“art” is at the core of “earth” so is:
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?