Reading the Rainbow: Transmission Spectroscopy Explained

If a planet passes in front of a star, and if starlight filters through the planet's thin layer of gas, then some photons will be absorbed by that gas; if we measure which colors are missing, we can identify the atmosphere's composition.

If we want to see detailed light patterns, we must separate the mixed starlight into its individual colors or wavelengths; if a spectrograph uses a grating or prism to spread the light out, it allows us to see specific absorption lines.

If atoms or molecules have specific energy levels, they can only absorb photons of an exact energy; if those photons are removed from the starlight, then those specific colors will be missing from the rainbow, creating dark gaps called absorption lines.

If different molecules absorb different parts of the light spectrum, and if water vapor has a unique and known pattern of absorption, then we can confirm its presence by finding that specific pattern in the transit data.

If the physics of electron shells is the same everywhere in the universe, then a specific element like Oxygen will always absorb the same colors; if we see those colors missing, then we have identified that element just like reading a barcode.

If an atmosphere contains gases or hazes that stop blue light but let red light pass through, then the planet will block more of the star's blue light; if more light is blocked, the "shadow" or transit depth will appear deeper in that color.

If light has properties of both waves and particles, then we can treat a beam of light as a stream of photons; if each photon has a specific energy level based on its color, then only certain photons can be absorbed by specific atoms.

If a hot, dense object like a star's core emits light at all possible wavelengths, and if nothing has blocked that light yet, then we see a solid rainbow; if the rainbow is complete, it is called a continuous spectrum.

If we define the spatial period of a wave as the distance between its crests, then that physical measurement is the wavelength, which determines the light's color and energy.

If an instrument is equipped with a spectrograph and can monitor a star during a transit, then it can perform spectroscopy; JWST, Hubble, and VLT are built for this, while the ISS and Mars Rover have different primary missions.

If a molecule can only absorb energy in specific "quantized" amounts, and if a photon's energy does not fit one of those amounts, then the molecule cannot catch it; if the photon is not caught, it continues traveling through the atmosphere to our detector.

If molecules vibrate and rotate at specific frequencies, and if those frequencies correspond to the energy found in infrared light, then the infrared part of the spectrum is the most useful place for scientists to look for these "life-related" gases.

If we want to see how the atmosphere affects starlight, we must look at how much light is blocked at each color; if we plot the percentage of light lost (transit depth) for every wavelength, we can see which colors the atmosphere is "thickest" in.

If a gas is heated until it glows, it will release energy as light only at specific wavelengths; if we see these specific colors as bright lines without the rest of the rainbow, then it is an emission spectrum.

If every element has a unique set of absorption lines, and if sodium is famous for having two very strong lines in the yellow part of the spectrum, then seeing a yellow "spike" in transit depth suggests sodium is in the atmosphere.

If a planet has thick clouds or high-altitude hazes, they act like a solid wall that blocks all light wavelengths equally; if all light is blocked, the specific "barcodes" of the gases below cannot be seen, making the spectrum look like a flat line.

If an object is relatively cool (like a planet), it does not glow with visible light like a star; if it has a temperature, it must emit thermal radiation; if that radiation is low energy, it will primarily appear in the infrared range.

If the atmosphere of an Earth-sized planet is very thin, then the amount of starlight it filters is extremely small; if the change is only 0.001%, then the telescope must have "parts per million" precision to detect the signal.

If light is governed by the equations E=hf and c=f(wavelength), then frequency and wavelength define the energy; because energy determines the "color" we see, color is also a factor. Photons have no mass or volume.

If we have captured a pattern of missing light from an exoplanet, then we must determine what it means; if we compare our results to laboratory data of gases like Methane or CO2, then we can confirm what is in the planet's air.