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Transmission Spectroscopy: Studying Planets from the Comfort of Home

Written by Annika Colman
Edited by Shivangi Roy
Illustrated by Aeysha Munawwarah

A host star is seen in the distance at the top of the page. Its orbiting exoplanet passes by in front, larger and closer to the viewer. Underneath it reads "studying planets from the comfort of home", and directly below that reads the title "Transmission Spectroscopy" where "Spectroscopy" is overlaid on top of an absorption spectrum.

I’m sure that if I said the word astronomy, your first thought would go to spaceships and galaxies and far away stars. Those are certainly the most exciting aspects of the field. But one of the things that people don’t usually think about when it comes to planets is how we figure out what they’re made of. We’ve most certainly never been able to reach that far, much less take samples of the planet’s surface or atmosphere. How do we know what particles atmospheres of planets hundreds of thousands of kilometers away have?

The answer, as it turns out, is something called transmission spectroscopy, a technique that falls under the wide umbrella of astrochemistry. We use this method when we want to find out the composition of an atmosphere. Before we get into just how this spectroscopy works, we’ll have to establish some background knowledge. We know that atoms and molecules absorb light of different wavelengths. Each molecule has a different absorption rate, like a light sucking fingerprint, unique to that specific particle. That means that when we measure light through, say, a glass of water, we can see what wavelengths of light that should be there are not actually present. A great example would be an apple. It appears red to us because when hit by light with all the visible spectrum, it absorbs most of those colored wavelengths only reflecting the wavelengths of light that we see as red, giving it that recognizable hue¹. So, now that we’ve established how molecules absorb specific wavelengths of light, we can get back to how we measure what molecules are in the atmospheres of specific exoplanets (planets orbiting stars outside of the solar system).

An exoplanet takes up the top right corner of the page. Beside it reads text saying "1) light from host star passes through atmosphere." Below it and to the left is an image of the Hubble Space Telescope, beside which reads "2) capture the light with our telescopes." At the bottom of the page, there is a graph of a spectrum, with "amount of light" on the y-axis and "wavelength of light" on the x-axis. Dips in the spectrum are labelled with the molecule they correspond to (e.g O3, H2O). Above the graph reads "3) look for absorbed wavelengths, to determine the types of molecules present."

Transmission spectroscopy in space is the same concept as how it would work on earth, just on a much larger scale. We can shine a spectrum of light on molecules and figure out what wavelengths have been filtered through and what ones have not. However, we can assume and say that something like an exoplanet is a LOT further away than our apple. Then comes the problem of how we capture light from something that is quite literally light years away. The answer is, of course, telescopes. We have lots of telescopes in our planet’s atmosphere, and so we can detect light coming from very far away ² ³. But that still doesn’t explain what kind of light exactly we need.

When an exoplanet orbits around its star, rays of light from that star will both hit the planet itself, and filter through the atmosphere around the planet. What we care about is those rays of light that make it through the planet’s atmosphere: these are the ones that our telescopes will eventually catch, and these are the ones that will reveal to us the planet’s composition. When our telescopes catch the light, we can examine it and then determine which wavelengths of light are missing² ⁴. Since every molecule absorbs different wavelengths of light, we can use this information to figure out the composition of the exoplanet’s atmosphere, without ever having to venture beyond the comfort of Earth!

It gets even better though: not only can we figure out the composition of exoplanet atmospheres, we can also figure out its density! The only time we can reliably find the light absorbed by a planet’s atmosphere is when it is orbiting in front of its star in an eclipse. By using measurements with different absorption levels of light as the planets pass in front of their star, they will appear either bigger or smaller, giving scientists an insight into their depth and density metrics ².

So, why do we care about exoplanets thousands of light years away from us? More than that, why do we care about what kind of tiny molecules float around in their atmospheres? Well, knowing the composition of an exoplanet’s atmosphere tells us whether it can support life. In both our search for possible new homes for ourselves, and for other lifeforms out there in the universe, the knowledge of what planets to look for when it comes to being able to sustain life is important. Transmission spectroscopy narrows down our search, allowing us to quickly and accurately determine which planetary atmospheres might support life. It is an interesting technique for an even more interesting topic, and for anyone who is even the slightest bit interested in what else is out there – be it aliens or real estate opportunities – then transmission spectroscopy is a term you should know and explore.

Bibliography

  1. Laboratory Manual for CHM 135H1. 2025. University of Toronto.
  2. NASA Goddard. 2013. “NASA | Alien Atmospheres.” YouTube. December 3, 2013. https://www.youtube.com/watch?v=CcUhVCMAhAI.
  3. Robertson, Paul M. 2020. “Measuring Exoplanet Atmospheres with HPF.” The Habitable Zone Planet Finder. May 1, 2020. https://hpf.psu.edu/2020/04/30/gj-3470/.
  4. NASA, and Marty McCoy. 2025. “Spectroscopy 101 – Types of Spectra and Spectroscopy.” NASA Science. September 2025. https://science.nasa.gov/mission/webb/science-overview/science-explainers/spectroscopy-101-types-of-spectra-and-spectroscopy/.

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