Complex sentence: By the end of the 19th century, the word "planet" had a more firm definition: it applied only to objects in the Solar System, a number small enough that any differences could be dealt with on an individual basis.
Split into simpler sentences: By the end of the 19th century, the word "planet" had settled into a comfortable working term. It applied only to objects in the Solar System; a number small enough that any differences could be dealt with on an individual basis.