1. Planets
Ch. 21 Notes

2. Astronomical Unit
The average distance between the Earth and the sun; approximately 150 million kilometers AU
Most useful unit for measuring distances within the solar system

3. Planets VS. Dwarf Planets
Must be round
Orbit the sun
Have cleared out the region of the solar system along its orbit
Object that orbits the sun
Has enough gravity to be spherical
Has not cleared the area of its orbit
5 known: Pluto, Eris, Ceres, Makemake, and Haumea
List the planets in order of increasing distance from the sun (Science Starter)
Where is the largest gap between planets? Mars and Jupiter

4. Inner Planets Mercury, Venus, Earth and Mars Small Dense
Rocky surfaces
Called “Terrestrial Planets”
Mercury is the only inner planet without an atmosphere

5. Inner Planets Which planet is the largest?
Which planet has the most moons?
Which planet is most similar to Earth in size?

6. Smallest terrestrial planet Closest to the sun
Mercury
Smallest terrestrial planet
Closest to the sun
Temps. Range from 430 Celsius to -170 Degrees Celsius
Surface: flat plains
Atmosphere: None
Because Mercury’s mass is small, its gravity is weak. Gas particles can easily escape into space.
Temps. Range from 430 to -170 C because during the day it’s facing the sun and at night is not

7. Venus Earth’s twin Retrograde: spins clockwise
Atmosphere: think and it is always cloudy, mostly made up of carbon dioxide
Hottest planet, covered in rock
Greenhouse effect: the trapping of heat near a
planet’s surface by certain gases in
the planet’s atmosphere
Earth’s twin because of size and mass
Clouds are made of sulfuric acid
You would not be able to breathe on Venus because of the CO2
CO2 traps heat
1970 first probe on Venus: Venera 7 lasted only a few mins. before it blew up from high temp/pressure
In 1990, The Magellan probe was able to map out all of Venus’ surface using radar instruments

8. Earth
The only planet in the solar system where you could live easily. Earth has liquid water and a suitable temperature range and atmosphere for living.
Like Venus, Earth experiences a greenhouse effect. Without the atmosphere, Earth would be much cooler.

9. Mars “Red planet” Has ice (two polar caps)
Atmosphere: 95% Carbon Dioxide
Temperatures on the surface range from -140C to 20 C
Volcanoes
Mars has 2 very small moons (Phobos and Deimos)
Known as red planet because of the breakdown of iron-rich rocks, leaving rusty dust behind
Mars is too cold for liquid water now and had liquid water in the past
You can walk on Mars however you need your own oxygen supply
Mars use to be much warmer due to a thicker atmosphere
We have been to Mars with Rovers called Spirit and Opportunity
Phownix mission took samples of soil

10. Jupiter, Saturn, Uranus, and Neptune Gas Giants Many moons
The outer planets
Jupiter, Saturn, Uranus, and Neptune
Gas Giants
Many moons
Surrounded by a set of rings
These planets do not have a solid surface
Rings: a thin disk of small particles of ice and rock

11. Jupiter Largest and most massive planet
Atmosphere: thick atmosphere made up of hydrogen and helium
Great Red Spot
Moons: at least 63
Galileo Galilei found Jupiter’s largest moon in 1610

12. Saturn 2nd largest planet Atmosphere: thick, made up of hydrogen and
helium
Has the most spectacular rings of any planet made of ice and rock
Moons: at least 61 moons
Has the lowest density of any planet. If you could build a bathtub big enough, Saturn would float!

13. Uranus Blue-green in color due to methane in its atmosphere
Surrounded by dark, thin, flat rings
Moons: at least 27
Rotates from top to bottom
Astronomers think that billions of years ago, an object hit Uranus and knocked it on its side

14. Neptune Similar in size and color to Uranus Cold, blue planet
Atmosphere: contains no clouds
Moons: at least 13 (largest: Triton)
Blue comes from methane in atmosphere
Neptune’s interior is hot due to energy left over from it’s formation.