I’m always baffled when some article says Oh there’s this planet 120 light years away and its atmosphere is made of hydrogen. How tf can we know that just by looking at something this far? Same goes for things that are closer.
I’m always baffled when some article says Oh there’s this planet 120 light years away and its atmosphere is made of hydrogen. How tf can we know that just by looking at something this far? Same goes for things that are closer.
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It’s because of Fraunhofer lines.
You can split visible light into a rainbow spectrum using a prism. In the spectrum, you will usually see gaps at certain colors / wavelengths. When we first did this with sun light, we noticed that the gaps are in the same spots as that of a gas found on Earth. Hence we named the gas ‘helium’ after the sun god Helios.
Every element — if you make it glow — has these characteristic gaps (or peaks, depending on whether the gas absorbs or emits the light).
The light is passed into a spectroscope. Which reads of the spectrum of the light. Each element has its own unique spectrum. So, you just match the spectrums with the known one of elements. In fact the NIST gov web site lists them all. I have used their tables.
https://www.nist.gov/pml/atomic-spectra-database
https://en.wikipedia.org/wiki/Optical_spectrometer
I haven’t read the paper for the most recent discovery (which I assume motivated this question), just the articles published.
That said, usually it’s done by looking at how the light from a star changes when the planet passes in front of it. Using the difference in the light from the star when there is a planet in front of it versus when there isn’t, we can assume any changes to the light are caused by the atmosphere of the planet.
We know how certain molecules absorb or emit certain wavelengths of light, so we can associate changes with certain molecules.
Every gas has a certain light fingerprint. So one gas may have 50% blue 20% red 30% green, another gas may have 10% blue 20% red 70% green.
We have instruments that can measure each color and then match it up to a specific gas.
Here’s the actual fingerprints
https://images.app.goo.gl/TEwW9xLauCTFG7Tv7
transit spectroscopy.
when an exoplanet crosses in front of its star (from our perspective), aka a transit, we not only see a dip in the star light, but when analysing the whole spectrum of wavelengths, aka spectroscopy, we can see that certain specific wavelengths are much more dipped than the rest. these dips at specific wavelengths form a sort of barcode signature that we can match to a database of absorption signatures for known atoms and molecules we can and have studied in the lab.
atoms like hydrogen, sodium and so on have unique absorption signatures due to their electronic structure. because they all have a different number of electrons in a specific configuration , those electrons behave differently when interacting with light. for compounds, the shape of the molecule can have unique vibrational modes when interacting with light, so similarly, a collection of atoms like hydrogen and oxygen combined (water) therefore also have unique signatures, often very different from that of their constituent elements. all this works out for us because unique means easier to identify and all we need to do is to build the database in the lab and go look at the sky to find it.
for an exoplanet the key information we need is in the edge of the silhouette (which is actually the surface/atmosphere of the planet) of the transiting planet and just look for the same signatures, which suggests the presence of these things.
in 1932 Venus was identified through land based spectroscopy to be mostly covered in CO2, and confirmed later by direct probe sampling 30 years later. though they didn’t need to do transit method because the sunlight bouncing off Venus any time of night was sufficient to observe significant absorption in 2-2.7, 4.3 and 15um , which is what CO2 does.
When light passes through a molecule some of that light is absorbed by the molecule. Every single element and every molecule behaves differently and has its own distinct wavelengths it absorbs. Here’s Hydrogen’s. The little black bars are areas of the visible spectrum where hydrogen absorbs the light.
What NASA detects is light from the star the planet is orbiting, passing through the atmosphere of the planet, and then some of that light getting absorbed, and the rest passing through. That gives us an absorption spectrum. If that spectrum matches a known pattern, we can conclude that the element/molecule is present.