Solar System – Odds & Ends

So what was that “retrograde motion”stuff all about? Why do the planets

appear to move backwards sometimes?

http://mars.jpl.nasa.gov/allaboutmars/nightsky/nightsky04/

http://www.mhhe.com/physsci/astronomy/applets/Retro/frame.html

It’s an ILLUSION caused by the earth “passing” other planets as we all orbit the sun.

We would not see this if earth was at the center.

Dwarf Planets

Eris and its moon1500 miles wide

(drawing)

Ceres – the largestobject in the asteroid belt;

600 miles wide (photo)

Pluto and its moon1430 miles wide(drawing)

The Kuiper Belt – billions of icy objectsorbiting the sun, beyond Neptune’s orbit.

Pluto and Eris are both part of the Kuiper Belt.

The Oort Cloud – even farther out – billions of icy objects orbiting the sun. Home of comets that

occasionally visit the inner Solar System

The orbit of Sedna is extremely elliptical. It takes 10,500 years to circle the sun!

http://www.spitzer.caltech.edu/video-audio/681-ssc2004-05v1-Orbit-of-Sedna

What about somethinglike Sedna – outsideeven the Kuiper Belt. Is Sedna a planet?

Rest of Solar System!

What makes something a planet?Why did Pluto get kicked out?

Possible definitions of “planet”:(from Mike Brown – discoverer of Eris, Quaoar, and Sedna - and the

“killer” of Pluto - “www.gps.caltech.edu/~mbrown/ )

1) Purely historical: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto are planets. No scientific basis.

2) Historical plus: Mercury through Pluto are planets, as is any newly discovered object larger than Pluto. But why is Pluto’s size the cutoff?

No scientific basis.

Possible definitions of “planet”:

3) Gravitational rounding: Any object which is round and which directly orbits the sun is a planet. Scientifically based.

Pluto would be a planet, but so would Ceres, Eris, Quaoar, Sedna, and perhaps a dozen other Kuiper belt objects. But what’s so special about roundness?

Possible definitions of “planet”:

4) Population classification: If the object is a “solitary individual” it is a planet. If it’s part of a whole population of objects ranging in size, it is not.

So: Ceres – just part of the asteroid belt.Pluto, Eris, Quaoar – just part of the Kuiper BeltSedna – just part of the “inner Oort Cloud” (they think)

But Mercury, Venus, Earth, etc……. No other objects anywhere near their size in their region of the solar

system. They are definitely PLANETS.

(This one seems pretty good, according to Mike Brown.)

SO: Our Solar System has 8 planets

and 3 “dwarf planets”

(But why isn’t Sedna a dwarf planet then??)