Complex sentence: From then until the Kepler mission most known extrasolar planets were gas giants comparable in mass to Jupiter or larger as they were more easily detected, however the catalog of Kepler candidate planets consists mostly of planets Neptune sized and smaller down to sub-Mercury sized.
Split into simpler sentences: From then until the Kepler mission most known extrasolar planets were gas giants comparable in mass to Jupiter or larger as they were more easily detected. The catalog of Kepler candidate planets consists mostly of planets Neptune-sized and smaller down to sub-Mercury sized.