HOW DO WE KNOW WHAT EXOPLANETS ARE MADE OF?

“Never, by any means shall we be able to study the chemical composition or mineralogical structure of the stars,” said French philosopher Auguste Comte in 1835. He was wrong, as was demonstrated within two years of his death. Atoms and molecules, when heated, shine with light at characteristic wavelengths (energies). And, if they are in the cool atmosphere of a star, then they absorb light at those very same wavelengths. This creates a series of black lines like a supermarket barcode in the stellar ‘spectrum’. In the same way, when an exoplanet moves in front of its star, so that the starlight passes through the planet’s atmosphere on its way to Earth, there is the potential to see the characteristic barcode of the substances in the planet’s atmosphere.

So far, this technique has revealed a number of substances such as sodium, carbon monoxide, carbon dioxide and water in the atmospheres of extrasolar planets. The detection of molecular oxygen, an unstable gas, would indicate its continuous creation by living things.