8.3: The Solar System: The Sun and the Planets
pg. 313
Key Concepts:
1. Careful observation of the night sky can offer clues about the motion
of celestial objects.
2. Celestial objects in the Solar System have unique properties.
3. Some celestial objects can be seen with the unaided eye and can be
identified by their motion.
4. The Sun emits light and other forms of radiant energy that are
necessary for life to exist on Earth.
5. Satellites have useful applications for technologies on Earth.
6. The study of the night sky has influenced the culture and lifestyles of
many civilizations.
- The Sun, its eight planets and moons make up our Solar System.
- Planets are close to the Sun. (Mercury 58 million km, Neptune 4 billion km.
- At 28 000 km/h, a space craft will take 50 years to travel across the Solar
System.
Figure 1: This drawing shows the Solar System, but does not illustrate true
relative distance or sizes.
Measuring Distances in the Solar System
Astronomical Units: approximately 150 million kilometers; the average
distance from Earth to the Sun.
- The Astronomical Unit (AU) is equal to 1.5 x 108 km.
- The Astronomical unit is equal to the distance from the Sun to Earth. All
other planets are compared to Earth.
- Jupiter is 5.2 AU from the Sun; therefore it is 5.2 X 1.5 X 108 km. Jupiter
is 780 000 000 km from the Sun.
Planets Big and Small
- Other then the Sun the planets are the largest objects in the Solar System.
- Without the use of an optical aid, we can see Venus, Mars, and Jupiter at
night.
- The inner solar system consists of Mercury, Venus, Earth, and Mars,
Terrestrial Planets.
- The giant planets, beyond Mars are; Jupiter, Saturn, Uranus, and Neptune,
make up the outer Solar System. These are gaseous planets.
Dwarf Planets
Dwarf Planets: a celestial object that orbits the Sun and has a spherical
shape but does not dominate its orbit.
- To be considered a planet a celestial object must express the following
characteristics;
* be in orbit around a star (such as the Sun)
* have enough mass to be pulled into a stable sphere shape by gravity.
* dominate its orbit (i.e., its mass must be greater than anything else that
crosses its orbit)
Figure 2: Pluto does not meet the criteria for a planet because its tilted orbit
crosses Neptune’s orbit.
- Pluto was once considered a planet.
- After 2006, the definition of a planet changed, Pluto was no longer
considered a planet.
- It was newly categorized as a Dwarf Planet.
- Pluto orbits the Sun and is spherical in shape, but it does not dominate its
orbit.
- There are now 5 Dwarf Planets: Pluto, Ceres, Haumea, Makemake, and
Eris.
- These five lie outside the orbit of Neptune.
- Scientists estimate there are 200 to 2000 dwarf planets, most should be
found within the Kuiper belt.
Smaller Members of the Solar System
- The solar system also contains smaller celestial objects, rock, metal, and
ice composition.
Asteroid Belt
- Objects composed of rock and metal are called Asteroids.
- They orbit the Sun, yet they are too small to be planets.
- Most are found in the Asteroid Belt, located between Mars and Jupiter.
- Some asteroids can be as large as 950 km in diameter, and irregular in
shape.
Meteoroids
- Objects smaller then asteroids are called meteoroids, and are also made up
of metal and rock.
- The size of a meteoroid ranges from dust particles up to the size of cars.
- Meteoroids that enter the Earth’s atmosphere are called meteors and burn
up before they reach the Earth’s surface.
- Meteorites are meteors that actually reach the Earth’s surface, crashing into
the ground creating craters.
Meteor Showers
- On average a meteor enters our atmosphere every 15 minutes.
- There are time periods where the number increases per minute. Meteor
showers or shooting stars travel at a speed of 1.5 X 105 km/h.
Comets
Comets: a chunk of ice and dust that travels in a very long orbit around the
Sun.
- Comets are large chunks of ice, dust and rock.
- Comets orbit the Sun.
- Comets can be classified as either short or long period comets.
- Short period comets originate from a region outside Neptune and travel
around the Sun within 200 years.
- Hailey’s comet orbits the sun every 75 – 76 years.
- Long period comets originate outside of Pluto’s orbit and takes more then
200 years to orbit the Sun.
- Hale-Bopp takes 2380 years to orbit the Sun.
- When a comet gets closer to the Sun, the solid ice becomes a gas
(sublimates), gas and dust particles are released.
- These materials surround the nucleus (icy nucleus as sublimation occurs)
creating a Coma, a gaseous cloud 1000’s of kilometers wide.
- The Sun’s radiation and solar wind acts upon the coma causing tails to
form.
- The gaseous tail forms, directly pointing away from the Sun.
- The dust tail trails from the direction of which the comet came from.
- Gravity can pull comets into a planet or the Sun.
Figure 8: Most comets have two tails – a gaseous tail and a dust tail
Check Your Learning:
Questions 1 – 9, page 317
Summary:
- The average distance between the Sun and Earth is defined as one
astronomical unit (AU).
- The four planets closest to the Sun are the terrestrial planets. These small,
rocky planets are considered part of the inner solar system. The four planets
farthest from the Sun are gas giants. These large, gaseous planets are part of
the outer solar system.
- Pluto has bee reclassified as a dwarf planet, based on its physical properties
and motion.
- Besides the planets, there are smaller objects in the solar System that orbit
the Sun, such as asteroids, comets, and meteoroids.