Wait!  Come back!  This isn’t nearly as confusing as it sounds.  You’ll have this down cold in five minutes.  Really.

Okay, let’s say astronomers have just found a planet circling a star 50 light years away from us.  That’s not too much of a stretch; that’s the distance 51 Pegasi b sits out from the Earth.  Anyway, back to our new planet.  Our scientists announce that the planet has hydrogen, oxygen, and nitrogen in its atmosphere.

Emission spectrum of Hydrogen

Emission spectrum of Iron

Cool!

Wait… what?

How did they do that?  How do they know what’s in a planet’s atmosphere, short of going there and taking a good, solid sniff?  For that matter, how do they even know for certain the planet has an atmosphere?  It takes some serious equipment to be able to find the planet, much less its atmosphere.

(Absorption lines in the Solar Spectrum)

You know how these planets are being discovered, right?  One very good way is to study the light intensity of a star and watch for little dips in the level which would signify something passing between you and the light source, blocking out part of the light you see.  Bingo.  Once a planet has been found, you watch the light as it passes close to your object, and if there is an atmosphere there, it will change the “tone”, or “quality” of light you’re perceiving.  Just a tiny bit… but enough.  That’s spectrophotometry; the image and comparison of various spectra for scientific analysis.

The concept itself isn’t difficult to master, once you get beyond the formidable name.  You know that light passing through water looks different depending on what’s in the water.  Well, an atmosphere is nothing really except an extremely… puffy… fluid.

Diagram by Kevin Saff - this shows how carbon in the environment impacts carbon levels in the atmosphere

Now, you know that every element has its own “signature” on the light spectrum, right?  We have applications for that in every day science here on Earth.  You’re especially familiar with the concept if you follow forensics.  That’s how you tell whether or not your victim has traces of arsenic in his/her body; arsenic has its own “signature” on the light spectrum based on what’s absorbing or reflecting light.

You take this tiny bit of light that’s passed through the atmosphere of your distant planet, separate it out (like the way a prism separates out visible light), and you have the signature of everything that’s in your planet’s atmosphere.  It’s as easy as if we were reading a list of ingredients; in fact, that’s in essence exactly what we are doing.

The science doesn’t stop here, of course.  If we knew that certain life forms (like… ours, maybe?) leave chemical “markers” on the atmosphere, markers that are there only in the presence of this particular life form; and then we find those markers in the atmosphere of another planet…

… wow.