Lesson 20: The Nebular Theory of Solar System Formation
In this lesson, we will look at four main features of our solar system
including the ordered patterns of motion in the Sun, planet and moons. We will
then look at how our solar system may have formed, as proposed in the Nebular
Theory. We will also discuss the origin of the solar nebula that formed into
our solar system.
When trying to develop a scientific theory on how our solar system was formed,
scientists focus on four major features that
stand out about the general structure of our solar system.
The first is the patterns of motion among the large bodies - the Sun,
planets and large moons all orbit and rotate in an organized way.
Second is that the eight planets clearly divide into two groups - the
terrestrial planets that are small and rocky, and are close together and close
to the Sun, and the jovian planets that are large and gas-rich, and are farther
apart and farther from the Sun.
The third major feature of our solar system is the vast numbers of asteroids and
comets that orbit the Sun,
with some being large enough to qualify as dwarf planets.
Finally, the last feature is the existence of notable exceptions to the
generally orderly solar system. For example, Earth is the only terrestrial
planet with a LARGE moon. Mars has two moons, but they are not even twenty
miles across and are not big enough for gravity to mold them into a sphere.
Mercury and Venus have no moons. Another exception to the general rules of our
solar system is Uranus and how it is tipped on its side.
As we mentioned,
the most important feature of our solar system is probably the clear patterns
of motion among the large objects.
All planetary orbits are nearly circular and lie practically in the same plane.
All planets orbit the Sun in
the same direction - counterclockwise when viewed from above the Earth's North
Pole.
Most planets rotate in the same direction in which they orbit with small axis
tilts. The exceptions are Venus, which rotates in the opposite direction and
Uranus, which is tilted dramatically on its side. The Sun also rotates the same
direction as the planets.
Most of the large moons exhibit similar properties in orbiting their planets
such as orbiting in the equatorial plane of their planet in the same direction
that the planet rotates.
What is the nebular theory of solar system formation?
In around 1755,
German philosopher Immanuel Kant proposed the idea that
our solar system formed from the gravitational collapse of an interstellar
cloud of gas.
An interstellar cloud of gas is known as a nebula,
so this came to be known as the nebular hypothesis.
Subsequent experiments and
models including more recent discoveries about the physics of planet formation
showed that this hypothesis offered natural explanations for all four general
features of our solar system. Due to the overwhelming evidence supporting this
hypothesis, it has now achieved the status of scientific theory.
The Nebular Theory states that our solar system formed from a cloud of gas,
usually called the solar nebula.
The origin of the nebula can be traced back to
the Big Bang. The Big Bang produced basically only two elements - hydrogen and
helium. Stars formed from these original elements and produced heavier elements
which were released when they eventually died. After billions of years of the
birth and death of stars, the universe is still mostly made up of hydrogen and
helium.
The solar nebula that
formed into our solar system likely contained about 98% hydrogen and helium,
and 2% all other elements combined.
Spectroscopy analysis of old stars supports these ideas because the older the
star, the less heavier elements it appears to contain - which is in line with
the idea that the universe originally contained little more than hydrogen and
helium.
Other evidence is found in observing nebulas such as the Orion Nebula,
in which many stars are in various stages of formation.