Notes: The Solar System

The Formation of the Solar System1. A gas cloud collapses under the influence of

gravity.

2. Solids condense at the center, forming a protostar.

3. A falttened disk of matter surrounds the protostar, which has begun to shine.

4. As gravity pulls in on the protostar, its temperature rises, and fusion starts.

5. Dust and debris accrete – clump together. Planetesimals (proto-planets) dominate

their region of space.

6. Once the leftover gas, dust, and debris has been collected/removed, the solar system takes on a more familiar form.

We have found around 100 extrasolar planets.

Most are Jupiter-sized and around stars larger than the sun.

A few are believed to be Earth-like, just much bigger.

They are referred to as superearths.

A Survey of the Solar System 8 (9) planets: Mercury, Venus, Earth, Mars,

Jupiter, Saturn, Uranus, Neptune (Pluto).

All planets have elliptical orbits.

All revolve counter clockwise around the

sun.

Except for Venus, Uranus, and Pluto.

The sun is 99% of the mass of the solar system.

Jupiter and Saturn make up 90% of the mass of

the planets.

Video: How Big Is the Solar System?

Planet Building The planets formed from the same

protostar material as the sun.

The rocky planets formed from the

clumping together of dust grains in the

protostellar cloud – a process called

accretion.

The first rocky clumps are called

planetesimals.

As planetesimals collide, they form

protoplanets.

The rocky, inner planets form as the

dust is accreted.

The gassy, outer planets form as gases

accrete.

Clearing the Nebula The remains of the sun’s nebula were

cleared away:

By the pressure created by radiation

from the sun.

By the solar wind.

The ignition of the sun would sent a wave of

energy that would’ve cleared a lot of debris.

By being swept up by planets.

As evidenced by the craters of Mercury and

the Moon.

By being “sling shot” out of the solar

system after close encounters with

planets.

Comparative Planetology Two types of planets:

Terrestrial – “Earth-like”.

Small volume, low mass, higher density, mostly

rock.

Have thin atmospheres.

Except Mercury.

Are relatively close together.

Also called “Inner Planets”.

Include: Mercury, Venus, Earth, and Mars.

Jovian – “Jupiter-like”.

Large volume, high mass, low density, mostly

gas.

Have no discernible solid surface.

Are relatively far apart.

Also called “Outer Planets” and “Gas Giants”.

Include: Jupiter, Saturn, Uranus, and Neptune.

Space Debris Some of the uncollected debris from the

solar system’s formation still exist today.

Asteroids: rocky bodies, several

kilometers across.

Found mostly between Mars and Jupiter.

Comets: chunks of rock and ice.

Found mostly beyond Neptune.

Meteoroids: smaller chunks of rock.

Found floating throughout space.

Asteroids The last remains of the planetesimals

from 4.6 bya.

They are small, irregular objects at least

1 km in size.

Most can be found in the apparent

empty space between Mars and Jupiter.

This area is called the Asteroid Belt.

Believed to be a planet that was torn

apart by Jupiter’s large gravity.

Comets Icy, rocky objects in highly elliptical orbits

around the sun.

Three parts:

Icy nucleus: icy/rocky core.

Coma: thin atmosphere around the nucleus.

Created as material evaporates off the nucleus

as the comet approaches the sun.

The solar wind creates a tail of this same

material.

Two tails:

Ion tail: ionized gas pushed away from the

comet by the solar wind.

Pushed straight away from the sun.

Dust tail: dust set free from vaporizing ice.

Carried away from the comet by the sun’s

radiation pressure.

The Origin of Comets Comets are believed to originate in the

Oort Cloud.

This is a spherical cloud of several trillion

icy bodies.

The gravitational influence of passing stars

may dislodge some orbits and send them

hurtling toward the sun.

Gives them orbital periods of ~200 years.

They have extremely elliptical orbits.

A second source of icy bodies is the

Kuiper Belt.

Meteoroids Probably left over planetesimals.

Smaller (car-sized or less), rocky

bodies in space.

Called a meteor if it’s in the

Earth’s atmosphere.

We call it a shooting star.

The streak is the meteor burning up

in the atmosphere.

Most meteors appear in showers

which peak at certain times of the

year.

Typically orbit in the same paths

as comets.

Called a meteorite if it collides

with Earth’s surface.

About 2 strikes a day produce visible

impact craters.

Models of the Solar System Early Models

Aristotle’s model Earth centered; the sun, stars, and planets orbit around the Earth

Didn’t explain why planets appear to move backward (retrograde)

Models of the Solar SystemPtolemy’s model Earth centered;

planets move in epicycles Planets move in small

epicircles as they revolve around the earth in larger circles –explains retrograde movement

Models of the Solar System

Copernicus’ model Sun centered; the stars

and planets revolve around the Sun in the same direction but at different speeds and distances from sun

Explains retrograde motion

Tycho Brahe (1546-1601) From Danish nobility, was the king’s personal

astronomer.

Was given an island, where we created an observatory.

From this observatory, he made some of the most detailed

observations of the night sky ever.

Those observations are still used today.

He was the epitome of the naked-eye observers.

He was a staunch supporter of Ptolemy, and had a ton

of data, but didn’t have the ability to create a sufficient model from it.

He was a jerk.

Got into numerous duels.

During one, part of his nose was chopped off.

It was his jerkiness that got him kicked out of his own

country.

That, however, ultimately led him to meet Kepler, and

change everything.

Johannes Kepler (1571-1630) Born into poverty, was able to enter the clergy and go

to school.

While in college, overheard a speech from Copernican

supporters and became hooked on math and astronomy.

Met and began working for Tycho.

Tycho was afraid of Kepler’s intellect and that he’d show him up, so he only allowed him access to his data on Mars.

Once Tycho died, Kepler was given access to the rest of the data.

In trying to describe why Mars showed retrograde

motion, he came upon a paradigm-shifting discovery:

The orbits are ellipses NOT circles.

He also boldly exclaimed that “truth can only be found

through experimentation”.

Further studies lead him to creating 3 Laws of

Planetary Motion.

The 1st Law of Planetary Motion The orbits of planets and other celestial

bodies around the Sun are ellipses.

Ellipse: A curve for which the sum of the

distances from any point on it to two

points inside is always the same.

Those two points are called its foci.

The widest diameter of the ellipse is its major

axis. Half of that is its semimajor axis.

Its shape is called its eccentricity.

It ranges from 0 (perfect circle) to 1.

The orbits of most planets are nearly

circular.

o Earth’s eccentricity = 0.0167

Kepler put the sun at one focus.

The other was just a point in space. Eccentricity e = c/a

c

The 2nd Law of Planetary Motion A planet will cover equal areas in equal amounts of time, traveling at

different speeds in it’s orbit.

Also called the Equal Area Law.

This means that the planet’s orbital velocity will vary throughout the year.

The planet moves faster when it is closer to the sun.

June 15th

July 15th

January 15th

December 15th

(30 days) (30 days)

Sun

Equal areas

Earth’s orbit

3rd law: The law of periods

Mathematical formula P^2 = a^3

If you know the period of a planets orbit (P) then you can determine that planets distance from the sun (a)

Planets far away from the sun have longer periods than those close to the sun and they move slower Mercury, the innermost planet, takes

only 88 days to orbit the Sun but the outermost planet (Pluto) requires 248 years to do the same.

This law, not an apple, led Newton to his law of gravitation

P2 = a3

The 3rd Law of Planetary Motion

Final Thoughts Kepler’s Laws provide a way for followers of Copernicus to explain

motions.

Using them made it possible to make incredibly accurate predictions.

However, they only describe, they don’t explain the forces involved.

Kepler left that for Newton to describe.