recall scale of solar system

Planets are tiny compared to distances between them…

Sun is the size of a CD (~12 cm)– then:

Earth – 15 mMars – 23 mJupiter – 78 mSaturn –143 mUranus – 287 m

Sun

• Over 99.9% of solar system’s mass• Made mostly of H/He gas (plasma)• Converts 4 million tons of mass into energy each second

Mercury• made of metal and rock; large iron core • desolate, cratered; long, tall, steep cliffs• very hot and very cold: 425°C (day), –170°C (night)

Venus• nearly identical in size to Earth; surface hidden by thick clouds • hellish conditions due to an extreme greenhouse effect:• even hotter than Mercury: 470°C, both day and night• atmospheric pressure equiv. to pressure 1 km deep in oceans• no oxygen, no water, …• perhaps more than any other planet, makes us ask: how did it end

up so different from Earth?

Earth• An oasis of life• The only surface liquid water in the solar system; about 3/4 of

surface covered by water• A surprisingly large moon

Earth and Moon to scale

Mars• Looks almost Earth-like, but don’t go without a spacesuit!• Giant volcanoes, a huge canyon, polar caps, more…• Water flowed in the distant past; could there have been

life?

Jupiter

• Much farther from Sun than inner 4 planets (more than twice Mars distance)

• Also very different in composition: mostly H/He; no solid surface.

• Gigantic for a planet: 300 � Earth mass; >1,000 � Earth volume.

• Many moons, rings…

Moons can be as interesting as the planets themselves, especially Jupiter’s 4 large “Galilean moons” (first seen by Galileo)

• Io (shown here): active volcanoes all over• Europa: possible subsurface ocean• Ganymede: largest moon in solar system — larger than Mercury• Callisto: a large, cratered “ice ball” with unexplained surface features

Saturn

• Giant and gaseous like Jupiter

• most spectacular rings of the 4 jovian planets

• many moons, including cloud-covered Titan

• currently under study by the Cassini spacecraft

Saturn

Rings are NOT solid; they are made of countless small chunks of ice and rock, each orbiting like a tiny moon.

Artist’s conception

Saturn

Cassini probe arrived July 2004

(Launched in 1997)

casini trajectory.mov

Uranus

• much smaller than Jupiter/Saturn, but still much larger than Earth

• made of H/He gas, hydrogen compounds (H2O, NH3, CH4)

• extreme axis tilt — nearly tipped on its “side” — makes extreme seasons during its 84-year orbit.

• moons also tipped in their orbits…

Neptune

• Very similar to Uranus (but much smaller axis tilt)

• Many moons, including unusual Triton: orbits “backward”; larger than Pluto.

Pluto• A “misfit”: far from Sun like large jovian planets, but much

smaller than any terrestrial planet.• Comet-like composition (ices, rock) and orbit (eccentric,

inclined to ecliptic plane, long -- 248 years).• Its moon Charon is half Pluto’s size in diameter• Best current photo above; New Horizons mission launch 2006,

arrival 2015…

What features of our solar system provide clues to how it formed?

The Sun, planets, and large moons orbit and rotate in an organized way

counterclockwise seen from above the north pole)

Terrestrial planets are small, rocky, and close to the Sun. Jovian planets are large, gas-rich, and far from the Sun.(What about Pluto?)

Rocky asteroids between Mars & Jupiter

Icy comets in vicinity of Neptune and beyond

Asteroids and comets far outnumber the planets and their moons

A successful theory of solar system formation must allow for exceptions to general rules

Summary: Four Major Features of our Solar System

What theory best explains the features of our solar system?

According to the nebular theory our solar system formed from a giant cloud of interstellar gas

(nebula = cloud)

Where did the solar system come from?

The cloud of gas that gave birth to our solar system resulted from the recycling of gas through many generations of stars within our galaxy.

What caused the orderly patterns of motion in our solar system?

Start with a cloud of gas and dust…As gravity forced the cloud to become smaller, it began to spin faster and faster

As gravity forced the cloud to become smaller, it began to spin faster and faster

Conservation of angular momentum

As gravity causes cloud to shrink, its spin increases

Conservation of angular momentum

Collisions flatten the cloud into a disk.

The orderly motions of our solar system today are a direct result of the solar system’s birth in a spinning, flattened cloud of gas.

Collisions between gas particles in cloud gradually reduce random motions

Collisions between gas particles also reduce up and down motions

Spinning cloud flattens as it shrinks

We see evidence for spinning disks of gas and dust around other stars, especially newly formed stars

Four Unexplained Features of our Solar System

√ Why do large bodies in our solar system have orderly motions?

--> 2) Why are there two types of planets?

3) Where did the comets and asteroids come from?

4) How can we explain the exceptions the the ‘rules’ above?