Transcript The Planets of the Solar System

Chapter 27
Planets of the solar System
The Nebular Hypothesis
• In 1796: Laplace’s hypothesis states that the
sun and the planets condensed about the
same time out of a rotating cloud of gas and
dust.
The Origin of the Solar System
Four Challenges
1. Patterns of Motion
Planets orbit in the same direction...
...in nearly the same plane...
...in nearly circular orbits.
Most planets rotate in the same direction.
Most moons orbit in the same direction.
2. Categorizing Planets
Planets are either rocky or gas-rich.
The Origin of the Solar System
Four Challenges
3. Asteroids and Comets
Most asteroids are found between Mars and Jupiter.
Most comets have highly elliptical orbits.
4. Exceptions to the Rules
What about Pluto’s elliptical orbit and composition?
What about the odd rotation of Venus and Uranus?
Formation of the Solar System
• The solar system is thought to have formed from a
cloud of gas and dust in a process know as accretion.
• Our Sun is thought to be a second generation star.
• What does that mean?
Formation
•
•
http://observe.phy.sfasu.edu/courses/ast105/lectures105/chapter06/formation_protoplanet_disk
.htm
http://observe.phy.sfasu.edu/courses/ast105/lectures105/chapter06/accretion_and_planets.htm
• During the first few million years, matter in the
accretion disk of our protosun coalesced (joined
into a single mass)…
• ...in the larger objects called planetesimals, with
diameters of about 100 km.
• We see evidence of
accretion disk
around other stars.
• For example, b
Pictoris.
• Collisions of planetesimals
dominated the early solar
system…
• ...and these objects
combined to form our
planets.
• We see evidence of early
collisions in our solar
system in the form of
impact craters on the
planets and their moons.
Uh oh…
• In addition to the 8 major planets, there are at least
100 moons in our solar system.
• While some of these moons are spherical, most
look roughly like potatoes.
• There is still minor debris left over from the
formation of the solar system:
– asteroids and comets.
Section 2
Models of the Solar System
Early Models of the Solar System
• 2000 years ago Aristotle suggested an
Earth-centered or geocentric solar system.
• Around 130 AD Ptolemy proposed changes
to the model to account for problems with
Aristotle’s model.
• In 1543 Copernicus proposed a suncentered or heliocentric model.
Giants of Science
Tycho Brahe & Johannes Kepler
These two scientists showed that the
Universe was not some ideal
perfection as Ptolemy proposed and
worked towards acceptance of
Copernicus’ heliocentric model
Tycho Brahe
– made the most accurate observations of
stars and planets up to that time.
– was a flamboyant Danish nobleman who
wore a silver nose when part of his nose
was cut off in a duel!
Tycho Brahe (1546-1601)
Tycho Brahe and Uraniborg
• He lived in a
mansion/observatory on an
island off the coast of Denmark.
• The mansion had very
sophisticated equipment (but no
telescopes!) to help him and his
assistants to measure the
positions of stars and planets.
• He named the mansion
Uraniborg (Sky Castle).
Some of the equipment used at Uraniborg
Tycho Brahe’s Discoveries
• As a young man he proved that
comets had to be farther from Earth
than the Moon.
• He also proved that a star which
appeared to brighten dramatically
over a few weeks was also beyond
the Moon.
• Both observations showed that the
heavens could change like the Earth.
• He also came up with his own
compromise model of the Universe.
Brahe’s compromise:
All the planets went around the Sun
while the Sun moved around a fixed Earth
Tycho Brahe & Johannes Kepler
• A few years before he died,
Brahe hired Johannes Kepler to
help in analyzing the data he had
collected.
• Brahe started him out on his
hardest problem: determine the
orbit of Mars.
• Mars has the largest observed
retrograde motion and no
circular orbit could be found to
match Brahe’s observations.
Brahe and assistants making observations
Kepler’s Models
After years of work, the most
accurate circle he could find
for Mars’ orbit still left an
error of 8 arcminutes (about
1/4 the angular size of the full
Moon).
Johannes Kepler (1571-1630)
“If I had believed that we could ignore
these eight minutes [of arc], I would have
patched up my hypothesis accordingly. But
since it was not permissible to ignore,
those eight minutes pointed the road to a
complete reformation in astronomy”
- Kepler
Kepler’s Breakthrough
• Kepler’s key discovery
– planets do not orbit in circles
but rather in ellipses.
– the Sun was not at the center
of the ellipse but rather at
one focus.
• With this breakthrough he
obtained excellent agreement
between his model and
observations.
Properties of Ellipses
• Each point marked by a tack is
called a focus.
• The farther apart one focus is from
another the more eccentric the
ellipse.
• The line cutting the ellipse in half
that passes through each focus is
called a major axis. Half the major
axis is called a semimajor axis.
• The semimiajor axis is the average
distance of the planet from the Sun
Kepler’s 3 Laws of Planetary Motion
• These laws describe the observed planetary
motions but do not describe why these motions
occur as they do.
Kepler’s First Law of Planetary Motion
The orbit of each planet around the
Sun is an ellipse with the Sun at one
focus.
– There is nothing at the other
focus.
– The average distance of the
planet from the Sun is the
semimajor axis.
– Throws out Ptolemy’s perfect
circular orbits.
Kepler’s Second Law of Planetary Motion
• As a planet moves around
its orbit, it sweeps out
equal areas in equal times.
– A planet travels faster
when it is nearer the Sun
and slower farther away
– Throws out Ptolemy’s
uniform motion
Kepler’s Third Law of Planetary Motion
• The amount of time it takes
a planet to orbit the Sun is
related to the size of its
orbit by P2(years) = a3(AU)
– 1 AU (astronomical unit) is
the semimajor axis of the
Earth’s orbit. Earth’s average
distance from the Sun.
– It doesn’t matter how
elliptical the orbit as long as
the average distance is the
same
Touring Our Solar System
A Trip Through the Solar System
The Inner or Terrestrial Planets
• Mercury, Venus, Earth and Mars share certain
characteristics:
– All are rocky bodies.
– All have solid surfaces.
– Except for Mercury all have at least a thin
atmosphere
• They are called Terrestrial planets because of their
resemblance to Earth.
The Inner or Terrestrial Planets
Mercury - named after the speedy
messenger of the Roman gods
•
•
•
•
•
•
Closest planet to the sun
Revolution around the sun = 88 Earth days
Rotation on its axis = 59 Earth days
Crater-covered surface with steep cliffs
Almost no atmosphere
Temperature range
– as high as 427 degrees C
– as low as -170 degrees C
Venus - named after the Roman
goddess of beauty and love
• Venus is the second planet from
the sun and has an orbital period
of 225 days.
• Venus rotates very slowly, only
once every 243 days.
• Venus and Earth are of almost
the same size, mass, and density,
but differ greatly in other areas.
Visible and radar illumination
Venus - named after the Roman
goddess of beauty and love
• Second planet from the sun
• About the same size as Earth
• Thick, cloudy atmosphere
– sulfuric acid
– carbon dioxide
• Highest temperature range
of inner planets
– as high as 480 degrees C
Visible and radar illumination
Venus - named after the Roman
goddess of beauty and love
• Surface pressure = 91 times more than
Earth’s
• Surface has…
– deep canyons and tall mountains
– craters
– vast plains
• Revolution around the sun = 224 Earth days
• Rotation on its axis = 243 Earth days
Visible and radar illumination
Venus - named after the Roman
goddess of beauty and love
Greenhouse effect
• Venus’ atmosphere is
95% CO2.
Earth
• Earth is the third planet from the
sun.
• The orbital period of Earth is 365
1/4 days. Earth completes one
rotation on its axis every day.
• Earth has one large moon.
• Geologic records indicate that
over the last 250 million years,
Earth’s surface has undergone
many changes.
Earth
• Third planet from the sun
• Revolution around the sun = 365
days
• Rotation on its axis = 24 hours
• Because the axis of the Earth is
tilted, this creates distinct
“seasons” throughout the year
Earth
• Temperature range depends on the
location, altitude and season
• Gravitation pull of the moon creates tide
changes (rise and fall of the ocean levels)
• Surface –
–
–
–
Mountains
Plains
Deserts
Heavy vegetation
Mars - named after the Roman
god of war
• Mars is the fourth planet
from the sun.
• Mars is about 50% farther
from the sun than Earth is.
• Its orbital period is 687
days
• it rotates on its axis every
24 hours and 37 minutes.
• Mars’s seasons are like
Earth’s seasons because the
same axis.
Mars - named after the Roman
god of war
• Fourth planet from the sun
• Surface
– rocky
– large craters
– soil is similar to Earth’s
soil in many ways
– Has the largest volcano
in the solar system,
Olympus Mons.
Mars - named after the Roman
god of war
• Very thin CO2 atmosphere, polar caps of
mostly frozen CO2. Since its atmosphere is
thin and cold there is very little greenhouse
effect.
• High winds often create dust storms
• Temperate falls well below 0 degrees C all
the time
Mars - named after the Roman
god of war
• About half the size of Earth. No
geological activity likely now.
No magnetic field.
• Evidence of massive water
erosion some time in the past.
Scientists are searching for
liquid water now.
• Two satellites, Phobos and
Deimos
– (possibly captured asteroids)
Mars - named after the Roman
god of war
The Outer or Jovian Planets
• Jupiter, Saturn, Uranus and Neptune share certain
characteristics:
– All are large, gaseous bodies.
– All have very thick atmospheres, with possibly
liquid interiors and solid cores
– All have rings
• They are called Jovian planets because of their
resemblance to Jupiter.
The Outer or Jovian Planets
Jupiter - named after the king of
the Roman gods
• Fifth planet from the sun
• Made of mainly
– hydrogen
– helium
• Temperature range – very cold at the cloud tops
– as high as 30,000 degree C at the core
Jupiter - named after the king of
the Roman gods
• Jupiter is the largest planet in the solar
system and has a mass more than 300
times that of Earth.
• The orbital period of Jupiter is almost
12 years. Jupiter rotates on its axis
faster than any other planet—once
every 9 h and 50 min.
• Jupiter has at least 60 moons.
• It also has several thin rings that are
made up of millions of particles.
Jupiter - named after the king of
the Roman gods
• Atmosphere
– hydrogen
– helium
– ammonia
– methane
• Great Red Spot
– hurricane-like storm
– (as much as 20,000 years old)
Jupiter - named after the king of
the Roman gods
• Very high atmospheric pressure
• Giant magnetic field
– created by the liquid metallic layer
– called magnetosphere
Saturn - named after the Roman
god
• Sixth planet from the sun
• Surrounded by rings
– made of icy particles
– has at least seven major rings
• Made of mainly
– hydrogen
– helium
Saturn - named after the Roman
god
• The orbital period of Saturn
is 29.5 years.
• Saturn rotates on its axis
every 10 h and 30 min.
• Saturn is very cold and has an
average cloud-top
temperature of –176°C.
• Saturn has at least 60 moons.
Saturn - named after the Roman
god
•
•
•
•
Violent atmospheric storms
Very cold
Has a large magnetic field
Lowest density of all the planets
Uranus - named after the father
of Saturn in Roman mythology
• Seventh planet from the sun
• Atmosphere
– hydrogen
– helium
– methane
• Temperature range
– as low as -220 degree C at the
cloud tops
Uranus - named after the father
of Saturn in Roman mythology
• Extreme atmospheric pressure
– atmosphere is 11,000 kilometers
thick
• Rotates on its axis at a 90 degree
angle
– appears laying on its side
• Rings of methane ice surround it
Neptune - named after the Roman
god of the sea
• Eighth planet from the sun
• Atmosphere
– hydrogen
– helium
– methane
• Temperature
– as low as -220 degrees C
Neptune - named after the Roman
god of the sea
• Surface
– ocean of water and liquid
methane
– rocky core
• Five rings surround Neptune
– made of dust particles formed
from meteorites
Pluto - named after the Roman
god of the underworld
•
•
•
•
•
Ninth planet from the sun
Smallest planet
Least dense planet
Seems to be made primarily of methane ice
Thin atmosphere (only on the sunny side)
– methane ice evaporated to form this
Pluto - named after the Roman
god of the underworld
• Pluto may have been a
satellite of Neptune that
was displaced from its
original orbit and split
into two pieces.
Charon
Pluto