'It's huge, and it's been hidden for this whole time': Gigantic, glow-in-the-dark cloud near Earth surprises astronomers

Astronomers have discovered the closest known molecular cloud to Earth, giving them a rare close-up view of the cosmic recycling of matter that fuels the creation of new stars and planets.

Named "Eos" after the Greek goddess of dawn, the newfound cloud is an enormous, crescent-shaped blob of hydrogen gas located just 300 light-years from Earth. At roughly 100 light-years wide, it spans the equivalent of about 40 Earth moons lined up side by side, making it one of the largest structures in the sky.

"It's huge, and it's been hidden for this whole time," Blakesley Burkhart, an associate professor in the Department of Physics and Astronomy at Rutgers University in New Jersey who led the discovery, told Live Science.

Despite its enormous size and relatively close proximity to Earth, Eos had thus far eluded detection due to its low content of carbon monoxide (CO) — a bright, easily detectable chemical signature astronomers typically rely on to identify molecular clouds.

Instead, the researchers detected Eos through the fluorescent glow of hydrogen molecules within it — a novel approach that could reveal many similarly hidden clouds throughout the galaxy. "There definitely are more CO-dark clouds waiting to be discovered," Burkhart said.

Molecular hydrogen is the most abundant substance in the universe. By discovering and studying hydrogen-filled clouds like Eos, astronomers could uncover previously undetected hydrogen reservoirs, thereby allowing them to more precisely gauge the amount of material available for star and planet formation across the universe.

The researchers reported the discovery in a paper published April 28 in the journal Nature Astronomy.

"This cloud is literally glowing in the dark"

Burkhart discovered Eos while analyzing 20-year-old data from a spectrograph aboard the Korean Science and Technology Satellite-1, which was launched into Earth orbit in 2003 to map the distribution of hot gas in the Milky Way.

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Similar to how a prism splits visible light, the spectrograph on board the satellite broke down far-ultraviolet light into a spectrum of wavelengths. This enabled scientists to identify emissions from different molecules. In what seemed to be an empty region of the sky, data cataloging of the hydrogen molecules revealed Eos to be "literally glowing in the dark," Burkhart said in a Rutgers statement.

"It was very serendipitous," she told Live Science. "I was looking at this data and saw this structure. I was like, 'Huh, I don't know what that is. That's unique.'"

Eos has been sculpted into its crescent shape through interactions with a nearby colossal feature in the sky — the North Polar Spur, a vast region of ionized gas that extends from the plane of the Milky Way all the way toward the northern celestial pole. Eos' shape aligns perfectly with the North Polar Spur at high latitudes, Burkhart said, indicating that the energy and radiation from this massive structure, likely driven by past supernovas or stellar winds, have interacted and influenced the surrounding gas, including Eos.

Simulations tracing Eos' evolution — particularly how its molecular hydrogen reservoir is torn apart by incoming photons and high-energy cosmic rays from the North Polar Spur and other sources — suggest it will evaporate in about 6 million years, the new study found.

A follow-up study of Eos searched for signs of recent or ongoing star formation with data from the European Space Agency's recently retired Gaia space telescope. The findings, which have yet to be peer-reviewed, suggest the cloud has not undergone any substantial bursts of star formation in the past. However, it remains uncertain whether the cloud will begin to form stars before it dissipates, Burkhart said.

Burkhart and her colleagues are developing a mission concept for a NASA spacecraft named after the newly discovered molecular cloud. This proposed Eos space telescope would observe in far-ultraviolet wavelengths to measure the molecular hydrogen content in clouds across the Milky Way, including its namesake, to conduct a census of the formation and destruction of molecular hydrogen gas.

"There's still tons of open questions," she said. "We're just getting started."