From Universe Today:
And this was the technique that Redfield and his team used to measure the atmosphere. "Take a spectrum of the star when the planet is in front of the star," explains Redfield. "Then take a spectrum of the star when it’s not. Then you divide the two and get the planet’s atmospheric transmission spectrum. Each time the planet passes in front of the star the planet blocks some of the star’s light. If the planet has no atmosphere, it will block the same amount of light at all wavelengths. However, if the planet has an atmosphere, gasses in its atmosphere will absorb some additional light."
It occurs to me that this approach has an interesting flaw. If we were to find a planet whose body was transmissive for some reason (the example that springs to mind is a planet made entirely of glass or of some translucent or transparent mineral like quartz or diamond), the assumption that the solid body of the planet blocks light equally at all wavelengths is no longer true.
What’s the likelihood of such a body existing? Quite low, I should think; after all, many things that we think of as transparent would probably be opaque at planetary sizes. But then the universe is a big place, and there are plenty of strange and unlikely sounding objects out there already.
I suppose most such configurations are likely to show up in the line spectra, but even then my guess is that they could easily be mis-interpreted. That, of course, is one of the hazards of all of this type of research… because we can’t actually resolve the planets themselves, we don’t really know what we’re looking at. It’s all best-guess and assumptions. In a few hundred years’ time, I’m sure people will look back on the present-day planet hunters with considerable amusement, just as many of us do today at the unusual beliefs of some of our ancestors.