Transmission Spectroscopy

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In an earlier post (exoplanet detection methods) I had explained the Radial Velocity method of finding an exoplanet. This particular method is possible only because of spectrographs like the HARPS and ESPRESSO. These spectra are then analyzed to find and identify a particular planet and it's atmosphere. 

Transmission spectroscopy is a technique used in astronomy to study the atmospheres of exoplanets, which are planets outside of our solar system. It involves measuring the amount of starlight that passes through the planet's atmosphere during a transit and analyzing the resulting spectrum to detect and characterize the molecular composition and physical properties of the atmosphere.

Transmission spectroscopy is used to study exoplanet atmospheres and understand their physical and chemical properties. By detecting the absorption or emission lines of molecules in the atmosphere, astronomers can infer the temperature, pressure, composition, and structure of the atmosphere. Although this is not to be mistaken with emission spectroscopy. This information can help in determining the habitability of exoplanets and identifying potential targets for further study.

In addition to searching for biosignatures, exoplanet transmission spectroscopy can also reveal information about the temperature, pressure, and density of an exoplanet's atmosphere, as well as the presence of clouds or other atmospheric features.

There are several types of transmission spectroscopy, the most important ones are:

1) Broadband transmission spectroscopy

This involves measuring the flux of the starlight before, during, and after a planetary transit across a broad wavelength range. This provides information on the overall atmospheric properties of the planet. This provides a continuous spectrum of absorbed light that can reveal the presence of multiple components in a sample, each with their own characteristic absorption features. Broadband spectroscopy can provide a general overview of the composition of a sample, but it may not be suitable for identifying specific components or features.


2) Narrowband transmission spectroscopy

Narrowband transmission spectroscopy, on the other hand, involves measuring the absorption of light at specific wavelengths. This technique is useful for identifying specific components or features in a sample, as the spectral resolution is high enough to distinguish between closely spaced absorption peaks. However, narrowband spectroscopy may miss important information about other components or features that are not included in the selected wavelength bands.


In the context of exoplanet research, both broadband and narrowband transmission spectroscopy are used to study the atmospheres of exoplanets. Broadband spectroscopy is useful for detecting broad features in the transmission spectrum of an exoplanet's atmosphere, such as the presence of molecular absorption bands due to water or carbon dioxide.

Narrowband spectroscopy, on the other hand, can be used to identify specific absorption features associated with particular atmospheric components, such as the sodium D-line feature seen in the transmission spectrum of the atmosphere of HD 209458b.

However, exoplanet transmission spectroscopy is a challenging technique, as the signal from the exoplanet's atmosphere is very faint compared to the bright light of the host star. Additionally, many exoplanets have thick or complex atmospheres that can make it difficult to identify specific atmospheric components. Despite these challenges, transmission spectroscopy has already provided valuable insights into the atmospheres of several exoplanets and is expected to play a key role in future exoplanet research.

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