The planets were named after Roman gods. Demeter is not one of them.
Grains
Demeter's symbol is wheat.
What is Neptune's symbol?
demeters signs are a corn husk the four seasons and anything to do with nature
See link.
persphene
demeters downfall was her daugter pereshpohne was taken and demeter was shearchig so demters crops were faling
go here: http://solarsystem.nasa.gov/multimedia/gallery/Pluto_Sym-browse.jpg
Ceres.
Demeter's son is Persephone, who is also known as Kore. She is the goddess of the harvest and agriculture in Greek mythology.
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Yes yes he was.