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Solar System
Astronomers have always noticed planets, the stars,
and the moons.
We will use the powerful and still emerging perspective
of comparative planetology to understand better the
conditions under which planets form and evolve.
Solar System
comparative planetology
The systematic study of the similarities and differences
among the planets, with the goal of obtaining deeper
insight into how the solar system formed and has
evolved in time.
Solar System
solar system
The Sun and all the bodies that orbit it—
Mercury, Venus, Earth, Mars, Jupiter, Saturn,
Uranus, Neptune, (and Pluto) their moons, the
asteroids, and the comets.
Solar System
Comets appear as long, wispy strands of light
in the night sky that remain visible for periods
of up to several weeks, then slowly fade from
view.
Solar System
Meteors, or "shooting stars" are sudden bright
streaks of light that flash across the sky, usually
vanishing less than a second after they first
appear.
Asteroids or "minor planets" orbiting the Sun,
mostly in a broad band (called the asteroid belt)
lying between Mars and Jupiter.
Meteor
Solar System
ORBITAL
SEMIMAJOR
AXIS (A.U.)
ORBIT
PERIOD
(Earth
years)
MASS
(Earth
masses)
RADIUS
(Earth radii)
Mercury
0.39
0.24
0.055
Venus
0.72
0.62
Earth
1
Moon
NUMBER
OF KNOWN
MOONS
ROTATION
PERIOD*
(days)
0.38
0
0.82
0.95
1
1
—
—
Mars
1.5
Ceres
(asteroid)
OBJECT
AVERAGE DENSITY
(kg/m3)
(g/cm3)
59
5400
5.4
0
-243
5200
5.2
1
1
1
5500
5.5
0.012
0.27
—
27.3
3300
1.9
0.11
0.53
2
1
3900
3.9
2.8
4.7
0.00015
0.073
0
0.38
2700
2.7
Jupiter
5.2
11.9
318
11.2
16
0.41
1300
1.3
Saturn
9.5
29.4
95
9.5
18
0.44
700
0.7
Uranus
19.2
84
15
4
17
-0.72
1300
1.3
Neptune
30.1
164
17
3.9
8
-0.67
1600
1.6
Pluto
39.5
248
0.002
0.2
1
-6.4
2100
2.1
Comet HaleBopp
180
2400
1.0x 10-9
0.004
—
0.47
100
0.1
Sun
—
—
332,000
109
—
25.8
1400
Solar System
Orbital Semi-Major Axis
The major axis of an ellipse:
Longest diameter, a line that runs through the widest
points of the shape.
The semi-major axis is one half of the major axis, and
thus runs from the centre, through a focus, and to the
edge of the ellipse.
The distance of each planet from the Sun is known
from Kepler's laws once the scale of the solar system is
set by radar-ranging on Venus.
Solar System
Orbit Period
Length of time the object takes to orbit the sun.
A planet's (sidereal) orbital period is easily measurable
from repeated observations of its location on the sky,
so long as Earth's own motion around the Sun is
properly taken into account.
Solar System
Planet Mass
The masses of planets with moons may be calculated by
application of Newton's laws of motion and gravity, just by
observing the moons' orbits around the planets.
The masses of Mercury and Venus (as well as those of our
Moon) are a harder to determine because these bodies have
no natural satellites
Solar System
Planet Mass
We observe their influence on other planets or nearby bodies.
Mercury and Venus produce small but measurable effects on
each other's orbits, as well as that of Earth.
The Moon causes small "wobbles" in Earth's motion as the two
bodies orbit their common center of mass.
Solar System
Rotation Period
Length of time for an object to rotate completely around its
axis.
A planet's rotation period is determined simply by watching
surface features appear and disappear again as the planet
rotates. For some planets this is difficult to do, as their
surfaces are hard to see or may even be nonexistent
Solar System
The planets’ paths are all ellipses, with the Sun at (or very
near) one focus.
Most planetary orbits have low eccentricities.
The exceptions are the innermost and the outermost worlds,
Mercury and Pluto.
High eccentricities indicate more oval and less circular shapes.
Accordingly, we can think of most planets' orbits as circles
centered on the Sun.
Solar System
Maybe future space voyagers travel far enough from Earth to gain this
perspective on our solar system
Except for Mercury and Pluto, the orbits of the planets lie nearly in the
same plane. As we move out from the Sun, the distance between the orbits
of the planets increases. The entire solar system spans nearly 80 A.U.
Solar System
AnAstronomicalRuler.MOV
Solar System
The Titius-Bode law seemed to "predict" the radii of the
planetary orbits remarkably well.
Even the asteroid belt between Mars and Jupiter appeared to
have a place in the scheme, which excited great interest
among astronomers and numerologists alike.
There is apparently no simple explanation for this empirical
"law."
Solar System
On large scales, the solar system presents us with a sense of
orderly motion.
The planets move nearly in a plane, on almost concentric and
nearly circular paths
They move in the same direction around the Sun, at steadily
increasing orbital intervals.
However, the individual properties of the planets themselves
are much less regular.
Solar System
•A clear distinction can be drawn between the inner and the
outer members of our planetary system based on densities
and other physical properties.
•The inner planets—Mercury, Venus, Earth, and Mars—are
small, dense, and rocky in composition.
•The outer worlds—Jupiter, Saturn, Uranus, and Neptune (but
not Pluto)—are large, of low density, and gaseous.
Solar System
Diagram, drawn to scale, of the relative sizes of the planets and our Sun. Notice
how much larger the joviian planets are than Earth and the other terrestrials and
how much larger still is the Sun.
Solar System
•The terrestrial worlds lie close together, near the Sun
•the jovian worlds are widely spaced through the outer solar system.
•The terrestrial worlds are small, dense, and rocky;
•the jovian worlds are large and gaseous, being made up
predominantly of hydrogen and helium (the lightest elements), which
are rare on the inner planets.
•The terrestrial worlds have solid surfaces;
•the jovian worlds have none (their dense atmospheres thicken with
depth, eventually merging with their liquid interiors).
Solar System
•The terrestrial worlds have weak magnetic fields, if any;
•the jovian worlds all have strong magnetic fields.
•The terrestrial worlds have only three moons among them;
•the jovian worlds have many moons each, no two of them
alike and none of them like our own.
•Furthermore, all the jovian planets have rings, a feature
unknown on the terrestrial planets.
•Despite their greater size, the jovian worlds all rotate much
faster than any terrestrial planet.
Solar System
TheTerrestrialPlanetsI.MOV
Solar System
The Terrestrial Planets
TERRESTRIAL PLANETS
close to the Sun
high density
closely spaced
orbits
slower rotation
small masses
weak magnetic
fields
small radii
few moons
predominantly rocky
no rings
solid surface
Solar System
TheGasGiantsI.MOV
Solar System
Gas Giants
JOVIAN PLANETS
far from the Sun
no solid surface
widely spaced orbits
low density
large masses
faster rotation
large radii
strong magnetic fields
predominantly gaseous
many moons
many rings
Solar System
•Beyond the outermost jovian planet, Neptune, lies one more
small world, frozen and mysterious.
•Pluto doesn't fit well into either planetary category.
•“Indeed, there is debate among planetary scientists as to
whether it should be classified as a planet at all. In both mass
and composition, it has much more in common with the icy
jovian moons than with any terrestrial or jovian planet.
Astronomers speculate that it may in fact be the largest
member of a newly recognized class of solar system objects
that reside beyond the jovian worlds.”