A meteorite is any fragment of rock from the solar system that passes through the atmosphere and survives its impact with Earth. About 3 percent of meteorites found on Earth's surface are made almost entirely of the metals iron and nickel, and are called iron meteorites. When the parent body was partially molten, the heavy iron and nickel metals segregated from the less dense silicate minerals and concentrated in the core, just as metals sank to Earth's core while our planet was forming. Scientists believe that these dense metallic fragments may have come from the cores of certain minor planets, called M-type asteroids, when they were blasted apart by a collision with other bodies during the early days of the solar system, more than 4 billion years ago.