GJ 1214b is a strange planet. It has a radius about 2.7 times that of Earth and a mass roughly eight times greater. It’s bigger than a rocky planet, but not big enough to be a gas giant. Researchers classify it as a “sub-Neptune” and think that many such planets exist in our galaxies. But we don’t have one like it in our solar system, so it remains a mystery.

It also has a thick atmosphere, veiled by dense aerosols, that seemed impenetrable — until now. With the cutting-edge capabilities of the James Webb Space Telescope (JWST), researchers have detected possible signs of carbon dioxide (CO₂) and methane (CH₄) in its atmosphere — an exciting breakthrough that sheds light on this mysterious world.

Astronomers have debated for years whether these planets have hydrogen-rich atmospheres or are water worlds. But their thick blanket of clouds makes it almost impossible to study them. Almost.

Aerosols (clouds and haze particles) in planetary atmospheres interact with light in specific ways depending on the wavelength. They scatter and absorb light most efficiently at shorter wavelengths (like visible or ultraviolet light). This is why earlier telescopes, like the Hubble Space Telescope, which worked primarily in these ranges, could not penetrate the haze.

Infrared light, however, has longer wavelengths. These wavelengths are less affected by scattering from aerosols. By observing the infrared spectrum, JWST instruments like NIRSpec can “see through” the haze to detect the underlying molecular signatures of the planet’s atmosphere.

But when researchers led by Everett Schlawin at the University of Arizona and Steward Observatory and Kazumasa Ohno at the National Astronomical Observatory of Japan pointed the JWST at GJ 1214b, they were pretty surprised. Turns out, it’s neither hydrogen nor water: it’s carbon dioxide.

A good mini-Neptune to study

Sub-Neptunes, like GJ 1214b, are the most common type of exoplanet discovered to date. They seem to occupy a varied size range between Earth and Neptune and can have very different types of atmospheres. It’s this very diversity that makes it so difficult to classify them. Are they scaled-up versions of Earth, with rocky interiors and dense atmospheres? Or are they miniature versions of Neptune, dominated by thick gas layers? Atmospheric spectroscopy, the technique used by JWST, is the key to answering these questions.

Interpreting JWST findings is not so easy, however. The data reflects a complex interplay of factors that make it impossible to draw straightforward conclusions. For example, aerosols or hazes in a planet’s atmosphere can mimic or mask spectral features, making it difficult to discern which molecules are truly present. Additionally, the spectral signals are faint and may overlap, requiring sophisticated models to untangle contributions from different gases. Noise in the data and uncertainties about the planet’s exact conditions, such as temperature, pressure, or cloud composition, further complicate the analysis.

Schlawin says decoding the atmospheric signature is a bit like looking through a thick novel and searching for a changed sentence. And even this is only possible because GJ 1214b is such an outstanding sub-Neptune.

It orbits close to its star, with a year lasting just 1.6 Earth days, and experiences temperatures around 600 Kelvin (327°C). These conditions make it an ideal candidate for studying atmospheric properties with the transit method. The transit method measures the dimming of a star as the planet passes in front of it, allowing astronomers to analyze starlight filtered through the planet’s atmosphere and identify its chemical composition.

Its proximity to Earth (only 40 light-years away) and its relatively small host star make it even better for precise observations.

Aerosols on exoplanets

One of the most fascinating aspects of GJ 1214b is its hazy atmosphere. Aerosols, which can form from photochemical reactions or condensation of gases, play a crucial role in shaping an atmosphere’s appearance. On Earth, aerosols contribute to phenomena like smog and cloud formation. On GJ 1214b, they act as a veil, obscuring the planet’s deeper atmospheric layers.

The aerosols on GJ 1214b might be composed of complex organic molecules, sulfur compounds, or exotic materials not found on Earth. Their presence complicates efforts to analyze the planet’s atmosphere but also provides a unique opportunity to study how such particles form and evolve on other worlds. They could give important clues regarding how sub-Neptunes form and evolve.

Carbon dioxide is a robust marker of high metallicity, a term used in astronomy to describe atmospheres rich in elements heavier than hydrogen and helium. Planets with high metallicity are often thought to originate closer to their host star, where heavy elements are abundant. However, such planets may migrate outward or inward during their lifetimes, interacting with other planets or the protoplanetary disk.

There are still many unknowns, but this breakthrough is just the beginning. Future observations of GJ 1214b and similar planets will refine our understanding of their atmospheres and interior structures. Thanks to JWST’s advanced capabilities, we can peeer into planets like this one better than ever before and build a comprehensive picture of planetary diversity in the galaxy.

Journal Reference:

Everett Schlawin et al, Possible Carbon Dioxide above the Thick Aerosols of GJ 1214 b, The Astrophysical Journal Letters (2024). DOI: 10.3847/2041-8213/ad7fef

Kazumasa Ohno et al, A Possible Metal-dominated Atmosphere below the Thick Aerosols of GJ 1214 b Suggested by Its JWST Panchromatic Transmission Spectrum, The Astrophysical Journal Letters (2025). DOI: 10.3847/2041-8213/ada02c