“art” is at the core of “earth” so is:
carte
carts
darts
farts
garth
harts
karts
marts
parts
party
tarts
tarty
warts
warty
Fun that art would be the core of farts, party, and warty.
Then for 7-letter words, art being exactly at the center:
dearths, hearted, hearten, hearths, pearter, peartly, quartan, quarter, quartes, quartet, quartic, quartos, smarted, smarten, smarter, smartie, smartly, swarths, swarthy,
Do we even dare try 9-letter words?
bipartite, copartner, departees, departing, departure, imparters, impartial, imparting, inearthed, nonartist, unearthed, unearthly, updarting, upstarted.
I didn’t think 9-letter words would be possible, so maybe there are some 11-letter words. Indeed there are three!
compartment, nonpartisan, unchartered.
We have truly gone into unchartered territories! However, it bottoms out at 13-letter words. Or at least the Scrabble Word Finder bottoms out. Now, what shall we do with all these words?
unchartered has lots of meaning for having “art” in the middle.