Hi, Rocket Witch . From what I saw in the picture, it's probably a temperature bug. Not sure if 850 K would make the game renders object glow properly. I haven't tried it myself, but this is my lead hypothesis. Try heating the object up. Another way is that if your object is too far away from the parent object, it will start to 'glow', aka, the lighting system will start to break down. Not sure how far, but probably also related to the parent body's radius, considering that my pulsar planets are also glowing despite orbiting closer to their parent object than most planets in Sol System. This is unlikely to be the cause in this case, but it's what I knew.
I believe that you cannot make planets that orbit inside stars which is selectable. However, you can test it by yourself, but making Sol 200 times larger. With mass being the same, there should be no problem for Mercury and Venus to retain something in their orbit even if they're already inside the radius of the star, providing that they're still there and not removed.