PLANETSCh. 21 Notes

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

PLANETS VS. DWARF PLANETS

Planets 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

INNER PLANETS

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

an atmosphere

INNER PLANETS

MERCURY

• Smallest terrestrial planet• Closest to the sun• Temps. Range from 430 Celsius

to -170 Degrees Celsius• Surface: flat plains• Atmosphere: None

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

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.

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)

THE OUTER PLANETS

Jupiter, Saturn, Uranus, and Neptune

Gas Giants Many moons Surrounded by a set of rings

JUPITER

Largest and most massive planet Atmosphere: thick atmosphere made up

of hydrogen and helium Great Red Spot Moons: at least 63

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!

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

NEPTUNE

Similar in size and color to Uranus Cold, blue planet Atmosphere: contains no clouds Moons: at least 13 (largest: Triton)