Download A Planetary Overview

The planets
Lecture 5: A Planetary Overview
Solar System Roll Call
• The Sun the is largest and brightest
Sun
object in the solar system
• The Sun is hot (5800 K on surface)
• The Sun is gaseous and converts
matter into energy in core
• The Sun has the greatest influence on
the rest of the solar system (light, solar
wind…)
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Lecture 5: A Planetary Overview
Solar System Roll Call
• Mercury is the smallest planet in the solar
Mercury
system
• It rotates every 58.6 days and revolves every
88 days and is tidally locked to the Sun
• The produces 88 days of day and 88 days of
night, making temperatures extreme (425°C
to -150°C).
• One spacecraft has visited Mercury and
another is on its way
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Lecture 5: A Planetary Overview
Solar System Roll Call
Venus
• Venus is often called Earth’s “twin” because it is
nearly the same size as the Earth. But it’s nothing like
the Earth…
• It rotates backwards (or upside down) very slowly
• It is covered with an atmosphere of mostly CO2 which
allows a runaway greenhouse effect to occur raising
the temperature to 470°C (880°F) planetwide
• Its surface pressure in 90 times greater than the
Earth and there are clouds of sulfuric acid near the
surface of the planet
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Lecture 5: A Planetary Overview
Solar System Roll Call
Earth
• Earth is only world that we know of that has or
had life on it
• It is the only world with a significant amount of
oxygen in the atmosphere
• It is the only world with significant amounts of
liquid water
• It is the closest planet to the Sun to have a
moon and our Moon is quite large compared to
the Earth
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Lecture 5: A Planetary Overview
Solar System Roll Call
Mars
• Mars may bear the closest resemblance to the
Earth
• It has a thin atmosphere of mostly CO2
• It has polar caps made of CO2 and water-ice
• In the past, water very likely flowed on the surface
• It has great geological wonders such as a great
canyon and the largest volcano in the solar system
• It has two tiny moons
• It is the most studied extraterrestrial planet and has
several spacecraft present and proposed to land or
orbit Mars.
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Lecture 5: A Planetary Overview
Solar System Roll Call
Jupiter
• Jupiter is largest planet in the solar system
and is made mostly of gas with a Earth sized
rocky-ice core in the center
• It has more than 300 times the diameter and
1000 times the volume of the Earth
• Its atmosphere has many storms many of
which have lasted for hundreds of years
• Its four largest moons (of 63) have
interesting properties too (active volcanoes,
subsurface water, magnetic fields)
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Lecture 5: A Planetary Overview
Saturn
Solar System Roll Call
• Saturn is another gaseous giant planet
with a spectacular ring system
• The ring system is made of millions of
ice-dust chunks orbiting around the
planet
• Saturn has over 50 moons, a few of
them midsize moons and one large
one, Titan, which has a significant
atmosphere.
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Lecture 5: A Planetary Overview
Solar System Roll Call
Uranus
Neptune
• Uranus (YUR-uh-nus) is a smaller gas giant with a greenblue color due to methane
• It has several dozen moons a few of which are midsize
• The entire system (planet, rings, moons) is tilted on their
side
• It has been visited by only one spacecraft (Voyager 2)
• Neptune is just a bit smaller than Uranus and bluer in
color
• It has a dozen moons, one of which is large (Triton). Triton
is the largest moon to go backward around the planet
• It has been visited by only one spacecraft (Voyager 2)
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Lecture 5: A Planetary Overview
Solar System Roll Call
Pluto
• Pluto (and the other Dwarf Planets) are round
object which orbit around the Sun
• Pluto was discovered as a planet in 1930, but was
an oddball world. One of its 3 moons is half its size
(Charon). It will be visited by spacecraft in 2015.
• Soon in the 1990s other objects out where Pluto
lived were being discovered. One of these, Eris,
was found to be a little larger than Pluto
• In 2006, the phrase “dwarf planet” was defined for
these objects and asteroids (like Ceres) which were
round but were found “nearby” other solar system
objects
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Lecture 5: A Planetary Overview
Solar System featurs
•
Looking at the general characteristics, there
are 4 features which stand out:
1.
2.
3.
4.
Patterns of motion among large bodies
Two major types of planets
Asteroids and comets
Exceptions to the rules
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Lecture 5: A Planetary Overview
Distances In The Solar System
Measuring Distances in the Solar System
• Copernicus used geometry to determine
relative distances to the planets.
• Today we measure planetary distances
using radar.
• Average distances to the planets from
the Sun range from .387 AU for Mercury
to 39.53 AU for Pluto.
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Lecture 5: A Planetary Overview
Feature 1: Patterns
of Motion
• All planetary orbits are ellipses, but all are
nearly circular.
orbits
• Each of the planets revolves around the Sun
in the same direction.
• All planets - except Venus, Uranus - rotate in
a counterclockwise direction.
• Most of the satellites revolving around
planets also move in a counterclockwise
direction, though there are some exceptions.
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Lecture 5: A Planetary Overview
Feature 1: Patterns
of Motion
• Inclination of a planet’s orbit is the
tilts
angle between the plane of a planet’s
orbit and the ecliptic plane (the plane of
the Earth’s orbit).
• The elliptical paths of all the planets are
very nearly in the same plane
(inclination about 0°), though Mercury’s
orbit is inclined at 7° and Pluto’s at 17°.
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Lecture 5: A Planetary Overview
Planet Diameters
Diameters of Non-Earth Planets
• Diameters are determined from distances (from
the Earth to the planet) and the planet’s angular
size via the small angle formula (Cosmic
Calculations 2.1)
• Diameter of Sun (1.39 × 106 km) is over 100
times that of Earth (1.3 × 104 km).
• Jupiter’s diameter is 11 times that of Earth.
• Pluto’s diameter is 1/5 that of Earth.
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Lecture 5: A Planetary Overview
Planet Masses
Mass of the Planets
• Kepler’s third law was reformulated by Newton
to include masses (Cosmic Calculations 4.1):
a3/p2 = K (M1 + M2)
• Newton’s statement of Kepler’s third law allows
us to calculate the mass of the Sun.
• Consider the orbits of planets around the Sun.
Since one of the masses to the Sun (the other
being a planet), the sum of the two is essentially
equal to the mass of the Sun, and the equation
can be rewritten as:
a3/p2 = KM
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Lecture 5: A Planetary Overview
Planet Masses
• We can do the same sort of calculation for
planets as long as they have satellites
orbiting them
• The masses of 7 of the 9 known planets can
be calculated based on the distances and
periods of revolution of these planets’ natural
satellites.
• For Mercury and Venus, which do not
possess any natural satellites, accurate
determinations of their respective masses
had to await orbiting or flyby space probes.
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Lecture 5: A Planetary Overview
Feature 2: Classifying
the Planets
• The planets (except Pluto) fit into two groups:
the inner terrestrial planets and the outer
Jovian planets.
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Size, Mass, and Density
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• The Jovian planets have much bigger
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diameters and even larger masses than the
terrestrial planets.
• Terrestrial planets are more dense, however.
• Earth is the densest planet of them all.
Inside
the
planets
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Lecture 5: A Planetary Overview
Classifying the Planets
Satellites and Rings
• The Jovian planets have more satellites than
the terrestrials.
• 4 Jovian planets: 163 total satellites as of
September 2007 (63 for Jupiter, 60 for
Saturn, 27 for Uranus, and 13 for Neptune).
• 4 terrestrial planets: 3 total satellites.
• Pluto has 3 satellites.
• Each Jovian planet has a ring or ring system.
None of the terrestrial planets do.
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A comparison of planetary characteristics
Terrestrial
Jovian
Near the Sun
Small
Mostly solid
Low mass
Slow rotation
No rings
High density
Thin atmosphere
Few moons
Far from the Sun
Large
Mostly liquid & gas
Great mass
Fast rotation
Rings
Low density
Dense atmosphere
Many moons
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Lecture 5: A Planetary Overview
Feature 3: Asteroids
and Comets
Asteroids
Asteroids
• These rocky bodies orbit the Sun, but are
much smaller than planets. Most lie between
Mars and Jupiter
Comets
comets
• Small icy (water, ammonia, methane) objects
which occasionally visit the inner solar system
and become visible
• Comets originate from two regions: the Kuiper
Belt and the Öort Cloud
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Lecture 5: A Planetary Overview
Feature 4: Exceptions
to the Rules
Asteroids
• There are objects in the solar system that are
unusual or have characteristics which are unusual as
compared to the rest of the solar system. Some
examples:

Venus and Uranus rotate differently (backwards and on its
comets
side, respectively)
 Small moons of Jupiter and Saturn and the large moon
Triton (around Neptune) revolve in the opposite direction of
the rotation of the host planet.
 While other terrestrial planets have no moons (Mercury,
Venus) or tiny moons (Mars) The Earth’s moon is large
compared to the Earth.
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Lecture 5: A Planetary Overview
The Formation of the Solar System
Evolutionary Theories
• All evolutionary theories have their start with
Descartes’s whirlpool or vortex theory
proposed in 1644.
• Using Newtonian mechanics, Kant (in 1755)
and then Laplace (around 1795) modified
Descartes’s vortex to a rotating cloud of gas
contracting under gravity into a disk.
• The Solar Nebula Hypothesis is an example
of an evolutionary theory.
Solar
Nebula
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Lecture 5: A Planetary Overview
The Formation of the Solar System
Catastrophic Theories
• Catastrophic theory is a theory of the
formation of the solar system that involves an
unusual incident such as the collision of the Sun
with another star.
• The first catastrophic theory - that a comet
pulled material from the Sun to form the planets
- was proposed by Buffon in 1745.
• Other close encounter hypotheses have been
proposed too.
• Catastrophic origins for solar systems would be
quite rare (relative to evolutionary origins) due to
the unusual nature of the catastrophic incident.
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Lecture 5: A Planetary Overview
Solar Nebula Hypothesis
Towards a Solar Nebula Hypothesis
• The nebular cloud collapsed due the force of gravity
on the cloud. But the cloud does not end up
spherical (like the sun) because there are other
Cloud
collapse2
processes going on:
Heating – The cloud increases in temperature, converting
gravitational potential energy to kinetic energy. The sun would
form in the center where temperatures and densities were the
greatest
 Spinning – as the cloud shrunk in size, the rotation of the disk
increase (from the conservation of angular momentum).
 Flattening – as cloud starting to spin, collisions flattened the
shape of the disk in the plane perpendicular to the spin axis

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Lecture 5: A Planetary Overview
Testing the Model
• If the theory is correct, then we should see
disks around young stars
• Dust disks, such as discovered around
beta-Pictoris or AU Microscopii, provide
evidence that conditions for planet formation
exist around many Sun-like stars.
AU Mircoscopii
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HD 141569A
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Disks around other stars
Lecture 5: A Planetary Overview
Solar Nebula Hypothesis
The Formation of Planets
•
As the solar nebula cooled and flattened into a disk some 200 AU
in diameter, materials began to “freeze” out in a process called
condensation (changing from a gas to a solid or liquid).
The ingredients of the solar system consist of 4 categories (with %
abundance):
•
1.
2.
3.
4.


Hydrogen and Helium gas (98%)
Hydrogen compounds, such as water, ammonia, and methane (1.4%)
Rock (0.4%)
Metals (0.2%)
Since it is too cool for H and He to condense, a vast majority of the
solar nebula did not condense
Hydrogen compounds could only condense into ices beyond the
frost line, which lay between the present-day orbits of Mars and
Jupiter
Frost line
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Lecture 5: A Planetary Overview
Solar Nebula Hypothesis
Building the Terrestrial Planets
• In the 1940s, Weizsächer showed that eddies would form in a
rotating gas cloud and that the eddies nearer the center would be
smaller.
• Eddies condense to form particles that grow over time in a
process called accretion. Materials such and rock and metal
(categories #3 and #4).
• These accreted materials became planetesimals, which in turn
sweep up smaller particles through collision and gravitational
attraction.
• These planetesimals suffered gravitational encounters which
altered their orbits caused them to both coalesce and fragment.
Only the largest planetesimals grew to be full-fledged planets.
• Verification of this models is difficult and comes in the form of
theoretical evidence and computer simulations.
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Lecture 5: A Planetary Overview
Solar Nebula Hypothesis
Building the Jovian Planets
• Planetesimals should have also grown in the
outer solar system, but would have been
made of ice as well as metal and rock.
• But Jovian planets are made mostly of H and
He gas…
• The gas presumably was captured by these
ice/rock/metal planetesimals and grew into
the Jovian planets of today.
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Lecture 5: A Planetary Overview
Solar Nebula Hypothesis
• Stellar wind is the flow of nuclear particles
from a star.
• Some young stars exhibit strong stellar
winds. If the early Sun went through such a
period, the resulting intense solar wind
would have swept the inner solar system
clear of volatile elements.
• The giant planets of the outer solar system
would then have collected these outflowing
gases.
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Lecture 5: A Planetary Overview
Solar Nebula Hypothesis
Explaining Other Clues
• Over millions of years the remaining
planetesimals fell onto the moons and
planets causing the cratering we see today.
This was the period of heavy bombardment.
• Comets are thought to be material that
coalesced in the outer solar system from the
remnants of small eddies.
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Lecture 5: A Planetary Overview
Solar Nebula Hypothesis
• The formation of Jovian planets and its
moons must have resembled the
formation of the solar system. Jupiter
specifically:


Moons close to Jupiter are denser and
contain fewer light elements;
Moons farther out decrease in density and
increase in heavier elements.
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Lecture 5: A Planetary Overview
The Exceptions to the Rule
• Captured Moons – satellites which go the
opposite way were likely captured. Most of
these moon are small are lie far away from
the planet.
Giant impact
Moon
• Giant impacts – may have helped form the
Moon and explain the high density of Mercury
and the Pluto-Charon system. Furthermore,
the unusual tilts of Uranus and Venus can
also be explained by giant impacts.
Solar Nebula Theory
Summary
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Lecture 6: A Solar System Overview
Radioactivity
Radioactivity
Half-life
• Certain isotopes (elements which contain
differing number of neutrons) are not stable and
will decay into two or more lighter elements
• The time it takes for half of a given isotope to
decay is called the half-life
• By noting what percentage a rock (or human
body) has left of a radioactive element can
enable us to estimate the age of that object. This
process is called radioactive dating. See
Cosmic Calculations 6.1
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Lecture 6: A Solar System Overview
Radioactivity
Half-life
Earth rocks, Moon rocks, and meteorites
• The oldest Earth rock date back to 4 billion years
and some small grains go back to 4.4 billion
years. Moon rock brought back from the Apollo
mission date as far back as 4.4 billion years.

These tell us when the rock solidified, not when the
planet formed
• The oldest meteorites, which likely come form
asteroids, are dated at 4.55 billion years,
marking the time of the accretion of the solar
system
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Lecture 5: A Planetary Overview
Brown
Dwarf
Planetary Systems Around Other Stars?
COM
Jupiter
Sun
• Photographing planets around stars directly is very difficult
since planet merely reflect (visible) light from the nearby stars.
Using the infrared part of the spectrum, we can detect large
objects known as brown dwarfs which are neither stars or
Astrometric
Jupiter
planets
Sun
• Stars exhibiting a discernable wobble from gravitation tugs
can be evidence of an unseen companion - such as a large
planet or group of planets. One can try to look for positional
changes in the sky form this star – the astrometric
technique, but this is difficult.
Doppler
• Since 1995, this Doppler Technique has found evidence of
over 170 planets orbiting stars in the near vicinity of the Sun.
Doppler
• Some of the extrasolar planets can be detected when the
Velocity
transit the star. The star’s brightness dims just a bit during the curve
transit.
• Web link: http://exoplanets.org/
transit
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Lecture 5: A Planetary Overview
Planetary Systems Around Other Stars?
• Comparisons to our Solar System


Many of these planets are more massive than Jupiter
Many of these planets are closer to their star than
Mars is to the Sun
Mass
• These discoveries are in part due to a selection effect –
these are the easiest to detect

Jovian sized planets close to the star is not consistent
with the standard solar nebular model. So how does
one form a “hot Jupiter”?
• Planetary migration – the gas giant form in the cooler, outer
region of the nebular disk, but due to friction (and a loss of
angular momentum) from the nebular disk, the planet in
brought to a much closer distance.
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Orbits
Planetary
migration
38
The End
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Lecture 5: A Planetary Overview
Planetary Atmospheres & Escape Velocity
The Atmospheres of the Planets
• Ten times the average speed of molecules at
a particular temperature provides a good
measure of whether a planetary body will
atmospheric
retain a gas for billions of years.
speed
• Because of their size (and mass) the Jovian
planets have retained almost all of their
gases.
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Lecture 5: A Planetary Overview
Planetary Atmospheres & Escape Velocity
• Escape velocity is the minimum velocity an
object must have in order to escape the
gravitational attraction of an object such as a
planet.
Vesc
2GM

R
atmospheric
speed
• Earth’s escape velocity is 11 km/s. The
Moon’s escape velocity is only 2.5 km/s.
Jupiter’s escape velocity is 59 km/s
• Phobos (a moon of Mars) is so small that its
escape velocity is about 50 km/hr (13.9 m/s).
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The Solar Nebula Hypothesis
A rotating cloud of
gas contracts and
flattens …
to form a thin disk
of gas and dust
around the forming
sun at the center.
Planets grow from
gas and dust in the
disk and are left
behind when the
disk clears.
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Lecture 5: A Planetary Overview
Solar Nebula Hypothesis
• An object shrinking under the force of gravity
heats up. High temperatures near the newly
formed Sun (protosun) will prevent the
condensation of more volatile elements.
Planets forming there will thus be made of
nonvolatile, dense material.
Planet
Building
• Farther out, the eddies are larger and the
temperatures cooler so large planets can form
that are composed of volatile elements (light
gases).
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Lecture 5: A Planetary Overview
Solar Nebula Hypothesis
• Problem: The total angular momentum of
the planets is known to be greater than that
of the Sun, which should not occur according
to conservation laws (i.e. the present Sun is
spinning too slowly).
• Solution: As the young Sun heated up, it
ionized the gas of the inner solar system.


The Sun’s magnetic field then swept through the
ions in the inner solar system, causing ions to
speed up.
As per Newton’s third law, this transfer of energy
to the ions caused the Sun to slow its rate of
rotation.
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Lecture 5: A Planetary Overview
Solar Nebula Hypothesis
• A rotating, contracting disk of gas will speed
up according to the law of conservation of
angular momentum.

Angular momentum of an object is the product
of that object’s mass (m), speed of rotation (v),
and distance from the center of rotation (r).
A.M. = m×v×r

Demo
Conservation of angular momentum means that
(in the absence of an outside force) as the
distance to the spin axis decreases (contraction),
the speed increases.
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