Unit 13Planets

What is a planet?

Old Definition

• A planet is a body that orbits a star, shines by reflecting the star's light and is larger than an asteroid.

New Definition

The International Astronomical Union (IAU), defines a planet as an object that orbits a star, is large enough to have settled into a round shape and, crucially, "has cleared the neighborhood around its orbit."

How did they form?

• Remember the nebular hypothesis?• Our solar system started 4.6 billion years

ago as a cloud of dust rotating in space.• 10% of the cloud made up the platelike disk around the sun. • As the cloud spun large pieces of debris would

collide to form planetesimals.

What are the 2 groups?

• Rocky

• Gas Giants

Terrestrial (Inner-Rocky)

• Rocky crust

• Dense mantle layers

• Very dense cores

Jovian (Outer-Gas Giants)

• Much larger

• No solid surfaces

• Mainly hydrogen and helium

• They all have rings

Mercury

• Closest to the Sun• No atmosphere• Surface temp between -280oF and +800oF• Hot enough to melt lead and zinc• Almost cold enough for liquid oxygen to form• Day is nearly 59 days long• Year is 88 days

Mercury

Venus

• Hotter than Mercury, even though almost twice as far from the Sun

• Its day (243 days) is longer than its year (224 days

• Atmosphere is about 96% CO2

• Air pressure 90 times that of Earth• Sulfuric acid clouds

Venus

Earth

• Only planet with chocolate• Only planet with liquid water• Only known life, so far, in the solar system

Mars

• A bit more than half the diameter of Earth• Surface area a bit less than land mass of Earth• Day is about 24.5 hours, Year about 22.5

months• Atmosphere less than 1% of Earth’s• 2 moons• Phobos, average diameter, 13.8 miles • Deimos, average diameter, 7.8 miles

Mars

Phobos

Deimos

Jupiter

• King of the planets• Weighs more than all the other planets

combined• Over 1300 Earths could fit inside• Magnetic field is 20,000 times Earth’s• Nearly 90% hydrogen• At least 63 moons

Jupiter

Moons of Jupiter

• Biggest moon, Ganymede, larger than Mercury

• Europa, bigger than Pluto, nearly as large as the Moon

• Io, most volcanically active body in the solar system. Larger than the Moon

• Callisto, almost as large as Mercury• These 4 moons were discovered by Galileo

Ganymede

Io

Callisto

Europa