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Webb’s Impact on Exoplanet Research

What are planets around other stars like? What are they made of? How did they form? How different are they from each other and the planets in our solar system?

Building on the legacy of NASA’s Hubble Space Telescope, Spitzer Space Telescope, and other ground- and space-based observatories, the James Webb Space Telescope is expanding our understanding of exoplanet atmospheres. A few are highlighted below.

  • WASP-96 b: The first exoplanet transmission spectrum collected by Webb showed clear signs of water vapor that previous spectra only hinted at. It is the first transmission spectrum that includes wavelengths longer than 1.6 microns with high resolution and accuracy, and the first to cover the entire wavelength range from 0.6 microns (visible red light) to 2.8 microns (near-infrared) in a single shot. WASP-96 b is a gas giant exoplanet.
  • TRAPPIST-1 b: Webb performed the first thermal emission observation on any planet as small as Earth and as cool as the rocky planets in our solar system. These observations suggest that the planet does not have a significant atmosphere.
  • WASP-39 b: Webb detected carbon dioxide and sulfur dioxide in the atmosphere of a gas giant exoplanet for the first time. The detection of sulfur dioxide in the atmosphere of this “hot Saturn” provides evidence of photochemistry – chemical reactions initiated by energetic stellar light.

Exoplanets: From Small and Rocky to Gas Giant

Between the early 1990s, when the first exoplanets were discovered, and the beginning of Webb’s observations in 2022, more than 5,000 exoplanets had been detected in the Milky Way, with more discovered every year. Over the past three decades, researchers have identified a wide range of exoplanets, including rocky planets orbiting within the so-called "habitable zones" of their stars; have observed the planet-forming disks that surround young stars; and have detected molecules in exoplanet atmospheres.

In its first 18 months, Webb has begun to expand our knowledge through successful observations of a wide variety of exoplanets.

Illustration showing a portion of a cloudy planet set against the black background of space.
Gas Giants: (Artist Concept) WASP-17 b. As the term suggests, gas giants are large planets made primarily of hydrogen and helium gas surrounding a relatively small rocky core. These planets are most similar to Jupiter and Saturn. Gas giants like WASP-17 b, which orbit close to their parent stars, are often referred to as “hot Jupiters,” because of the extremely high temperatures of their gaseous atmospheres. Using Webb, researchers detected evidence for quartz nanocrystals in the clouds of WASP-17 b.
Illustration: NASA, ESA, CSA, Ralf Crawford (STScI)
Illustration of a planet and its cool red dwarf star. In the foreground on the right is the planet, which fills most of the frame. The planet is various shades of blue, with wisps of white scattered throughout. The left edge of the planet (the side facing the star) is lit, while the rest is in shadow. In the background at the lower left is the star, which appears smaller. The star has a bright red glow. Also in the background is another planet, which appears as a small crescent. The black background of space is speckled with a few small stars.
Neptune-like: (Artist Concept) Sub-Neptune K2-18 b. Neptune-like exoplanets, like gas giants, have dense atmospheres dominated by hydrogen and helium gas. These planets, however, have smaller radii and rocky, metallic cores that make up a greater share of their total mass. As their name suggests, these planets are similar to Neptune in their radii. K2-18 b, seen here, is an example of a special type of Neptune-like exoplanet called a sub-Neptune, which have smaller radii than Neptune and don’t exist in our own solar system. K2-18 b may also be a member of a class of exoplanets that have been theorized, but remain unconfirmed by observations: ocean worlds. These are planets with a large fraction of water in the form of oceans either above or beneath their surfaces. Using Webb, researchers detected both carbon dioxide and methane in K2-18 b’s atmosphere.
Illustration: NASA, ESA, CSA, Joseph Olmsted (STScI); Science: Nikku Madhusudhan (IoA)
Occupying the left two-thirds of the image is a foreground planet mostly in shadow. On the right side, a tan crescent shows subtle surface features. A thin, tenuous blue atmosphere lines the planet’s limb. On the right, a small red globe represents a red dwarf star. Its surface is mottled with small, dark spots resembling sunspots. Both planet and star are on a mostly black background speckled with hundreds of faint, distant stars.
Super-Earths: (Artist Concept) GJ 486 b. Super-Earths are larger than Earth, but not as large as Neptune. These planets can be rocky and metallic and can have water as a major constituent. Some of these planets have atmospheres, while some do not. In fact, Webb observed GJ 486 b to determine if it had an atmosphere. Its observations hint at the presence of water vapor in the system, but future observations will determine if the vapor is coming from the planet itself or the coolest areas of its parent star. Despite the name of these exoplanets, super-Earths only compare to Earth in radius and volume; there are nothing like them in our solar system.
Illustration: NASA, ESA, CSA, Joseph Olmsted (STScI), Leah Hustak (STScI)
Illustration of a rocky exoplanet and its active red dwarf star.
Terrestrial/Rocky: (Artist Concept) TRAPPIST-1 b. Like Super-Earths, rocky worlds are mostly rocky and metallic, but they have radii similar to or smaller than that of Earth. Some rocky exoplanets have atmospheres, while some do not. All of the seven exoplanets in the TRAPPIST-1 system are rocky, but TRAPPIST-1 b, the innermost planet, receives more energy from its parent star than any other planet in the system. This made it ideal for studying in infrared wavelengths with Webb to provide important information about its sibling planets. Webb was able to help researchers determine that the dayside of TRAPPIST-1 b is roughly 450 degrees Fahrenheit (200 degrees Celsius) and has no significant atmosphere. While TRAPPIST-1 b isn’t within this system’s habitable zone, TRAPPIST-1 e, f, and g are, much like Venus, Mars, and Earth in our own solar system. Scientists have observed these three worlds, and are currently analyzing data to search for signs of an atmosphere.
Illustration: NASA, ESA, CSA, Joseph Olmsted (STScI); Science: Thomas Greene (NASA Ames), Taylor Bell (BAERI), Elsa Ducrot (CEA), Pierre-Olivier Lagage (CEA)

How Webb Observes Exoplanets

Since Webb's first exoplanet observations in June 2022, Webb’s unique sensitivity and precision have allowed researchers to study the details of gas giant atmospheres, hints about the properties of small, rocky exoplanets, and even how some exoplanets might have formed.

Infographic titled “Rocky Exoplanet TRAPPIST-1 b Secondary Eclipse Light Curve” showing a diagram of a secondary eclipse and a graph of change in brightness over time.
This secondary eclipse light curve shows the change in brightness of the TRAPPIST-1 system as the innermost planet, TRAPPIST-1 b, moves behind the star. Download the TRAPPIST-1 b light curve.
Illustration: NASA, ESA, CSA, Joseph Olmsted (STScI); Science: Thomas Greene (NASA Ames), Taylor Bell (BAERI), Elsa Ducrot (CEA), Pierre-Olivier Lagage (CEA)

Directly observing planets that are close to their stars is difficult for Webb, due to the stars’ overwhelming brightness. However, while stars are much brighter, both stars and planets glow in infrared light. As a planet passes in front of its star from our perspective, it blocks a small amount of starlight, decreasing the total amount of light we observe from the system. This is known as a transit. Conversely, when a planet passes behind its star, the star blocks the planet’s light, creating a smaller dip in the light from the system, known as a secondary eclipse. Observing an exoplanet as it orbits and measuring these changes in brightness allows us to create what's known as a light curve.

Researchers captured a secondary eclipse light curve for TRAPPIST-1 b and TRAPPIST-1 c using MIRI's (Mid-Infrared Instrument) time-series photometry mode, which was able to measure the light emitted by both the star and the dayside of the planet over time.

By subtracting the brightness of the star from the combined brightness of the star and planet, astronomers were able to calculate how much infrared light is coming from the planet's dayside – its thermal emission. These data were then used to calculate the planet's temperature.

Webb’s observation of TRAPPIST-1 b marked the first thermal emission observation of any planet as small as Earth and as cool as the rocky planets in our solar system. Further, its observation of TRAPPIST-1 c's dayside temperature confirmed it as the coolest rocky exoplanet ever characterized, based on its thermal emission.

We can also observe the changes of brightness across the planet's entire orbit. These lead to a light curve known as a phase curve.

Graphic showing how the observed total brightness of a star-planet system changes as the planet orbits the star.
This simplified diagram of an exoplanet phase curve shows the change in total brightness of a star–planet system as the planet orbits the star. As a planet passes in front of its star, the planet blocks a small amount of starlight, decreasing the total amount of light we observe from the system, and causing a dip in the phase curve. As a planet passes behind its star, the star blocks the planet's light, creating another dip. Download this exoplanet phase curve diagram.
Image: NASA, ESA, CSA, Dani Player (STScI), Andi James (STScI), Gregory Bacon (STScI)

One of Webb's first phase curves came from observations of the sub-Neptune GJ 1214 b. The phase curve allowed researchers to map the planet's dayside and nightside temperatures, which revealed a shift from 535 degrees Fahrenheit on its day side to 326 on its night side (or from 279 to 165 degrees Celsius).

With these data, researchers were able to determine that the atmosphere of this planet must be made of heavier molecules, such as water or methane, since they tend to transfer heat more slowly than lighter molecules, like hydrogen. The data also point to clues about the formation of this planet. Researchers think it may have started with a hydrogen-rich atmosphere and lost that hydrogen over time, leaving heavier molecules behind or it was rich in heavier elements to begin with.

Transmission Spectroscopy: Measuring Exoplanet Atmospheres

Webb can use transmission spectroscopy to characterize the composition of an exoplanet's atmosphere. Transmission spectroscopy compares the light filtered through the exoplanet's atmosphere to the light coming from the parent star. Different types of chemicals in the atmosphere absorb different colors of the starlight spectrum, so the colors that are missing tell astronomers which atoms and molecules are present. By viewing infrared light with its sensitive spectrographs, Webb can pick up chemical fingerprints that can’t be detected in visible light.

Learn how Webb uses transmission spectroscopy to study the atmospheres of exoplanets.
Video: NASA, ESA, CSA, Leah Hustak

Get video details and downloads in the Video gallery, or download video captions (VTT, 4 KB), and transcript of the audio description (Word Doc, 21 KB).

An example of these transmission spectra can be found from an observation of the WASP-39 system. Three of Webb’s instruments, NIRSpec (Near-Infrared Spectrograph), NIRISS (Near-Infrared Imager and Slitless Spectrograph), and NIRCam (Near-Infrared Camera), took transmission spectra of WASP-39 b, a gas giant, from 0.6 microns (visible red light) to 5.3 microns (near-infrared light).

The results show that this planet’s atmosphere contains several molecules of interest, which absorb starlight and appear on the spectrum in the form of peaks. In fact, this observation marks the first time sulfur dioxide and carbon dioxide have been detected in the atmosphere of an exoplanet. The presence of sulfur dioxide, a molecule produced from chemical reactions triggered by high-energy starlight, gave researchers the first evidence of photochemistry, or the chemical effects of light, in an exoplanet.

Graphic titled “Hot Gas Giant Exoplanet WASP-39 b Atmosphere Composition” includes the NIRSpec PRISM spectra with an illustration of the planet and its star in the background. The graph shows the amount of light blocked in percent on the y axis versus wavelength of light in microns on the x axis. The y axes range from 2.00 percent (less light blocked) to 2.35 percent (more light blocked). The x axes range from less than 0.1 microns to 5.5 microns. Data points are plotted as white circles with grey error bars. A curvy blue line represents a best-fit model. The graph features labeled highlights for sodium, water, carbon monoxide, and carbon dioxide.
WASP-39 b is a hot gas giant exoplanet that orbits its star closer than Mercury orbits the Sun. The y-axis shows the percentage of light blocked by the planet and its atmosphere, while the x-axis shows the wavelength of light that's being observed in microns. The detection of carbon dioxide in this planet's transmission spectrum demonstrated Webb's capacity to find this molecule in the thinner atmospheres of smaller, rocky planets, especially across the 3- to 5.5-micron range. Download the WASP-39 b PRISM spectra.
Credit: NASA, ESA, CSA, J. Olmsted (STScI).
Graphic titled “Hot Gas Giant Exoplanet WASP-17 b Composition of Cloud Particles, MIRI Low-Resolution Time-Series Spectroscopy” showing a transmission spectrum with evidence for quartz crystals.
Webb observed the WASP-17 system using MIRI's low-resolution spectrograph for nearly 10 hours to collect more than 1,275 measurements before, during, and after the transit. A peak at 8.6 microns led astronomers to think that silica particles are absorbing some of the starlight passing through the atmosphere. Download the WASP-17 b spectra.
Illustration: NASA, ESA, CSA, Ralf Crawford (STScI); Science: David Grant (University of Bristol), Hannah Wakeford (University of Bristol), Nikole Lewis (Cornell University)

Using its time-series spectroscopy mode, MIRI observed WASP-17 b for ten hours, and captured a transmission spectrum that showed that its atmosphere contains clouds rich in nanoparticles of crystalline silica, better known as quartz. Silicates, like quartz, make up the bulk of Earth and other rocky objects in our solar system, including the Moon. However, silicate grains previously detected in the atmospheres of exoplanets were rich with magnesium, not quartz alone.

Direct Observation

Four pull-outs from a bright star showing a bright spot next to an occulting disk in purple, blue, orange and red from left to right.
This image shows the exoplanet HIP 65426 b in different bands of infrared light, as seen from the James Webb Space Telescope: purple shows the NIRCam instrument's view at 3.00 microns, blue shows the NIRCam instrument's view at 4.44 microns, yellow shows the MIRI instrument's view at 11.4 microns, and red shows the MIRI instrument's view at 15.5 microns. The small white star shape in each image marks the location of the host star, HIP 65426, which has been subtracted using the coronagraphs and image processing. The bar shapes in the NIRCam images are artifacts of the telescope's optics, not objects in the scene. Download the HIP 65426 b infographic.
Image: NASA, ESA, CSA, Alyssa Pagan (STScI); Science: ERS 1386 Team, Aarynn Carter (UC Santa Cruz)

Many of the "images" of exoplanets that you may see are actually artist's concepts. They are depictions of what a planet might look like based on scientific data. While it is possible to take pictures of some exoplanets, planets near bright stars are just too dim to image without blocking the starlight.

Despite this difficulty, Webb has successfully imaged exoplanets. Both of Webb's imagers, NIRCam (Near-Infrared Camera) and MIRI have devices known as coronagraphs, which are tiny masks that block out starlight so that we can see the planet. Other direct imaging modes require researchers to subtract the known shape of a point-like star as seen through Webb's mirrors and other optics to reveal the light from the faint planet.

Webb's first direct image of an exoplanet was taken of HIP 65426 b, a gas giant. It is about 100 times farther from its parent star than Earth is from the Sun. This distance makes it easy for Webb to separate the planet from the star using a coronagraph, which allowed Webb to take direct images of HIP 65426 b in multiple infrared wavelengths.

What's Next for Webb in Exoplanet Research?

The James Webb Space Telescope has ushered in a new era in exoplanet research. To learn about the diversity of exoplanets and their atmospheres, Webb is continuing to study a range of exoplanets, from hot Jupiters to small rocky planets. As Webb deepens our understanding of exoplanet systems, we are able to better understand our own solar system. This includes the details of how planetary atmospheres form and evolve over time, what separates gas giants from Neptune-like and rocky planets, and how the unique conditions of each planet and star system shape their physical and chemical properties.

Exciting discoveries of molecules such as methane on K2-18, further our discussions of potentially habitable worlds. Astronomers plan to use the full suite of Webb's instruments to study exoplanets like these, which are abundant in methane, carbon dioxide, and water, and may be promising places to search for evidence of habitability.

Hubble is also continuing to gather new data, obtaining ultraviolet and visible light images and spectra of the host stars, which will us understand how they affect their exoplanets. Further, NASA's upcoming Nancy Grace Roman Space Telescope, will help Webb and Hubble by surveying large swaths of sky to detect a wide-range of worlds, from the mysterious super-Earths and sub-Neptunes to familiar terrestrial planets perhaps similar to our own.  

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