The Solar System's boundary is a hard-to-define location-after the end of the solar wind's influence, there's still the theorised Oort cloud, an icy sphere of comets orbiting the Sun a third of the way to our nearest neighbouring star.
'This distant source could be the signature of a "wall" of hydrogen, formed near where the interstellar wind encounters the solar wind or could be more distant.
"We assume there's something extra out there, some extra source of brightness", study author Randy Gladstone from the Southwest Research Institute told Gizmodo.
Hydrogen atoms which make up the barrier produce ultraviolet light which was picked up by the New Horizons probe, which beamed back incredible images of Pluto in 2015.
What New Horizons definitely sees, the researchers reported in a paper published August 7 in the journal Geophysical Research Letters, is some extra ultraviolet light - the kind the researchers would expect such a wall of galactic hydrogen to produce.
Just beyond the edge of that bubble, around 100 times farther from the sun than the Earth, uncharged hydrogen atoms in interstellar space should slow when they collide with solar wind particles. Along the way, it's been regularly scanning the sky with its Alice ultraviolet spectrograph.
New Horizons, an interplanetary space probe has recently captured an ultraviolet glow that seems to emanate from near the edge of the solar system. "If New Horizons will give us the opportunity, we will try to portray it".
In total, Alice performed seven scans between 2007 and 2017 looking for UV signals and found enough to confirm the measurements made three decades before by the two Voyager spacecraft. They said that New Horizons could actually have detected ultraviolet rays and not a hydrogen wall, as the scientists believe.
New Horizons will continue on, first past 2014 MU69 and perhaps past other Kuiper Belt Objects, provided it gets NASA's approval. Once this task is behind it, the mission will carry on surveilling the UV emissions at the edge of the solar system for the next 10 to 15 years, and hopefully will figure out exactly what causes it.
"If the ultraviolet light drops off at some point, then New Horizons may have left the wall in its rearview mirror", the researchers explained in an accompanying statement. However, if it fails to dissipate, it's possible the signature is coming from a source even farther away.