||6.1 A Brief Tour of the Solar System
6.2 Clues to the Formation of
Our Solar System
- What does the solar system look like?
Our solar system consists of
the Sun, nine planets and their moons, and vast numbers of asteroids and
comets. Each world has its own unique character, but there are many
clear patterns among the worlds.
Birth of the Solar System
- What features of our solar system provide clues to how it formed?
Four major features provide clues:
(1) The Sun, planets, and
large moons generally rotate and orbit in a very organized way.
With the exception of Pluto, the planets divide clearly into two groups:
terrestrial and jovian.
(3) The solar system contains huge numbers
of asteroids and comets.
(4) There are some notable exceptions to
these general patterns.
- What theory best explains the features of our solar system?
The nebular theory, which holds that the solar system formed from
the gravitational collapse of a great cloud of gas.
6.4 The Formation of Planets
- Where did the solar system come from?
The cloud of gas that gave
birth to our solar system was the product of recycling of gas through
many generation of stars within our galaxy. This gas consisted of 98%
hydrogen and helium and 2% everything else combined.
- What caused the orderly patterns of motion in our solar system?
gas cloud naturally tends to heat up, spin faster, and flatten out as it
shrinks in size. Thus, our solar system began as a spinning disk of gas.
The orderly motions we observe today all came from the orderly motion of
this spinning disk of gas.
- Why are there two types of planets?
Planets formed around solid
"seeds" that condensed from gas and then grew through accretion. In the
inner solar system, temperatures were so high that only metal and rock
could condense, which explains why terrestrial worlds are made of metal
and rock. In the outer solar system, cold temperatures allowed more
abundant ices to condense along with metal and rock. Icy planetesimals
grew large enough for their gravity to draw in hydrogen and helium gas,
building massive jovian planets.
- Where did asteroids and comets come from?
the rocky leftover planetesimals of the inner solar system, and comets
are the icy leftover planetesimals of the outer solar system.
- How do we explain the existence of our Moon and other "exceptions
to the rules"?
Most of the exceptions probably arose from collisions or
close encounters with leftover planetesimals, especially during the
heavy bombardment that occurred early in the solar system's history. Our
Moon is probably the result of a giant impact between a Mars-size
planetesimal and the young Earth.
- When did the planets form?
The planets began to accrete
in the solar nebula about 4.6 billion years ago, a fact we determine
from radiometric dating of the oldest meteorites.
- How do we detect planets around other stars?
So far, we are only able to
detect extrasolar planets indirectly by observing the planet's effects
on the star it orbits. Most discoveries to date have been made with the
Doppler technique, in which Doppler shifts reveal the gravitational tug
of a planet (or more than one planet) on a star.
- What have other planetary systems taught us about our own?
Planetary systems exhibit a surprising range of layouts, suggesting
that jovian planets sometimes migrate inward from where they are born.
This lesson has taught us that despite the successes of the nebular
theory, it remains incomplete.
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