© NatureLifePhoto / Flickr (New York City Skyline), Burkhart et al. 2025An artist's rendition of what Eos would look like from Earth if it were visible to the naked eye.
An international team of researchers has discovered a massive molecular cloud that, if we could see it in our sky, would appear to stretch as big as 40 moons. The cloud had been hiding relatively near to Earth, and the find promises to shed new light on the formation of stars and planets.
Molecular clouds are made up of gas and dust, and they give birth to stars when they collapse under the force of gravity. These regions are often rich in carbon monoxide and hydrogen, and scientists usually spot them with radio telescopes that detect carbon monoxide.
But this new cloud — named Eos after the Greek goddess of the dawn — contains very little carbon monoxide, which is what allowed it to elude researchers for so long. Instead, the crescent-shaped cloud was found using data from a retired instrument on a Korean satellite called the Far-Ultraviolet Imaging Spectrograph, which detected the ultraviolet light emitted by hydrogen.
© Burkhart, B., Dharmawardena, T.E., Bialy, S. et al.Top: FIMS/SPEAR FUV H2 fluorescence emission map13. Bottom: map of the ratio of H2 intensity to total FUV intensity. In both the top and bottom panels, the location of the Eos cloud is outlined and appears as a bright fluorescent feature that is not present in the FUV continuum. We determine the on-sky boundary for the Eos cloud on the basis of the magenta contours in the bottom panel, which outline the 10% ratio of H2 intensity.
Eos was found around 300 light-years away at the edge of the Local Bubble, a giant, gas-filled cavity in our galaxy that's about 1,000 light-years wide and includes our solar system. The team's findings were published in the journal Nature Astronomy on Monday.
The cloud's proximity to our planet makes this a surprising discovery, says Melissa McClure, an astrophysicist at the University of Leiden in the Netherlands who was not involved in the study, to Katie Hunt at CNN. Eos "is closer than any of the molecular clouds that we've known about previously," she explains.
"It's puzzling why there's something this big right in our solar neighborhood that we didn't see before," adds McClure. "It would be a bit like living in a suburb with above-ground houses and open lots in it and suddenly realizing that one of the open lots actually hosts a hidden underground bunker in it."
The discovery can help scientists better understand the gas and dust that make up the space between the stars in a galaxy, also known as the interstellar medium.
"We don't really know how stars and planets form," Burkhart says to Katrina Miller at the New York Times. "If we're able to look at molecular hydrogen directly, we're able to tell how the birthplaces of stars are forming — and also how they're being destroyed."
Scientists could also now use the same technique to find other previously hidden clouds within our local region of space. The Eos molecular cloud shares a name with a proposed NASA mission that many of the researchers are supporting, which would strive to detect fluorescent molecular hydrogen by picking up ultraviolet light.
"The use of the far ultraviolet fluorescence emission technique could rewrite our understanding of the interstellar medium, uncovering hidden clouds across the galaxy and even out to the farthest detectable limits of cosmic dawn," says Thavisha Dharmawardena, a NASA Hubble fellow at New York University and a shared first author of the study, in the statement.
The team recently posted a preprint paper, which has not yet been peer-reviewed, detailing a finding of the most distant molecular gas found yet, in data from the James Webb Space Telescope. So, with further research, there may be plenty more "hidden underground bunkers" to come.
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