Using the James Webb Space Telescope (JWST), scientists have discovered what may be the first verified “steam world”—an exoplanet enveloped in a warm, vapor-rich atmosphere, about 100 light-years away in the constellation Pisces.

This exciting addition to the catalog of over 5,700 confirmed exoplanets includes GJ 9827 d, a planet roughly twice the size of Earth with an atmosphere composed mainly of water vapor. This discovery confirms theories about the existence of such worlds, which were only hypothetical until now. Eshan Raul, a researcher from the University of Michigan, shared, “We were specifically looking for water worlds. If these exist, who knows what else is out there.”

GJ 9827 d, which scientists envision as having a thick, water-rich atmosphere and no flowing water or ice, could resemble Jupiter’s moons Europa and Ganymede if they were closer to the Sun. The exoplanet’s surface temperature is estimated at 660°F, vastly hotter than Earth’s 59°F average, and orbits close to its host star. This intense heat likely gives GJ 9827 d a layered, hydrogen-poor atmosphere, setting it apart from gas giants.

Caroline Piaulet-Ghorayeb, lead author and doctoral student at the University of Montréal, highlights that GJ 9827 d is the first exoplanet with an atmosphere where hydrogen is not the primary component. To study it, the JWST used transmission spectroscopy, analyzing starlight as it filters through planetary atmospheres to reveal each one’s unique molecular composition.