The atmosphere of a warm, rocky exoplanet can reveal secrets about its surface. The high temperatures and moderate pressures at the base of the atmosphere can create a thermochemical equilibrium between rock and gas. This links the composition of the surface to the composition of the atmosphere. 

For example, simulations have shown that the chemical equilibrium of simple molecules like carbon dioxide in the atmosphere of Venus can be used to probe the composition of its surface. 

The atmospheres of exoplanets can be diverse, with strong atomic and molecular signatures, including water vapor. A study of the atmosphere of the exoplanet WASP-107b found water vapor, sulfur dioxide, and silicate clouds, but no methane. These detections provide insights into the exoplanet’s chemistry and dynamics. 

In November 2022, NASA’s James Webb Space Telescope provided the most detailed analysis of an exoplanet atmosphere ever, with the analysis of WASP-39 b. The telescope also provided a molecular and chemical profile of the skies of a distant world.

As astronomers have begun to gather data on the atmospheres of planets, we’re learning about their compositions and evolution. Thick atmospheres are the easiest to study, but these same thick atmospheres can hide the surface of a planet from view

The most successful method for measuring the chemical composition of an exoplanet’s atmosphere is transit spectroscopy. 

Here’s how transit spectroscopy works: 

1. The planet passes in front of its star. 
2. Some of the star’s light passes through the planet’s atmosphere on its way to our telescopes. 
3. Different particles in the atmosphere absorb light of different wavelengths. 
4. We can infer the composition of the planet’s atmosphere by the periodic decrease in light detected from the star at various combinations of wavelengths. 

Direct imaging is another technique that can be used to determine an exoplanet’s atmospheric composition and temperature. This technique involves measuring the brightness of the planet at different wavelengths and analyzing its spectrum. 

The composition of the atmospheres of exoplanets can also be revealed by observing them using space-based telescopes, such as the Hubble Space Telescope, or from the ground using observatories like the Very Large Telescope or Keck.

Each atom and molecule have its own unique spectral fingerprint. If an atom or molecule is present in the exoplanet’s atmosphere it will imprint its signature on the spectrum of the starlight which can be measured to determine the atmospheric composition

For such warm rocky planets, broadly Venus-like planets, the high temperatures and moderate pressures at the base of their atmospheres may enable thermochemical equilibrium between rock and gas. This links the composition of the surface to that of the observable atmosphere

Atmospheric pressure is the force per unit area exerted by an atmospheric column. It can be expressed in millimeters of mercury, pounds per square inch, millibars, or standard atmospheres

The temperature required to turn rock into gas depends on the type of rock: 

• Sulfur: About 150°C 
• Normal low-temperature rocks: About 900°C 
• Rocks with median melting points: About 2000°C

Thick atmospheres are easier to study, but they can also hide the surface of a planet from view. For example, Venus has a thick atmosphere that makes it impossible to see the planet’s terrain. 

Venus’s atmosphere is 92 times thicker than Earth’s and is made up of 95% carbon dioxide. The atmosphere is the result of a combination of factors, including volcanic activity, the planet’s proximity to the Sun, and the lack of a magnetic field. The atmosphere traps heat and raises surface temperatures

We have discovered a huge diversity in the atmospheres of exoplanets from clear strong atomic and molecular signatures including water vapour, to scattering and muted or obscured features indicative of clouds high in the planet’s atmosphere.

The atmospheres of exoplanets can have a wide range of diversity, including strong atomic and molecular signatures like water vapor

Scientists study exoplanets by observing them as they pass in front of their host stars. The spectrum of light that passes through an exoplanet’s atmosphere during these transits can reveal the presence of water vapor and other chemical signatures. 

In 2019, researchers detected water vapor signatures in the atmosphere of a planet beyond our solar system that resides in the “habitable zone”. The habitable zone is the region around a star where liquid water could potentially pool on the surface of a rocky planet. 

In November 2023, European astronomers used the James Webb Space Telescope to find water vapor, sulfur dioxide, and sand clouds in the atmosphere of an exoplanet. They described the discovery as “crucial

There are several factors that scientists consider when determining whether an exoplanet, a planet outside of our solar system, is habitable or not. These include the planet’s distance from its star, the type of star it orbits, the planet’s atmosphere, and whether it has liquid water on its surface

According to Space.com, a planet must be relatively small and rocky, and orbit within the habitable zone of its star to be considered potentially life-friendly. 

According to BBC Sky at Night Magazine, other features that make a planet habitable include: 

• The right distance from its star 
• A more or less circular orbit 
• A stable rotational axis 
• Some water, but not too much 
• No hydrogen-rich atmosphere 
• Plate tectonics 
• Magnetic field 

According to PNAS, other requirements for life on an exoplanet include: 

• Temperature and state of water 
• Water availability 
• Light and redox energy sources 
• UV and Ionizing radiation 

According to Quizlet, a planet can only have a habitable surface if it: 

• Is within its star’s habitable zone 
• Is large enough to retain internal heat and have plate tectonics 
• Has enough of an atmosphere for liquid water to be stable on its surface 

According to Springer, a habitable planet must also have: 

• A favorable size or mass 
• Initial composition 
• Atmospheric properties 
• An active interior that can support geological activity to buffer climate 
• Replenish the atmosphere via outgassing 
• Generate a magnetic field to shield the planet

Astronomers are learning about the composition and evolution of planets’ atmospheres by gathering data on them. 

Here are some things we’ve learned about the atmospheres of planets: 

• Evolution The atmospheres of the inner planets have evolved since they formed. For example, Earth’s original atmosphere was probably similar to Venus, consisting of carbon dioxide and nitrogen. 
• Composition Most planets in our solar system have two or three main constituents that make up their atmosphere. For example, Venus and Mars have more than 98% carbon dioxide and nitrogen, while Earth has 99% nitrogen and oxygen. 
• Gas giants The atmospheres of gas giant planets are made up of mostly molecular hydrogen and helium. These planets have no solid planetary surface. 

Thick atmospheres are the easiest to study, but they can also hide a planet’s surface from view

There is evidence that extrasolar planets can have an atmosphere. Comparisons of these atmospheres to one another and to Earth’s atmosphere broaden our basic understanding of atmospheric processes such as the greenhouse effect, aerosol and cloud physics, and atmospheric chemistry and dynamics

Studying the atmospheres of other planets can help astronomers learn about how our atmosphere may change in the future. For example, astronomers can study how other planets’ atmospheres are affected by substances like dust, sulfuric acid, and carbon dioxide. 

Astronomers can also use the atmospheres of other planets to learn about the history of an exoplanet. For example, astronomers can use data detected during transits to determine the size and orbit of an exoplanet. They can also study the composition of gases around an exoplanet by looking at the light passing through its atmosphere. 

Astronomers can also use spectroscopy to determine the composition of planets and their atmospheres. Spectroscopy involves analyzing the light that a planet emits to determine what elements are present in its atmosphere.

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