It appears the only way to delete a planet post is to overwrite the original with a new post in its stead. I wrote a silly little thing to be up for a day, and expected it to disappear from the planet when I nuked the original post. But the planet doesn’t work that way and preserves the old stuff in perpetuity unless you edit the original and push it out in your feed. OK. I know now. Don’t post silly ephemeral posts as they will soon look ridiculous.