I always heard they were called “stars and planets,” but I’m pretty sure Mort Walker invented them, so he’s the authority. From the excellent weekly newsletter World Wide Words:
#@%$?*! Dave Aton pointed out an interesting word, which he found on the Ask section of typography.com. It’s the name given to those strings of random non-alphabetical characters that cartoonists use in speech bubbles to indicate profanity. They call them grawlixes. The word was the creation of the cartoonist Mort Walker. He first used it in 1964 in an article he wrote for the National Cartoonists Society in the US and which he then included in his 1980 book The Lexicon of Comicana, a satire on the comic devices that cartoonists use but which ironically became a textbook for art students. Other terms he invented for various comic-artist graphical conventions include waftarom, squean, spurl, neoflect, plewd, vite, dite, hite, direct-a-tron (and throwatron, sailatron, staggeratron, swishatron…), jigg, briffit, solrad, whiteope, indotherm, crottle eyed, neoflect, and three other ways to indicate maladicta – jarn, quimp, and nittle. And no, I’m not going to define any of them, not least because you can really only do it by illustrating them, as Walker did. Buy his book if you’re interested – it’s still in print.