Webb Spots Water on GJ 9827d: A Water World or Mini-Neptune?
September 17, 2025
NASA's James Webb Space Telescope has detected water vapor in the atmosphere of exoplanet GJ 9827d, raising pivotal questions about its true nature.
In a groundbreaking discovery, NASA's James Webb Space Telescope has detected water vapor in the atmosphere of GJ 9827d, the smallest exoplanet to date where such a finding has been made. Located 97 light-years away in the constellation Pisces, this super-Earth is roughly twice the diameter of our own planet. The presence of water sparks a fascinating debate among scientists: is GJ 9827d a true water world with a steamy atmosphere, or is it a 'mini-Neptune' shedding its original atmosphere under the intense radiation of its host star?
A Tale of Two Worlds
The planet orbits its red dwarf star in a blistering 6.2 days, resulting in surface temperatures soaring to approximately 800 degrees Fahrenheit (427 degrees Celsius), comparable to Venus. This intense heat makes it impossible for liquid water to exist, meaning any water detected is in the form of vapor. Webb's powerful instruments, including the Near-Infrared Imager and Slitless Spectrograph (NIRISS), allowed astronomers to analyze the starlight filtering through the planet's atmosphere, revealing the undeniable signature of H₂O. Now, researchers are exploring two primary scenarios to explain its composition.
- Water World Hypothesis: In this scenario, GJ 9827d could be a planet that formed farther from its star, where water ice was abundant, and later migrated inward. It would possess a significant fraction of its mass as water, enveloped in a steam-rich atmosphere.
- Mini-Neptune Remnant: Alternatively, the planet could be a former mini-Neptune that once had a thick, hydrogen-dominated atmosphere mixed with water. The star's intense radiation could be stripping away the lighter hydrogen and helium, leaving behind a denser, water-vapor-heavy atmosphere over a rocky core.
This discovery opens up an unprecedented opportunity to understand the composition and evolution of planets across the galaxy. Distinguishing between these two scenarios is crucial for our models of planetary formation.
What's Next for GJ 9827d?
The key to solving this puzzle lies in further observations. Scientists will use Webb to search for heavier elements and molecules, such as carbon dioxide and methane, in GJ 9827d's atmosphere. The relative abundance of these molecules will help determine if the atmosphere is primarily steam (indicating a water world) or a hydrogen-rich envelope that is slowly dissipating. Whatever the outcome, the study of GJ 9827d marks a significant step forward in our quest to understand the diverse array of worlds beyond our solar system.
Webb Spots Water on GJ 9827d: A Water World or Mini-Neptune? - FAQs
GJ 9827d is a 'super-Earth' exoplanet located 97 light-years away. It is about twice the diameter of Earth and orbits its star so closely that its surface temperature is around 800°F (427°C).
It is the smallest exoplanet to date where water vapor has been detected in the atmosphere. This finding provides a crucial opportunity to study the atmospheric composition of small, rocky planets outside our solar system.
The two leading theories are: 1) It is a true 'water world' with a thick, steam-dominated atmosphere. 2) It is the remnant core of a 'mini-Neptune' whose original hydrogen-rich atmosphere is being burned off by its star, leaving water vapor behind.
Scientists will use the Webb Telescope for further observations, searching for other molecules like carbon dioxide. If the atmosphere is almost pure steam, it supports the water world theory. If it's a mix with hydrogen, it's likely a mini-Neptune remnant.