Solar System

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Solar System Odds Ends
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So what was that retrograde motionstuff all
about? Why do the planets appear to move
backwards sometimes?
http//mars.jpl.nasa.gov/allaboutmars/nightsky/nig
htsky04/
http//www.mhhe.com/physsci/astronomy/applets/Retr
o/frame.html
Its 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.
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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)
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The Kuiper Belt billions of icy
objects orbiting the sun, beyond Neptunes
orbit. Pluto and Eris are both part of the
Kuiper Belt.
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The Oort Cloud even farther out billions of
icy objects orbiting the sun. Home of comets that
occasionally visit the inner Solar System
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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/681-s
sc2004-05v1-Orbit-of-Sedna
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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 Plutos size the cutoff?
No scientific basis.
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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 whats so special about roundness?
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Possible definitions of planet
4) Population classification If the object is a
solitary individual it is a planet. If its
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.)
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SO Our Solar System has 8 planets
and 3 dwarf planets (But why isnt Sedna a
dwarf planet then??)