The phrase "iron core" refers to the central part of something made of iron, like the core of the Earth or the center of an electromagnet. Full definition
Unlike the other rocky planets in our solar system, it has an immense iron core that makes up roughly 70 per cent of the planet's volume. (newscientist.com)
Earth and Mercury are both rocky planets with iron cores, but Mercury's interior differs from Earth's in a way that explains why the planet has such a bizarre magnetic field, UCLA planetary physicists and colleagues report. (sciencedaily.com)
The planet has a large iron core which generates a magnetic field about 1 percent as strong as that of the Earth. (sciencedaily.com)