Planets form from the gradual accretion of smaller pieces of material, referred to as planetesimals. There are two basic categories of planetesimal: differentiated and undifferentiated. Both start off as what Rayssa Martins, the study’s first author, describes as “big lumps of dust and other small rocky bits (often loosely) stuck together.” Undifferentiated planetesimals basically remain this way; differentiated planetesimals, by contrast, are characterized by having been subjected to enough heat to melt all their component parts together.