What is a planet?

For the Greeks…

planet = “traveler”

Huh? Traveller?

• They could see that most objects in the sky rotate together, like the mirror squares on a disco ball (see picture on next slide).

• But a few move differently from those many, each with its own motion.

• These few they called planets

• They could see five of these (no telescopes, naked eye only), plus Sun and Moon

Like the mirror squares on a disco ball…

Camera pointed north, with the lens open. Shows the stars rotating around earth.

From College Physics, 1968, Pg 259

Later additional criteria…

• Stars make their own light– Gravity crushes star enough to start nuclear

reactions (a star is a continuous hydrogen bomb going off)

• Planets are smaller than stars, only shine by light from stars

• Planets orbit around stars (suns)

So, nine planets …

1. Mercury 4. Mars 7. Uranus

2. Venus 5. Jupiter 8. Neptune

3. Earth 6. Saturn9. Pluto

(always doubts)

BUT in 2005 …

• Astronomers discovered Xena (permanent name is Eris)

• Larger than Pluto but obviously not a planetVery distant orbitTilted way out of line with other planetsOrbit is distant, much less roundMade out of only a little rock, but lots of ice

So, in 2006…

• The International Astronomical Union (IAU) redefined “planet”:

• Must “command” its orbital region – absorb or eject other nearby small bodiesEarly history of earth had many collisions as it

cleared out its orbitMoon covered with craters from these early

collisionsEarth was molten, so few craters remain

• Pluto and Eris now “Dwarf Planets”