Leaflet : Custom markers scaling when zooming on non-geographical map

I've been asked to build a fictional map website using Leaflet. The website would consist in a simple page filled with a map which has an jpeg background. The background is a photoshop illustration with orbiting planets.

What I'd like to do is using markers to make these plants glow on hover. I managed to do it but unfortunately when I zoom in the marker keeps its original size. Here's what I'm talking about :

Map with planet marker Marker not scaling when zooming in

How can I "stick" the marker to the map so that it scales with the zooming in and out ?