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Atmosphere Composition of Exoplanet K2-18 b (NIRISS & NIRSpec)

A graphic titled “Exoplanet K2-18 b: Atmosphere Composition.” The graphic shows the spectra of the exoplanet K2-18 b from NIRISS and NIRSpec in the form of a graph, with the vertical y-axis labeled as Amount of Light Blocked and the horizontal axis labeled as Wavelength of Light (microns). The spectra is plotted as dots with vertical short vertical lines across the plot, with the best-fit model as a blue jagged white line.There are green, yellow and light blue vertical columns of varying thicknesses scattered across the plot indicating where variations in the line represent the presence of methane, carbon dioxide, and dimethyl sulfide, respectively. Behind the graph is an illustration of the planet and star. The planet is large fuzzy blue-ish sphere off to the right, taking up half of the background. The red star is smaller at the bottom left of the entire graphic.

Spectra of K2-18 b, obtained with Webb’s NIRISS (Near-Infrared Imager and Slitless Spectrograph) and NIRSpec (Near-Infrared Spectrograph) displays an abundance of methane and carbon dioxide in the exoplanet’s atmosphere, as well as a possible detection of a molecule called dimethyl sulfide (DMS).

The detection of methane and carbon dioxide, and shortage of ammonia, are consistent with the presence of an ocean underneath a hydrogen-rich atmosphere in K2-18 b.

K2-18 b, 8.6 times as massive as Earth, orbits the cool dwarf star K2-18 in the habitable zone and lies 110 light years from Earth.

  • Release Date
    September 11, 2023
  • Science Release
    Webb Discovers Methane, Carbon Dioxide in Atmosphere of K2-18 b
    Read the release
  • Credit
    Illustration: NASA, ESA, CSA, Ralf Crawford (STScI), Joseph Olmsted (STScI); Science: Nikku Madhusudhan (IoA)

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Related Images & Videos

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.

Exoplanet K2-18 b (Illustration)

This illustration shows what exoplanet K2-18 b could look like based on science data. K2-18 b, an exoplanet 8.6 times as massive as Earth, orbits the cool dwarf star K2-18 in the habitable zone and lies 120 light years from Earth. A new investigation with NASA’s James Webb Space...

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Last Updated
Aug 28, 2025
Contact
Media

Laura Betz
NASA’s Goddard Space Flight Center
Greenbelt, Maryland
laura.e.betz@nasa.gov

Illustration Credit

NASA, ESA, CSA, Ralf Crawford (STScI), Joseph Olmsted (STScI)

Science Credit

Nikku Madhusudhan (IoA)

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