Complex sentence: Pluto is the largest and most massive member of the Kuiper belt; originally considered a planet, it was reclassified as a dwarf planet in 2006.
Split into simpler sentences: Pluto is the largest and most-massive member of the Kuiper belt. Originally considered a planet, Pluto's status as part of the Kuiper belt caused it to be reclassified as a dwarf planet in 2006.