2  Atmospheres of planets and their climates

If we can tell what stars are made of by looking for the missing colours, or wavelengths, of light in their spectra, then in theory we can do the same for planets. In both cases, what we can learn directly is restricted to the contents of the outer layers (i.e. the atmospheres of stars and planets). However, there’s one thing which makes applying this technique to planets really difficult. Stars are big and bright, so we can see them even when they’re a long way away, but planets are small and faint. It’s extremely difficult to detect the light coming directly from a planet, let alone split it up into all its various colours.

Instead, astronomers do something ingenious with transiting planets. When a planet transits, it blocks some of the starlight. You learned about this back in Week 4, and you also learned that the amount of light blocked tells you how big the planet is relative to the size of the star.

Remember, this is how the transit depth is calculated:

\text{transit depth} = \left(\frac{R_{\text{p}}}{R_{\text{star}}}\right)^2

When you studied transits in Week 4, the calculations assumed that the planet was solid. A lump of rock is completely opaque: no light gets through it. But even a planet like Earth – a rocky planet – is more than just a lump of rock. Except for Mercury, the rocky planets in our Solar System are surrounded by a shell of gas called an atmosphere.