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The Inner and Outer Planets
A solar story
Astronomical Unit
1 AU = 149,597,870.691 kilometers
(93,000,000 miles)
Definition: An Astronomical Unit is
approximately the mean distance between the
Earth and the Sun.
Why is it the mean distance?
Answer: Because orbits are elliptical
• Jupiter is 5.2 AU’s away from the sun
Mercury
• Smallest terrestrial planet
closest to the sun
• No moons
• Surface consists of flat
plains and craters
• Very little atmosphere
• Greatest temp. difference
860 F to –274F
Venus
• Similar to Earth in
Size
• Takes 8 months to
rotate on it’s axis and
7.5 months to revolve
around the sun.
• Planet rotates from
east to west.
Earth
• Unique because it has
liquid water (70%)
• Has enough gravity to
hold an atmosphere
• Atmosphere is made
out of nitrogen,
oxygen and carbon
dioxide.
• Has one moon
Mars
• Red because of the iron
rich rocks on the surface
• Atmosphere is more than
95% CO2
• Surface temps. range from
–140 to 20 C.
• There may have been
flowing water
• Has seasons because of its
tilted axis
• Has two moons
The Outer Planets
• Known as the gas
planets
• All the gas planets
have rings
• Rings are mostly ice
but are also composed
of rock and dust
Jupiter
• Largest most massive
planet
• Thick atmosphere of H
and He
• Great red spot the size of
Earth
• Prob. has a dense core of
rock
• Atmospheric pressure at
the core is 30 million X
that of Earth.
• Many moons most famous
are the Galileon moons (4)
Saturn
• Thick atmosphere of
H and He
• The density of Saturn
is less than water!
• Rings made of chunks
of rock and ice.
• Saturn’s largest moon
is titian
Uranus
• Blue green color
because atmosphere is
made up of methane
• Discovered in 1781 by
William Herschel
• Axis is tilted at 90
deg. and rotates in
about 17 hours
• Has at least 27 moons
Neptune
• Cold blue planet
• Discovered by
mathematical prediction
based on the orbit of
Uranus
• Dark spots are prob.
storms on the surface
• 13 moons
• Internal heat source.
Slowly shrinking
Pluto?
(The roman god of the underworld)
• Three moons
• Revolves around the
sun 1x every 248
Earth years
• Not considered a
planet today
So why isn’t Pluto a planet?
Is Pluto a planet? Does it qualify? For an object to be a planet, it needs to meet these
three requirements defined by the IAU:
•It needs to be in orbit around the Sun – Yes, so maybe Pluto is a planet.
•It needs to have enough gravity to pull itself into a spherical shape –
Pluto…check
•It needs to have “cleared the neighborhood” of its orbit – Uh oh. Here’s the rule
breaker. According to this, Pluto is not a planet.
What does “cleared its neighborhood” mean? As planets form, they become the
dominant gravitational body in their orbit in the Solar System. As they interact with
other, smaller objects, they either consume them, or sling them away with their
gravity. Pluto is only 0.07 times the mass of the other objects in its orbit. The Earth,
in comparison, has 1.7 million times the mass of the other objects in its orbit.
Any object that doesn’t meet this 3rd criteria is considered a dwarf planet. And so,
Pluto is a dwarf planet. There are still many objects with similar size and mass to
Pluto jostling around in its orbit. And until Pluto crashes into many of them and
gains mass, it will remain a dwarf planet. Eris suffers from the same problem.
Size comparison of planets
The next slide shows The Relative Sizes of the Largest
Solar System Objects - in the following order (to scale):
Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars,
Ganymede, Titan, Mercury, Callisto, Io, the Moon,
Europa, Triton, Eris, Pluto
Size comparison of planets
Size comparison of planets
compared to the Sun
100 Largest objects in the solar
system