A transmission spectrum is created when light passes through a substance, which absorbs some of that light. Imagine shining a bright, white light through a piece of colored glass. The light that emerges on the other side is missing the colors that were absorbed by the glass, revealing something about the glass’s properties.
This basic principle is the foundation of transmission spectroscopy. When starlight passes through a planet’s atmosphere, gases absorb certain wavelengths of that light. By capturing and analyzing this transmitted light, scientists can identify which wavelengths are missing and deduce the atmosphere’s composition.
Generating a Transmission Spectrum
To study planets outside our solar system, called exoplanets, astronomers rely on a phenomenon known as a transit. A transit occurs when an exoplanet passes directly in front of its host star from our vantage point on Earth. As the planet makes this journey, a fraction of the star’s light is filtered through the upper layers of the exoplanet’s atmosphere.
Telescopes are used to measure the star’s light continuously before, during, and after the planet transits. While the planet itself blocks a significant portion of the light, the valuable information is in the small, additional dimming caused by the atmosphere absorbing specific wavelengths.
To isolate the atmospheric signal, astronomers compare the spectrum of the starlight filtered through the planet’s atmosphere to the normal, unfiltered spectrum of the star. This comparison allows them to produce a transmission spectrum that reveals the specific wavelengths absorbed by the planetary atmosphere.
Decoding Spectral Fingerprints
A transmission spectrum is presented as a graph that plots the amount of starlight blocked against different wavelengths of light. The features of this graph are dips and troughs, known as absorption lines. These lines are created because each atom and molecule, such as sodium, water, or methane, absorbs light at a unique and predictable set of wavelengths. This distinct pattern acts as a “spectral fingerprint,” allowing scientists to identify the chemical components present in the exoplanet’s atmosphere.
The position of an absorption line on the graph indicates which element or molecule is responsible for absorbing the light. A deeper dip signifies a greater amount of absorption by that particular substance. For instance, a noticeable dip at the specific infrared wavelengths associated with water is a strong indicator of water vapor in the atmosphere being studied.
By matching the observed patterns to known spectral fingerprints from laboratory measurements, astronomers can piece together a chemical inventory of a distant world’s atmosphere. The relative depths of these features can also provide insights into the abundance of each detected molecule.
Unveiling Alien Atmospheres
The analysis of transmission spectra provides insights into the conditions on distant worlds. Identifying molecules like water vapor, methane, and carbon dioxide can help scientists understand an exoplanet’s climate and temperature. The presence of these gases is important for assessing a planet’s potential to support life.
An example is the exoplanet K2-18b, located 124 light-years away. This sub-Neptune sized planet orbits within its star’s habitable zone, the region where conditions might allow liquid water to exist on the surface. Initial observations with the Hubble Space Telescope hinted at the presence of water vapor in its atmosphere.
Subsequent, more detailed studies using the James Webb Space Telescope (JWST) confirmed the existence of methane and carbon dioxide. These findings have led to K2-18b being classified as a potential “Hycean” planet—a hypothetical type of world with a water ocean beneath a hydrogen-rich atmosphere.
Further analysis of K2-18b’s spectrum also suggested the possible presence of dimethyl sulfide (DMS), a molecule that, on Earth, is predominantly produced by marine life. While this detection is still under investigation, it highlights how transmission spectroscopy can guide the search for biosignatures, or signs of life, on other planets.
Observational Challenges and Advanced Telescopes
Transmission spectroscopy faces observational hurdles. A challenge is the presence of clouds and hazes high in an exoplanet’s atmosphere. These aerosols can scatter or block starlight across a wide range of wavelengths, creating a flat, featureless spectrum that masks the absorption signals from individual molecules below.
Another complication arises from the host star itself. Stellar activity, such as dark starspots or bright faculae on the star’s surface, can contaminate the measurements. These features can alter the baseline starlight, creating signals that mimic or obscure the true atmospheric absorption.
Advanced observatories like the James Webb Space Telescope (JWST) are designed to mitigate these issues. JWST’s high sensitivity and its access to a broader range of infrared wavelengths allow it to gather more light with greater precision. This capability helps it to peer through cloud layers more effectively and distinguish the faint planetary signals from stellar contamination.