An exoplanet transmissions spectrum is a difficult thing to interpret because the planetary spectrum (right) is derived as the temporal variations in the stellar spectrum, as the planet passes in front of its host star.

Here we have shown what the hypothetical exoplanet would look like at various wavelengths as it passes in front of its host star. The right hand box shows the exoplanet transmission spectrum, with a bar sliding over it to represent the wavelength of light passing through the atmosphere.

As the the opacity of the atmosphere (related to its molecular composition) increases, so does the fraction of light absorbed by the planet, which is the y-axis on the right figure. On the left, you will see that the atmospheric opacity increases when its molecules absorb a larger fraction of light.

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