Humans have been wondering whether we truly are alone in the universe for millennia. Part of our quest to understand our place has involved searching for other planets like Earth in the Solar System and beyond — but that's much easier said than done. Space, as it turns out, is big, and other planets are far, far away. Studying them in detail, then, is tricky.

A new study in Science gets around this roadblock by studying white dwarfs, which are not an obscure fantasy reference but are actually the remnants of low and medium mass stars. By studying the "pollution" in these white dwarfs, which occurs when rocky bodies crash into the stars earlier in their lives, Doyle and colleagues were able to estimate the rocks' geochemical and geophysical properties — specifically, what was are they made of and what their structures involve.

To estimate these properties, Doyle and colleagues relied on a key geochemical indicator: oxygen fugacity, which is a measure of the partial pressure of oxygen in rocks as they form. They measured six elements as oxides, including magnesium, silicon, aluminium, calcium and iron. As these oxides must have been in thermodynamic equilibrium with oxygen during formation, their abundances can be used to estimate an overall oxidation state. That estimate can then be linked to atmospheric composition, the geochemistry of its crust and mantle, and even the size of its core.

Ultimately, Doyle and colleagues determined that these rocky exoplanets were similar to Earth and Mars in terms of both composition and structure. So there are some exoplanets like ours in the universe...and this brings us one step closer to knowing how we fit in the universe.