Transmission Spectroscopy of Exoplanets

Jovian Explorer
12 min readJul 7


Wasp 39b artist’s impression © NASA



Transmission spectroscopy is a technique used to study the atmospheres of exoplanets by analyzing the changes in the light spectrum as it passes through the planet’s atmosphere during a transit event. By measuring the absorption of specific wavelengths of light, scientists can infer the presence of different molecules and particles in the exoplanet’s atmosphere.


The field of transmission spectroscopy in exoplanets gained prominence with the discovery of the first exoplanet around a Sun-like star in 1995. As astronomers began to detect more and more exoplanets, the need to understand their atmospheric properties became crucial. The first transmission spectroscopy measurements were carried out in the early 2000s, and since then, technological advancements have allowed for more precise and detailed observations.


Transmission spectroscopy provides valuable insights into the composition, structure, and physical processes occurring in exoplanet atmospheres. By studying the atmospheric properties of exoplanets, scientists can gain a better understanding of the formation and evolution of planetary systems, as well as the potential habitability of these worlds.

Transit of an Exoplanet © NASA


Transit Method Spectroscopy

The transit method is the most common technique used to detect exoplanets and perform transmission spectroscopy. It involves observing a star as an exoplanet passes in front of it, causing a slight dip in the star’s brightness. During a transit, a small fraction of the star’s light passes through the exoplanet’s atmosphere, providing valuable information about its composition.

To perform transmission spectroscopy, astronomers measure the star’s spectrum before, during, and after the transit. By comparing these spectra, they can identify the absorption features caused by the exoplanet’s atmosphere. This method allows for the detection of various molecules and particles, such as water vapor, methane, carbon dioxide, and aerosols.

Spectrum for Wasp 39b © JWST ERS, NASA

Interpretation of Results

Transmission Spectra

Types of Transmission Spectra

Transmission spectra can exhibit different shapes and patterns, providing clues about the atmospheric properties of exoplanets. There are primarily three types of transmission spectra: flat, featureless, and sloped.

- **Flat spectra** indicate a featureless atmosphere with no significant absorption or scattering. This suggests a cloud-free atmosphere or the presence of a thick blanket of clouds, preventing the detection of molecular absorption features.

- **Featureless spectra** show a smooth decline in the amount of light as a function of wavelength. This suggests the presence of a haze or aerosol layer in the atmosphere, which scatters light in all directions, obscuring the molecular signatures.

- **Sloped spectra** exhibit a gradual increase or decrease in the amount of light as a function of wavelength. This indicates the presence of molecular absorption features, providing valuable information about the atmospheric composition.

Factors Affecting Transmission Spectra

Several factors influence the shape and characteristics of transmission spectra. These include the exoplanet’s atmospheric composition, temperature, gravity, and the presence of clouds and haze. The abundance and distribution of molecules, as well as the physical properties of the exoplanet, play a significant role in shaping the observed spectra.

Applications of Transmission Spectra

Transmission spectra provide crucial information for understanding exoplanet atmospheres. They allow scientists to determine the presence and abundance of various molecules, including those that are potentially relevant to the existence of life, such as water vapor and oxygen. By studying the composition and structure of exoplanet atmospheres, scientists can assess their potential habitability and compare them to the atmospheres of Solar System planets.

Absorption Features

Transmission spectroscopy relies on the detection of absorption features in the exoplanet’s spectrum. These absorption features occur when specific molecules in the atmosphere absorb light at certain wavelengths, leaving characteristic imprints on the transmission spectrum.

By analyzing the absorption features, scientists can identify the molecular constituents of the exoplanet’s atmosphere and estimate their abundance. For example, the detection of water vapor absorption features can provide insights into the exoplanet’s water cycle and potential habitability.

Atomic and Molecular Signatures

Different atomic and molecular species leave distinct signatures in transmission spectra. These signatures arise from the absorption or scattering of light at specific wavelengths.

For instance, the presence of molecular oxygen (O2) and ozone (O3) can indicate the presence of life-supporting processes, as they are produced by biological activity on Earth. Similarly, the detection of methane (CH4) can be an indication of geological or biological processes, as methane is a byproduct of some forms of life.

Atmospheric water hidden in opaque clouds © NASA

Clouds and Haze

Clouds and haze in exoplanet atmospheres can significantly affect transmission spectra. Clouds consist of tiny liquid or solid particles suspended in the atmosphere, while haze refers to a collection of small, dispersed particles. Both can block or scatter light, making it challenging to detect molecular absorption features.

The presence of clouds and haze can lead to featureless transmission spectra or obscure the underlying molecular signatures. However, by characterizing the properties of clouds and haze, scientists can better understand their impact on the overall atmospheric structure and composition.

Planetary Parameters

The physical properties of exoplanets, such as their mass, radius, gravity, and temperature, influence the interpretation of transmission spectra and provide insights into the exoplanet’s atmospheric characteristics.


Radius: Basics

The radius of an exoplanet plays a fundamental role in understanding its atmosphere. A larger radius indicates a larger atmospheric volume, allowing for more extensive observations and increased sensitivity to atmospheric properties.

Radius: Measurement

Measuring the radius of an exoplanet accurately is crucial for the interpretation of transmission spectra. Various methods are used to determine the radius, including the transit depth, duration, and shape, as well as other complementary techniques, such as radial velocity measurements.

Radius Impact on Atmosphere

The exoplanet’s radius influences its atmosphere in several ways. A larger radius generally leads to a larger atmospheric scale height, which affects the distribution of molecular species with altitude. Additionally, the atmospheric escape rate can be influenced by the exoplanet’s radius, affecting the long-term evolution of its atmosphere.

Altitude Effects

The altitude within an exoplanet’s atmosphere can have a significant impact on its chemical composition. Different molecular species tend to be distributed at specific altitudes due to temperature and pressure gradients. By studying the altitude dependence of molecular species, scientists can gain insights into the vertical structure of exoplanet atmospheres.

Gas Composition

The composition of the exoplanet’s atmosphere is strongly influenced by its radius. Larger exoplanets with lower escape velocities are more likely to retain hydrogen and helium gas, leading to a primarily hydrogen-dominated atmosphere. Smaller exoplanets may have lost most of their primordial atmospheres and contain denser compositions with a higher proportion of heavier elements.

Climate Correlation

The radius of an exoplanet can also have implications for its climate. Larger exoplanets with thicker atmospheres may exhibit stronger greenhouse effects, leading to higher surface temperatures. Understanding the correlation between radius, atmospheric composition, and climate is crucial for assessing the potential habitability of exoplanets.


Gravity and Atmosphere: Effect on Planetary Composition

The strength of gravity on an exoplanet affects its ability to retain its atmosphere over time. The escape velocity, which depends on the exoplanet’s mass and radius, determines the minimum velocity required for atmospheric particles to escape the planet’s gravitational pull.

Gravitational Pull on Light

The strength of gravity can also affect the shape and properties of transmission spectra. Higher gravity can cause a larger atmospheric scale height, resulting in a more extended and denser atmosphere. This, in turn, can lead to broader and deeper molecular absorption features in the transmission spectrum.


Temperature Profiles

The temperature profile of an exoplanet’s atmosphere is a crucial factor in understanding its composition and physical processes. Different layers within the atmosphere can have distinct temperatures, creating thermal structures such as thermal inversions and stratospheres.

Atmospheric Strata

The vertical distribution of temperature within an exoplanet’s atmosphere can give rise to distinct strata or layers. These layers play a significant role in shaping the observed transmission spectra. For example, the presence of a stratosphere, where temperatures increase with altitude, can lead to enhanced molecular absorption features in certain wavelength regions.

Thermal Inversions

Thermal inversions occur when the temperature increases with altitude in a specific region of the atmosphere. This can be caused by the presence of certain molecules, such as carbon monoxide (CO) or titanium oxide (TiO), which absorb and re-radiate heat, creating a localized temperature increase. Thermal inversions can leave characteristic signatures in transmission spectra, providing insights into the atmospheric heating mechanisms.


Temperature also influences the chemical composition of an exoplanet’s atmosphere. Different molecules have specific temperature-dependent abundances and can exist in different chemical forms under varying temperature conditions. By studying the temperature signatures in transmission spectra, scientists can gain information about the molecular composition and thermal properties of exoplanet atmospheres.

Temperature Signatures

Temperature variations within an exoplanet’s atmosphere can manifest as distinct features in transmission spectra. These temperature signatures can be caused by thermal structures, such as thermal inversions, as well as by the redistribution of heat due to atmospheric circulation patterns. Analyzing these signatures can provide insights into the atmospheric dynamics and energy transport processes.

Temperature Variability

The variability of temperature over time is another important aspect to consider in the interpretation of transmission spectra. Temporal variations in temperature can result from various factors, including changes in stellar irradiation, atmospheric circulation patterns, and internal heat sources. By monitoring the temperature variability, scientists can better understand the dynamical and thermal processes occurring in exoplanet atmospheres.

JWST images © Hannah Wakeford


Hot Jupiters

Transmission spectroscopy has provided valuable insights into the atmospheres of hot Jupiter exoplanets. Hot Jupiters are gas giants that orbit very close to their host stars, leading to extreme temperature and radiation conditions.

By studying the transmission spectra of hot Jupiters, scientists have detected various molecules, including water vapor, carbon monoxide, carbon dioxide, and methane. These observations have helped constrain atmospheric models and provided clues about the formation, evolution, and migration of these giant exoplanets.


Transmission spectroscopy has also been used to study the atmospheres of super-Earth exoplanets, which are rocky planets with masses and radii larger than Earth but smaller than Neptune.

The characterization of super-Earth atmospheres is challenging due to their diverse compositions and atmospheric conditions. However, transmission spectroscopy has revealed the presence of molecules such as water vapor, hydrogen, and helium in the atmospheres of some super-Earths. These findings contribute to our understanding of planetary diversity and the transition from rocky to gas-dominated worlds.

Exoplanetary Atmospheres

Transmission spectroscopy has provided a wealth of information about the atmospheres of various types of exoplanets beyond hot Jupiters and super-Earths.

For example, the detection of water vapor and molecular absorption features in transmission spectra has been reported for temperate exoplanets in the potentially habitable zones of their host stars. These findings are significant for understanding the potential habitability of exoplanets and the search for life beyond our solar system.

Search for Life

One of the ultimate goals of transmission spectroscopy is to detect the presence of biosignatures — indicators of life — in exoplanet atmospheres.

The identification of biosignatures, such as the simultaneous presence of oxygen and methane, could provide strong evidence for the existence of life. However, the interpretation of transmission spectra in the context of biosignatures is complex and requires careful consideration of various factors, including the planet’s physical parameters, the presence of false positives, and potential abiotic processes that could mimic the observed molecular signatures.

DIfferent types of exoplanets found so far © NASA

Future Prospects

James Webb Space Telescope

The upcoming launch of the James Webb Space Telescope (JWST) is highly anticipated in the field of transmission spectroscopy. JWST is equipped with advanced instruments capable of high-precision spectroscopic observations, making it a powerful tool for studying exoplanet atmospheres.

With its enhanced sensitivity and larger wavelength coverage, JWST will enable detailed studies of diverse exoplanets, including probing the atmospheres of potentially habitable exoplanets. The data from JWST will provide valuable insights into atmospheric compositions, thermal structures, and the potential presence of biosignatures, further advancing our understanding of exoplanetary systems.

Spectroscopic Surveys

In addition to targeted observations, spectroscopic surveys will play a crucial role in expanding our knowledge of exoplanet atmospheres. Large-scale surveys, such as the Transiting Exoplanet Survey Satellite (TESS) and the upcoming Nancy Grace Roman Space Telescope, will provide a wealth of transit events for follow-up spectroscopic observations.

These surveys will enable statistical analyses of exoplanet atmospheres, helping to identify trends and patterns among different exoplanet populations. By observing a wide range of exoplanets with various physical properties, spectroscopic surveys will contribute to a comprehensive understanding of exoplanetary atmospheres and their diversity.

Detection of Biosignatures

Future advancements in transmission spectroscopy techniques and instruments aim to improve our ability to detect potential biosignatures in exoplanet atmospheres.

New observational strategies, such as observing multiple transits and performing time-resolved spectroscopy, will provide more precise measurements and enhance the detectability of weak molecular signatures. Additionally, the development of advanced models and statistical analysis methods will help distinguish between potential biosignatures and abiotic processes, further refining our search for life in the universe.

Exoplanet Characterization

Transmission spectroscopy will continue to be a critical tool for characterizing exoplanets and assessing their potential habitability.

By combining transmission spectroscopy with other techniques, such as emission spectroscopy, direct imaging, and atmospheric modeling, scientists will obtain a more comprehensive understanding of exoplanet atmospheres. This holistic approach will allow for the determination of atmospheric compositions, temperature structures, cloud properties, and the potential presence of biosignatures, advancing our knowledge of exoplanet environments and the prospects for finding life beyond Earth.


Transmission spectroscopy has revolutionized our understanding of exoplanet atmospheres, offering insights into their compositions, structures, and physical processes. By studying the transmission spectra of exoplanets, scientists have detected various molecules, identified atmospheric features, and begun to explore the potential for habitability and the existence of life beyond our solar system.

While significant progress has been made, there are still numerous challenges and unanswered questions in the field of transmission spectroscopy. Future advancements in observational capabilities, such as the launch of the JWST, combined with continued theoretical modeling and data analysis efforts, will drive the field forward, providing further discoveries and deepening our understanding of exoplanetary systems. By unlocking the secrets of exoplanet atmospheres, we gain valuable insights into the formation, evolution, and potential habitability of worlds beyond our own.


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