What is a planet?

1. What is a planet?

2. For the Greeks… planet = “traveler”

3. 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

4. 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

5. 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)

6. So, nine planets …

7. 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

8. 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”