Did you add it to the stars.tbl? If so, planets (and everything that needs to be rendered with an alpha blend mode, rather than an additive mode like nebulas are) there have to be $BitmapX: rather than $Bitmap:
Also, there are a variety of reasons to give planets an alpha channel (that is: an opacity channel, for transparency) instead of green-masking them. Green-masking is an "all or nothing" effect where no pixel can be partially transparent: either it's (0, 255, 0) or it's rendered fully opaque. It would look ugly in a mission and there's no real reason to do it anymore.
I'm not sure how to introduce you to alpha channels because I suck at explaining stuff, but look up planet tutorials in your preferred image editor; they will probably cover the subject.