Yes it is possible to add some additional levels. The problem is the maximum count of tiles for mode 3 - I have actually 225 tiles in use. So I'm afraid it's only possible to add tiles for 1 additional planet... (or I have to create additional levels with the existing tiles).
About the planet graphics: I found a website which offers pictures for all planets from our solar system in several resolutions. So it was easy to convert this pictures with TileStudio
I will save the reported bugs in the next days - also the problem with the half text in the last line. (I have not this problem on my CRT TV.)
Further ideas or game extensions are:
- better and more sound effects (at the moment I use changed sound effects from B.C Dash).
- meteors in higher levels
- vertical scrolling
I will publish also the source code. But first I will "polish" my sources and comments, fix some bugs and add some features (levels).