Difference in chemical composition between the stars 16 Cyg A and 16 Cyg B, versus the condensation temperature of the elements in the proto-planetary nebula. If the stars had identical chemical compositions then the difference (A-B) would be zero. The star 16 Cyg A is richer in all elements relative to star 16 Cyg B. In other words, star 16 Cyg B, the host star of a giant planet, is deficient in all chemical elements, especially in the refractory elements (those with high condensation temperatures and that form dust grains more easily), suggesting evidence of a rocky core in the giant planet 16 Cyg Bb.