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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
Ceres – the largest
object in the asteroid belt;
600 miles wide (photo)
Eris and its moon
1500 miles wide
(drawing)
Pluto and its moon
1430 miles wide
(drawing)
The Kuiper Belt – billions of icy objects
orbiting 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
What about something
like Sedna – outside
even the Kuiper Belt. Is
Sedna a planet?
Rest of 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/681ssc2004-05v1-Orbit-of-Sedna
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 Belt
Sedna – 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??)