NASA’s New Horizons mission has delivered a fresh view of our galactic neighbourhood by mapping the sky in Lyman-alpha light, a specific ultraviolet wavelength linked to hydrogen. The result is the most detailed chart of its kind, offering new clues about the interstellar medium and challenging some long-held assumptions along the way. The findings, led by a team at the Southwest Research Institute (SwRI) and published in The Astronomical Journal, were made using data gathered well beyond the main clutter of the solar system. New Horizons, which famously flew past Pluto in 2015, has since ventured far enough to take in a clearer picture of the Lyman-alpha background.
A Glow We Didn’t Expect
Lyman-alpha light is scattered by hydrogen atoms, making it a useful way to probe the gas and dust between stars. The Alice instrument aboard New Horizons conducted wide sky scans throughout 2023, eventually covering over 80% of the sky. What scientists found was surprising: the background brightness of Lyman-alpha light is around 10 times stronger than expected. “It’s likely the result of an interstellar greenhouse effect,” said Michael Shull of CU Boulder, a co-author on the study. Hydrogen atoms appear to scatter the light repeatedly, causing it to bounce through space like a cosmic game of pinball.
Debunking the Hydrogen Wall
One of the most unexpected outcomes was what the data didn’t show. Scientists had speculated for years about a possible ‘hydrogen wall’ forming at the edge of the heliosphere, the vast bubble dominated by the solar wind. It was thought that this wall, made of interstellar hydrogen, might contribute significantly to the Lyman-alpha background. But New Horizons found no sign of it. According to the team, any such wall plays little or no role in shaping what the spacecraft observed. That’s a major revision to earlier models of how the solar system interacts with its surroundings.
Glimpses into the Galactic Bubble
Instead, the emissions seem to be tied more closely to local structures in the interstellar medium, including the so-called ‘Local Hot Bubble’, a cavity in space likely carved out by ancient supernovae. “It’s amazing to think these features were shaped by exploding stars just a few million years ago,” said Shull.
Lead author Randy Gladstone added that understanding the Lyman-alpha background is key to building a better picture of how our solar system fits into the broader galactic environment. The data also lays groundwork for future studies of how light, gas, and matter move across interstellar space.
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