Do scientists really know what the core of planets and stars are made of?


Do scientists really know what the core of planets and stars are made of?

In: Earth Science

We have a pretty good idea.

Our knowledge of the Earth’s core is based on a few pieces of information:

– It must have a certain density, because Earth’s mass and volume are known.
– It must be made of relatively common elements, or there wouldn’t be enough to form such a large object (the core is approximately as heavy as the Moon)
– It must be highly electrically conductive, because Earth has a magnetic field.
– It must be a pretty exact amount of “springy” because we can observe earthquake waves bouncing off of it.
– It must be made of heavier stuff than the other components of Earth (in particular, the silicates that make up Earth’s surface), because it sank to the bottom of the once-molten planet.

The combination of these constraints makes us pretty confident that the Earth’s core is mostly iron, with some nickel. Iron is common, heavy, conductive, and rigid enough to produce the properties we observe.


Stars, on the other hand, are constrained by our understanding of particle physics and the observed frequencies of the elements they produce in the cosmos.

We know that stars are made almost entirely of hydrogen early in their lives, and produce other elements through fusion. This means that only elements producible through fusion should be common in stars, with heavier and hotter stars producing heavier elements (because it takes progressively more energy to cause fusion the higher up you go).

This means that light and young stars, like our Sun, must have cores primarily of hydrogen with some helium produced by fusion. Heavier stars fuse heavier elements, up to carbon, oxygen, neon, silicon, sulfur, and ultimately iron (all of which are common elements in the Universe). And because heavier elements exist, we know that some process must produce them, and stars are the only plausible mechanism based on our current understanding of the universe. In fact, there are *two* processes: the s-process in the core of very large stars and the r-process that occurs during supernovas.

Knowing how much of these elements are produced tells us a lot about the inner conditions inside stars, because we know how long they live and how fast elements are produced at certain temperatures and pressures. Actually, in one case, we discovered a new particle physics phenomenon because we knew it *had* to exist for stars to work the way we know that they do (and it turned out that [it does indeed exist](

More recently, we’ve been able to observe supernovas directly, which has given us more information about these rarer and less-well-understood processes.

We have a really, really, really good idea. We’re not gonna be 100% sure about the exact numbers, but we’re gonna have a really good estimate.

We use different wavelengths of light or xrays or radiowaves to measure how quickly they travel through different materials, which gives us a really good idea of how much of that material there is. We measure all those things on Earth and then approximate based on their gravity or atmosphere or whatever.

If there is some unknown material, we won’t be able to measure that. And because they’re so far away, we might be off by 0.0000001% here or there, just based on the math.