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Unit 2 - The Solar System
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In class notes from the second unit of grade 12 earth and space science, the solar system.
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Unit 2 - Earth & Space Science Notes, Structure of the Solar System
● For thousands of years humans believed in a geocentric model in which everything rotated
around Earth; however, there was no theory to explain why some planets appear to move
backwards across the sky for short periods (retrograde motion).
● In the 2nd century, Ptolemy explained these movements by proposing that planets moved in
epicycles. His theory suggested that planets had small circular orbits as they revolve around
the Earth. However, these epicycles did not always match the planetary motion that was
observed in the night sky.
● It was not until the 16th century that Copernicus proposed a heliocentric model (sun-centred)
and the retrograde motion could be explained because planets orbit at different speeds and
distances from the Sun.
The Sun
● The Sun is at the centre of the solar system. As a main sequence star, energy is produced by
the fusion of hydrogen to helium. The Sun is composed of gases and can be divided into six
layers - the core, radiative zone, convective zone, photosphere, chromosphere, and corona.
● The core of the Sun where fusion occurs will reach temperatures of 13, 600, 000 K. The
photosphere is the visible surface of the Sun, and it is the source of the Sun’s light. It has a
temperature of 6000 K. The light scatters into space and becomes less dense in the outer
atmospheric layers (the chromosphere and the corona), which extend millions of kilometres
from the core. The corona is visible when there is a solar eclipse.
● In 2006, the International Astronomical Union (IAU) developed a new classification system for
planets and objects in the solar system. The three classes created were the planets, small
solar system bodies, and dwarf planets.
The IAU Classification of Objects Orbiting Around the Sun:
1. A “planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for
itself-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round)
shape, and c) has cleared the neighbourhood arounds its orbit.
2. A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for
itself-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round)
shape, and (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
, 3. All other objects except satellites (moons) orbiting the Sun shall be referred to collectively as “Small
Solar-System Bodies.”
Planets
● The eight planets all have counter-clockwise orbits with the orbital planes being closely aligned
with the Sun’s equator.
● The four inner planets, Mercury, Venus, Earth, and Mars are all terrestrial planets and are
composed of heavier elements (Fe, Si, Mg, S, and Ni).
● The four outer planets, Jupiter, Saturn, Uranus, and Neptune are all gaseous planets
composed of mostly Hydrogen and Helium. All of the outer planets are at least four times
larger than Earth's diameter and Jupiter is 11.2 times larger. The planetary rings are a unique
feature to the outer gaseous planets, with the rings of Saturn being the most famous and
documented.
Dwarf Planets
● With the addition of Makemake in 2008, there are now four official dwarf planets. Pluto was
formerly considered the smallest of the “nine traditional planets,” until being reclassified as a
dwarf planet. Eris and Makemake are icy bodies discovered in the Kuiper belt. Finally, Ceres,
the largest asteroid in the asteroid belt, has enough mass to produce a hydrostatic equilibrium,
(spherical shape). Astronomers estimate that with better telescopes and better funding for
research, more than 200 more dwarf planets may be proven to exist in the outer reaches of the
solar system.
Small Solar System Bodies (SSSBs)
● Asteroids composed of rock and metals located in the asteroid belt between Mars and Jupiter.
There are possibly over 1,000,000 asteroids greater than 1 km in size
● Smaller asteroids less than 10 m in diameter are called meteorites
● Comets are composed of ice. When comets approach the Sun, long tails of water vapour can
be seen from Earth. Comets may originate in the Kuiper belt (a great ring of debris similar to
the asteroid belt but consisting of icy material).
Outer Limits of the Solar System
● Where are the outer limits of the solar system? The answer to this question has eluded
astronomers. The principle problem is how to define the limits - should it be the outer limit of
the Sun’s particles or the outer limit of the Sun’s gravitational influence? The influence of the
Sun’s particles is known to be roughly four times that of Pluto’s orbit; however the gravitational
influence may be 100 times farther.
● Beyond Neptune at 30 AU, lies the Kuiper belt between 30 AU and 50 AU. The objects found
in the Kuiper belt are referred to as KBOs (Kuiper Belt Objects) and their research has
emerged in the last 10 years.
● The “scattered disc” is considered to be an extension of the Kuiper belt. Objects found in the
scattered disc have much higher eccentricities (more elliptical orbits) than the KBOs. As a
result they extend much farther from the Sun, out to more than 100 AU. These objects are also
inclined to the ecliptic plane, with highly irregular orbits possibly due to gravitational forces
from Neptune. Eris, one of the small dwarf planets, is located in the disc at 68 AU.
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