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. Mercury4. Mars7. Uranus 2. Venus5. Jupiter8. Neptune 3. Earth6. Saturn 9. Pluto (always doubts)

BUT in 2005 … Astronomers discovered Xena (permanent name is Eris) Larger than Pluto but obviously not a planet Very distant orbit Tilted way out of line with other planets Orbit is distant, much less round Made 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 bodies Early history of earth had many collisions as it cleared out its orbit  Moon covered with craters from these early collisions  Earth was molten, so few craters remain Pluto and Eris now “Dwarf Planets”