Scanning Alien Skies: Transmission Spectroscopy Explained

If a planet transits a star, then a thin ring of starlight passes through its gaseous envelope; if the gases absorb specific photons, then the light reaching Earth contains "fingerprints" of those chemicals.

If transmission spectroscopy relies on light filtering through atmospheres, and if that only happens when the planet is between Earth and the star, then it must occur during the primary transit, not the secondary eclipse.

If we need to see how much light is absorbed at specific wavelengths, then we must use a device that disperses the light into a spectrum; that device is a spectrograph.

If a specific gas (like water vapor) is opaque to a certain wavelength, then it blocks starlight at that altitude; if that light is blocked, the shadow of the planet appears larger on the light curve at that specific color.

If molecules have unique vibrational modes that absorb infrared energy, and if H2O, CH4, and CO2 are common in planetary atmospheres, then they will leave distinct absorption signatures in the infrared spectrum.

If an atmosphere were 100% transparent to all light, then no photons would be absorbed; if no photons are absorbed, there would be no change in the spectrum to analyze.

If the scale height is larger, the atmosphere extends further into space; if the atmosphere is more extended, then more starlight filters through it, making the spectral signals easier to detect.

If the cooler gas in the atmosphere removes specific wavelengths from the continuous spectrum of the star, then the resulting data will show dark lines or "dips" known as an absorption spectrum.

If clouds or hazes are opaque across a wide range of wavelengths, then they block starlight before it can reach the deeper layers where specific gases are; if the gas-specific absorption is blocked, the resulting spectrum shows no distinct features.

If electrons can only exist in specific energy states, and if they must absorb a photon of an exact energy to move between states, then each atom or molecule will only absorb very specific wavelengths of light.

If we have more photons (bright star/big telescope) and a larger area of atmosphere to filter the light (puffy planet), then the signal will be stronger than the random background noise.

If a transmission spectrum shows a steep slope where more blue light is removed than red light, then the light is being redirected by small particles or molecules through Rayleigh scattering.

If the scale height and the excitation states of molecules are dependent on temperature, and if those factors change the shape of the absorption lines, then we can use the spectrum to calculate the atmospheric temperature.

If many chemicals (like oxygen) can be produced by both biological and geological processes, then seeing a single "biosignature" gas is not definitive proof of life without further context.

If the goal is to see how the planet's perceived size changes based on the color of light, then the graph must plot the wavelength (color) against the transit depth (size).

If starlight travels horizontally through the atmosphere's limb (the slant path), and if it hits a density where it can no longer pass through, then that height defines the opaque boundary of the planet.

If most important atmospheric molecules (H2O, CH4, CO2) have their strongest absorption features in the infrared, and if JWST is a giant infrared-optimized telescope, then it can see these signals much more clearly than previous tools.

If a planet has high gravity, then its atmosphere is pulled closer to the surface; if the atmosphere is compressed (small scale height), then the "ring" of light passing through the air is smaller and harder to detect.

If Hydrogen and Helium are the primary gases in the universe, and if astronomers call all other elements "metals," then a "high-metallicity" atmosphere is one rich in heavier elements like Carbon, Oxygen, and Nitrogen.

If we want to find out what was removed from the light, then we must first establish the "baseline" or continuous light source, which is the star's own spectrum (the background).