At 4.567 billion years old, refractory inclusions are the oldest known objects to have formed in the solar system. They are often referred to as calcium-aluminium-rich inclusions (CAI) because their chemistry is dominated by the refractory oxides of Ca and Al, as well as Ti, forming minerals such as hibonite (CaAl12O19), spinel (MgAl2O4), perovskite (CaTiO3), melilite (Ca2Al2SiO7), and a Ti-rich pyroxene called fassaite. These minerals are predicted to form in a condensing gas of solar composition leading to theories based on a hot homogeneous solar nebula where all the elements were originally in a gas state.
However, the discovery of isotopic anomalies in O, Ti, Ca, and Mg are inconsistent with a homogeneous hot solar nebula and potentially require more localised heating sources, or at least the retention of the isotopic anomalies on a localised scale. Refractory inclusions also contain isotopic anomalies attributed to the presence of short-lived radionuclides (such as 26Al, 53Mn, 60Fe, 129I) and indicate the injection of freshly synthesized material into the solar nebula. These radionuclides, particularly 26Al, were potentially a source of heat for planetesimals, causing them to melt.