Download The Solar System

The Solar System
Dr. Ken Rice
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The Solar System
Is the region around the Sun
comprising
•
8 planets
– Mercury, Venus, Earth, Mars, Jupiter,
Saturn, Uranus, Neptune.
•
3 Dwarf planets
– Ceres, Pluto, Eris
•
Asteroids
– Asteroid belt between Mars and
Jupiter
•
Satellites/moons and Rings
– All planets except Mercury and
Venus have satellites
– All Jovian planets have rings
•
Comets
– Short period : Kuiper belt
– Long peroid : Oort cloud
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What is a solar system planet?
• Planet vs Star
– Star : Generates energy via nuclear fusion – emits predominantly blackbody
radiation that depends on its temperature
– Planet : Shines largely because of reflected light.
• International Astronomical Union (IAU) definition of a Solar System
planet
– Orbits the Sun.
– Is massive enough that its gravity causes it to be spherical (hydrostatic
equilibrium)
– Has largely cleared its neighbourhood of other bodies.
• By this definition, Pluto is no longer a planet since it has not cleared its
neighbourhood of other bodies.
– Pluto, and other small spherical solar system bodies in orbit around the Sun
are now known as dwarf planets.
Spherical bodies in the solar system are in hydrostatic equilibrium. The inward force of
gravity is balanced by an outward pressure force.
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Solar System Properties
• All the planets (and most other bodies)
– orbit in the same direction.
– have orbital planes very close to the ecliptic.
• All planets have small eccentricities (e < 0.1).
– Comets, Kuiper belt objects and Oort cloud objects can have very large
eccentricities.
• Titius-Bode Law
– The planets’ semimajor axes (orbital radius) generally satisfy the following law
a (AU) =
n+4
10
where n = 0, 3, 6, 12, 24, 48 (n > 3, n = 2[n − 1])
– Missing planet when n = 24 (Asteroid belt?)
– No strong theoretical reason why this Titius-Bode law should hold
• Orbital resonances?
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Planetary orbits
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Planetary orbits and rotation
• All of the planets orbit in the same
direction.
• All of the planetary orbits lie very
close to the ecliptic.
• Apart from Venus and Uranus, all
of the planets rotate in the same
direction.
–
Collision (either after or during
the formation process) probably
caused the retrograde rotation of
Uranus and Venus.
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Types of planets
Planets in the Solar System can be
divided into two basic types
• 4 inner rocky/terrestrial planets
– Mercury, Venus, Earth Mars
• 4 outer gaseous/icy planets
(Jovian)
– Jupiter, Saturn, Uranus, Neptune
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The Scale of the Solar System
•
By observing Venus from different locations on
the Earth when it transits the Sun, we can
measure its parallax angle.
d = r sin θ ≈ rθ (radians) ⇒ r =
•
d
θ (radians)
Using Kepler’s third law
PVenus ( years) = aVenus (AU )
2
3
d
– Can determine the distance from Venus to the
Sun in AU (AU not yet known).
•
Path seen from
the south
Now know the distance from Venus to the Sun in
AU, and the distance from the Earth to Venus in
metres
r (metres)
r (metres) = (1 − aVenus )AU ⇒ AU =
(1 − aVenus )
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θ
Path seen from
the north
The Scale of the Solar System 2
• We can now use radar to determine the distance to Venus.
• When Venus is at its greatest elongation (the furthest from the Sun
when viewed from Earth), we can use trigonometry to determine the
distance from the Sun to the Earth
– Radar gives distance, d, to Venus.
– AU = d/cos(e)
• From this :
– Mass of the Sun
– Stellar parallax
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Determining planetary properties
•
Distance determined using Kepler’s third law
P 2 ( years) = a 3 (AU )
– Period can be determined observationally
– Remember synodic and sidereal periods
1
1
1
1
1
1
=
−
or
= −
Psyn PE PS
Psyn PS PE
•
Apply Kepler’s laws to orbiting satellites (man-made or natural) to determine planet’s masses
4π 2 asat
4π 2 asat
π 2 asat
≈
⇒ M pl =
P =
G (M pl + msat ) GM pl
GP 2
3
3
3
2
•
Radii determined by occultations of stars, moons or satellites
•
Densities
ρ =
M
4 / 3πR 3
– Terrestrial planets : 3400 – 5500 kg/m3
– Jovian planets : 700 – 1600 kg/m3
– Water : 1000 kg/m3 (Saturn could float on water)
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Dwarf planets
• Spherical bodies orbiting the Sun that do not dominate their
neighbourhoods!
• Currently 3 dwarf planets
– Pluto, Ceres, Eris
• Almost certainly more that have yet to be discovered.
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Satellites/Moons
• All the planets except Mercury and
Venus have satellites
• Earth
– Moon
• Jupiter and Saturn (Jovian)
– 63 and 47 respectively
• Galilean moons
– 4 satellites of Jupiter discovered by
Galileo
– Io, Europa, Ganymede, Callisto
• Some of the Jovian satellites are larger
than Mercury
– e.g., Ganymede, Callisto, Titan
• Most of the solar system moons orbit in
the same plane as the planets and in
the same direction.
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Rings
• All 4 Jovian planets have ring systems.
• Made of small particles, mainly water
ice.
• Regular collisions cause Saturn’s rings
to be very thin
– Only a few metres thick.
• Small moons in the rings open gaps
• Other gaps formed by resonances with
outer moons.
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Saturn’s new ring
•
New ring of dust found around
Saturn!
– It would appear larger than the
Moon if we could see it from Earth.
•
May be material ejected from
Saturn’s moon Phoebus.
•
May explain the dark regions on
one side of Iapetus
– Dark material is dust from the ring.
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Asteroids
• Small rocky bodies generally
a few km in size.
• Most orbit the Sun in the
Asteroid belt
– Region between Mars and
Jupiter.
• Irregular shapes
• Largest may be a few
hundred km in size
– Asteroid belt also contains
the dwarf planet Ceres.
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Comets
• Small ball of dust and ice
(“dirty snowballs”) that orbit
the Sun.
• Highly elliptical orbits.
• Short period
– Coplanar
– Kuiper belt
• Long period
– All directions
– Oort cloud
• Long tails
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Meteorites
• A rocky object that can reach the
ground is called a meteorite.
• Primitive meteorites
–
Remnants from the birth of the
solar system.
• Processed meteorites
–
Once part of a larger object that
has processed the primitive
material.
• Some appear to come from the Moon
or Mars
–
Ejected by collisions.
–
Panspermia!
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Kuiper belt and Oort cloud
• Kuiper belt
– Ring of material 30-50 AU from the
Sun.
– ~100000 bodies bigger than 100 km.
– Short period comets.
– Pluto and Eris.
• Oort cloud
– Cloud of material extending to about
50000 AU from the Sun.
– May contain a trillion objects/comets
– Long period comets.
– Sedna?
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Exploration of the Solar System
•
There are four main ways to explore the solar system directly
–
–
–
–
Flyby
Orbiter
Lander or probe
Sample return mission
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Mars Landers
•
Two rovers landed on Mars in early 2004
– Opportunity
– Spirit
•
Carry various instruments for determining conditions on Mars
– Cameras
– Spectrometers
– Magnetometers
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Combination missions
• Combines various different strategies
– Flybys before arriving at destination
– Orbiter
– Lander or probe
• Galileo – Jupiter
– Orbiter plus a probe
• Cassini – Saturn
– Flybys if Venus, Earth and Jupiter
– Orbiter
– Huygens probe that landed on Saturn’s moon Titan
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Sample return missions
•
•
First sample return mission were the Apollo missions to the moon
(manned).
More recently there have been unmanned sample return missions
– Genesis collected solar wind particles
• Crash landed in Utah when parachute didn’t open
– Stardust collected dust sample from comet Wild-2
• Safely returned in January 2006
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Basic properties of solar system bodies
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Time to think
Asteroids have irregular shapes because
a.
they suffer many collisions
b.
there hasn’t been enough time for them to settle into spherical shapes.
c.
Jupiter’s gravity prevents them from becoming spherical
d.
they are not massive enough for their own gravity to make them spherical.
e.
they spin very fast.
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