New Horizons may have seen a glow at the solar system’s edge

The ultraviolet signal may mark a wall of hydrogen where the sun’s influence wanes.

HELLO FROM THE OTHER SIDE The sun’s journey through the galaxy may build a wall of hydrogen near the edge of the solar system (curved line to the left of this illustration). The New Horizons spacecraft may have seen evidence of just such a wall.
HELLO FROM THE OTHER SIDE The sun’s journey through the galaxy may build a wall of hydrogen near the edge of the solar system (curved line to the left of this illustration). The New Horizons spacecraft may have seen evidence of just such a wall.

New Horizons, an interplanetary space probe has recently captured an ultraviolet glow that seems to emanate from near the edge of the solar system. Astronomers predicted that the glow might come from a long-sought wall of hydrogen that represents where the sun’s influence wanes.

“We’re seeing the threshold between being in the solar neighborhood and being in the galaxy,” says team member Leslie Young of the Southwest Research Institute, based in Boulder, Colo.

Even before New Horizons flew past Pluto in 2015 (SN: 8/8/15, p. 6), the spacecraft was scanning the sky with its ultraviolet telescope to look for signs of the hydrogen wall. As the sun moves through the galaxy, it produces a constant stream of charged particles called the solar wind, which inflates a bubble around the solar system called the heliosphere.

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. That build-up of hydrogen, or wall, should scatter ultraviolet light in a distinctive way.

New Horizons is the first spacecraft in a position to double-check the Voyagers’ observations. It scanned the ultraviolet sky seven times from 2007 to 2017, space scientist Randy Gladstone of the Southwest Research Institute in San Antonio and colleagues report.

As the spacecraft traveled, it saw the ultraviolet light change in a way that backings the decades-old perceptions. Each of the three rocket saw more ultraviolet light more remote from the sun than anticipated if there is no wall. Yet, the group alerts that the light could likewise be from an obscure source more remote away in the cosmic system.

Leslie Young of the Southwest Research Institute, based in Boulder, Colo said, “We’re seeing the threshold between being in the solar neighborhood and being in the galaxy.”

Space scientist David McComas of Princeton University said, “It is really exciting if these data are able to distinguish the hydrogen wall. That could help figure out the shape and variability of the solar system’s boundary.”

After New Horizons flies past the outer solar system object Ultima Thule on New Year’s Day2019 (SN Online: 3/14/18), the spacecraft will continue to look for the wall about twice each year until the mission’s end, hopefully, 10 to 15 years from now, Gladstone says.

If the ultraviolet light drops off at some point, then New Horizons may have left the wall in its rearview mirror. But if the light never fades, then its source could be farther ahead — coming from somewhere deeper in space, says team member Wayne Pryor of Central Arizona College in Coolidge.