When a baker kneads dough, the grains of flour can tell you which mill they came from. It's the same with Earth: the composition of iron in its depths preserves a memory of where the pieces that assembled the planet were born. Previously, it seemed that Earth's iron resembled iron from meteorites that came from the outskirts of the solar system — from cold, ice-rich regions. But a new, highly precise measurement showed that the resemblance is deceptive.
Iron has several varieties of atoms — isotopes. One of them, with 58 particles in its nucleus, is rarer than others. It was this one that revealed the difference. Iron in Earth's mantle and iron in meteorites from the outer part of the system differ in this rare isotope.
So, Earth did not assemble from snowballs that came from afar. Its iron came from the inner, warmer part of the solar system — from rocky bodies that orbited near the young star. Such bodies have not survived to this day as meteorites, but their material became part of our planet. Earth grew gradually, colliding with neighbors in the inner ring, like a snowball rolling along a rocky path, not through a snowdrift.
The isotope iron-58 makes up only 0.28% of all iron on Earth, but it was this that helped distinguish 'native' iron from 'foreign' iron.