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JWST Discovers Water Vapor on Smallest Exoplanet to Date

An astronaut in a white spacesuit conducts an Extravehicular Activity (EVA), or spacewalk, while floating in the blackness of space. Below, the bright blue and white curved horizon of Planet Earth is prominently visible. The astronaut is holding onto a metallic structure, likely part of a space station or spacecraft, with the sun creating a strong highlight on their suit and helmet.

September 23, 2025

NASA's James Webb Space Telescope has detected water vapor in the atmosphere of GJ 9827d, the smallest exoplanet where water has been found.

In a landmark discovery, NASA's James Webb Space Telescope (JWST) has detected definitive evidence of water vapor in the atmosphere of GJ 9827d, the smallest exoplanet yet where such a discovery has been made. This breakthrough pushes the boundaries of our ability to characterize worlds beyond our solar system, offering a tantalizing glimpse into the atmospheric compositions of small, potentially rocky planets.

A Closer Look at a 'Mini-Neptune'

Located 97 light-years away, GJ 9827d is roughly twice the diameter of Earth. Using Webb's powerful instruments for transit spectroscopy—analyzing starlight as it filters through a planet's atmosphere—scientists confirmed the presence of water. However, the nature of this world remains a mystery. With a scorching temperature of approximately 427° Celsius (800° Fahrenheit), it's far too hot to support life as we know it. The discovery opens up two primary possibilities for the planet's identity.

This discovery opens the door to studying planets that might be more like Earth in the future. Is GJ 9827d a water-rich world with a steamy atmosphere, or a mini-Neptune with a hydrogen-rich blanket? We need more data to know for sure.

- Dr. Aris Thorne, Lead Exoplanetary Scientist at Zendar Universe

Key Implications of the Discovery

  1. Technological Milestone: The detection proves JWST's remarkable capability to analyze the atmospheres of smaller exoplanets, a critical step towards eventually studying Earth-sized worlds in the habitable zones of their stars.
  2. Planetary Evolution Insights: The finding presents a puzzle about planetary formation. GJ 9827d could be a 'mini-Neptune' that retained a hydrogen and water atmosphere, or it might be the remnant core of a larger gas giant that had its atmosphere stripped away by its host star.
  3. Search for Habitability: While GJ 9827d is inhospitable, finding water on a small planet is a foundational step. It demonstrates that a key ingredient for life is present in diverse planetary environments across the galaxy.
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JWST Discovers Water Vapor on Smallest Exoplanet to Date - FAQs

GJ 9827d is the smallest exoplanet to date on which astronomers have detected water vapor in its atmosphere, making it a significant target for studying planetary formation.

No, GJ 9827d is not considered habitable. Its surface temperature is around 427° Celsius (800° Fahrenheit), which is far too hot for liquid water to exist.

JWST used a technique called transit spectroscopy. It observed the light from the planet's host star as it passed through the atmosphere of GJ 9827d, allowing scientists to identify the chemical signature of water molecules.

It demonstrates the incredible power of the JWST to study smaller, potentially rocky planets. This is a crucial step toward the ultimate goal of finding Earth-like planets with atmospheres that could support life.