Earth-like Planets have Earth-like Interiors

Mass-Radius Relation for Rocky Planets Based on PREM Credit: Harvard-Smithsonian Center for Astrophysics

Every school kid learns the basic structure of the Earth: a thin outer crust, a thick mantle, and a Mars-sized core. But is this structure universal? Will rocky exoplanets orbiting other stars have the same 3 layers? New research suggests that the answer is yes – they will have interiors very similar to Earth.

Zeng et al applied a computer model 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 6 known rocky exoplanets with well-measured masses and physical sizes. Despite their differences from Earth, all should have a nickel/iron core containing about 30% of the planet’s mass. In comparison, about a 1/3 of the Earth’s mass is in its core. The remainder of each planet would be mantle and crust, just as with Earth.
“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,” adds Zeng.

The new code also can be applied to smaller, icier worlds like the moons and dwarf planets in the outer solar system. Eg. by plugging in the mass and size of Pluto, the team finds that Pluto is about 1/3 ice (mostly water ice but also ammonia and methane ices).

The model assumes that distant exoplanets have chemical compositions similar to Earth. This is reasonable based on the relevant abundances of key chemical elements like Fe, Mg, Si, and O in nearby systems. However, planets forming in more or less metal-rich regions of the galaxy could show different interior structures. The team expects to explore these questions in future research.
https://www.cfa.harvard.edu/news/2016-03

http://arxiv.org/abs/1512.08827