Complex sentence: Several ground-based observations have searched for this hypothetical Fomalhaut "c" but have yet to find it, although they have limited the mass of such an object to be less than thirteen times the mass of Jupiter.
Split into simpler sentences: Several ground-based observations have searched for this hypothetical Fomalhaut "c" but have yet to find it. At very small, solar system-like scales any additional companions must have a mass less than thirteen times the mass of Jupiter.