Refractory materials in protoplanetary disks are best described as which?

• A. Gaseous substances
• B. Solids with high melting points
• C. Ice crystals
• D. Volatile liquids

Answer: (B)

Refractory materials are defined by their high resistance to heat; they have high melting and condensation temperatures, so they stay solid even in the hot inner regions of a protoplanetary disk. As a result, these materials condense and persist as solid grains close to the newborn star, forming the first solid building blocks of planets. The other options don’t fit because gases would not remain solid, ice crystals form only at cooler temperatures farther from the star, and volatile liquids vaporize in the hot inner disk.