Some of these don't show up for me (like Saturn). This might work better with a web-font? Although then you'd lose the harmonious platform emojis. I wonder if there's a way to check if a unicode character exists in a font and dynamically substitute, without using canvas.
Quick and dirty idea: maybe try rendering it to an offscreen span/div, get its size and compare it to a well known size of what gets substituted for a generic symbol if it doesn't exist, and if the size matches that, replace with something else.