Complex sentence: Since Gliese 581 b does not transit, nothing more can be said of it yet; for instance, given that Gliese 581 b orbits alongside two other planets and that Gliese 436 b (thus far) stands alone, their formation must have differed.
Split into simpler sentences: Since Gliese 581 b does not transit, nothing more can be said of it yet. At the least, given that Gliese 581 b orbits alongside two other planets and that Gliese 436 b (thus far) stands alone, their formation must have differed.