It's time for the human race to enter the solar system.
Dan Quayle
Arizona State University AST 111
Introduction to Solar Systems Astronomy
Frank Timmes
[email protected]
cococubed.asu.edu/class_pages/class_planet.shtml
Chapter 7 - Our Planetary System - Learning Goals:
Patterns in the Solar System
‣What features of our solar system
provides clues to how it formed?
Patterns in the Solar System
What features of our solar system provides clues to how it formed?
rs
Plut
Ea
rt
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M
a
All planets orbit the Sun in the same direction: counterclockwise when
viewed from above the Earth’s North Pole.
r c ur y
V en
Me
us
Nept
un
e
Ura
nu
s
rn r
e
Satu
Jupit
Most planets rotate in the same direction
they orbit: counterclockwise when viewed
from above the Earth’s North Pole.
The Sun rotates in
Mercury
Sun
Venus
the same direction in which
the
Mars
planets orbit.
o
All planetary orbits lie in nearly the same plane.
Almost all moons orbit their planet in the same direction as the planet’s
rotation and near the planet’s equatorial plane.
Earth
Venus
Sun
Mercury
Mars
Saturn
Uranus
Neptune
Jupiter
Pluto
Almost all the planets travel on nearly circular orbits, with a spacing that
increases with distance according to a fairly regular trend.
Most planets have fairly small axis tilts, usually less than 25º.
Question #1: Why are motions in the solar system generally so orderly?
Can we categorize the planets into groups?
Terrestrial
Jovian
Smaller size and mass
Larger size and mass
Higher density (rocks, metals)
Lower density (light gases)
Solid surface
No solid surface
Closer to Sun (and closer together) Further from Sun (and further apart)
Warmer surfaces
Cooler could tops
Few if any moons and no rings
Many moons and all have rings
Question #2: Why do the inner & outer planets
divide so neatly into two classes?
No description of our solar system would be complete without the most
numerous objects in the solar system: asteroids and comets.
Asteroids are small rocky bodies that orbit the Sun between the orbits of
Mars and Jupiter, primarily in the asteroid belt.
Their orbits generally lie close to the plane of the planetary orbits, although
they are usually tilted a bit more. Some have quite large eccentricities.
Some 500,000 asteroids are known. The largest asteroids have a radius of
about 200 km - much less than half of the Moon’s radius.
Comets are small, icy bodies residing in the Kuiper Belt and the Oort Cloud.
The Kuiper belt begins near
Neptune (~30 AU) and
extends out to ~100 AU.
Their orbits lie fairly close to
the ecliptic and are in the same
direction as the planets.
The Oort cloud is a huge, spherical region centered on the Sun that extends
about halfway to the nearest stars.
Comets in the Oort cloud comets
have orbits with random inclinations,
orbital directions, and eccentricities.
Question #3: Why are there a large number of asteroids & comets in different locations?
Some object patterns don’t fit the general patterns:
Mercury and Pluto have much larger eccentricities and inclinations.
Venus rotates backwards - clockwise rather than counterclockwise.
The rotational axes of Uranus and Pluto are substantially tilted.
2027 solstice
B
B
B
B
A
A
A
B
B
A
B
A
Pole A
in light
Pole B
in light
B
A
B
B
A
2006 equinox
1965 equinox
A
B
B
A
A
A
Rotation
A
1985 solstice
8º
Earth has an exceptionally large moon. Pluto’s moon is almost as big as Pluto.
While most Jovian moons orbit with the same orientation as the planet’s
rotation, a few orbit in the opposite direction.
Question #4: Why are there exceptions to the general patterns?
What features of our solar system provides clues to how it formed?
1) The Sun, planets, and moons generally rotate and orbit in the same way.
2) The eight official planets clearly divide into two groups, terrestrial and jovian.
3) The solar system contains huge numbers of asteroids and comets.
4) There are notable exceptions to these general patterns.