The interior structure of rocky exoplanets orbiting other stars will have interiors very similar to Earth - a thin outer crust, a thick mantle, and a Mars-sized core, researchers have shown using a computer model. "We wanted to see how Earth-like these rocky planets are. It turns out they are very Earth-like," said lead author Li Zeng of the Harvard-Smithsonian Centre for Astrophysics (CfA). Zeng and his team applied a computer model known as the Preliminary Reference Earth Model (PREM), which is the standard model for Earth's interior. They adjusted it to accommodate different masses and compositions, and applied it to six known rocky exoplanets with well-measured masses and physical sizes. They found that the other planets, despite their differences from Earth, all should have a nickel\/iron core containing about 30 percent of the planet's mass. In comparison, about a third of the Earth's mass is in its core. The remainder of each planet would be mantle and crust, just as with Earth, according to a CfA statement. "We've only understood the Earth's structure for the past hundred years. Now we can calculate the structures of planets orbiting other stars, even though we can't visit them," Zeng said. The model assumes that distant exoplanets have chemical compositions similar to Earth. This is reasonable based on the relevant abundance of key chemical elements like iron, magnesium, silicon, and oxygen in nearby systems.