Alien Air: Exoplanet Atmosphere Spectroscopy Quiz

If a planet passes in front of its host star, then a tiny fraction of starlight travels through the planet's atmosphere. If the gases in that atmosphere absorb specific wavelengths of light, then the resulting spectrum contains "absorption features" that act as chemical fingerprints.

If water vapor molecules have specific rotational and vibrational energy states, and if those states correspond to the energy of infrared photons, then water will leave its clearest signatures in the infrared wavelengths rather than visible light.

If the measurement of light occurs specifically during a transit event, and if spectroscopy is used to split that light into its component colors, then the combined term is transit spectroscopy.

If we are analyzing the gaseous envelopes of planets, then we look for volatile molecules (H2O, CH4, CO2) or vaporized metals (Na) that exist in the gas phase; if an ocean is liquid on the surface, it does not typically contribute to the spectral absorption of the upper atmosphere.

If a planet has low gravity and high temperature, its atmosphere extends further into space (high scale height). If the atmosphere is more extended, then more starlight passes through it during a transit, creating a stronger signal for detection.

If the planet is behind the star, it is occulted and cannot be seen. If we want to measure the light the planet emits (emission), we must measure the system light just before it disappears behind the star and then subtract the star's light alone.

If a telescope captures light, it needs a device to disperse that light into a rainbow or spectrum. If that device is designed for scientific data capture, it is called a spectrograph.

If an atmosphere contains small molecules or aerosols, then shorter wavelengths (blue) are redirected more effectively than longer wavelengths (red). If this happens, the planet appears "larger" or more opaque at blue wavelengths, creating a characteristic slope in the spectra.

If clouds or hazes are opaque, they block starlight from reaching the deeper layers of the atmosphere; if the star is dim or the planet is far away, fewer photons reach our sensors, lowering the signal quality.

If a planet has a clear atmosphere, we see deep absorption valleys; if it has a flat spectrum, it means something is blocking the light at all wavelengths equally. If this occurs, it usually points to a high-altitude cloud or haze layer.

If the width and depth of molecular absorption lines are temperature-dependent, then analyzing the "shape" of these lines allows researchers to calculate the kinetic temperature of the gases in the atmosphere.

If we want to know what the atmosphere "removed" from the light, we must know what the light looked like originally. If the star provides a smooth, continuous light source, that source is the continuum.

If gases like Methane and Oxygen are present together, they should react and disappear; if they persist, a source must be replenishing them. If that source is not geological, then it provides evidence for biological activity.

If a telescope is equipped with a spectrograph and positioned in space to avoid Earth's air, it can study exoplanets; Voyager 1 is a deep-space probe and does not observe distant exoplanet atmospheres.

If the star is millions of times brighter than the light filtering through the planet's air, then the star's light is a massive background signal. If we subtract the star's light (out-of-transit), then only the "filtered" component remains for analysis.

If gravity is high, the atmosphere is "squashed" closer to the surface (low scale height). If the atmosphere is thin and compressed, the cross-sectional area of the gas ring is smaller, making the spectral lines much harder to detect.

If the planet is orbiting at high speed, its spectral lines are shifted by its velocity. If the star is relatively stationary, we can use this Doppler shift to distinguish the two sets of signals from each other.

If we are searching for Earth-like environments, we look for "greenhouse" gases; if Methane and Water are both found on a temperate rocky planet, then the likelihood of a complex carbon-based biosphere increases.

If we have detailed spectral data, we can identify gases, detect cloud interference, model heat layers (T-P profile), and even see line broadening from rotation; however, we cannot yet see surface features like grass color.

If photosynthetic life on Earth reflects near-infrared light very strongly, it creates a sharp "jump" in the reflectance spectrum at 700 nm. If we detect this "Red Edge" jump in a planet's light, then it serves as a strong biosignature for plants.