Pressure at the Edge of the Solar System Is Far Higher Than Expected, Scientists Discover

Scientists have found that the pressure at the edge of the solar system is greater than expected after analyzing data collected by NASA's Voyager spacecraft—the only objects to have traveled to interstellar space, where the sun's magnetic field no longer has influence over the environment.

The edge of the solar system is a poorly understood region of space given its distance from Earth and the difficulty of conducting observations there. But a study published in The Astrophysical Journal is helping to cast new light on the dynamics of this mysterious zone.

In our solar system, the solar wind—a constant flow of charged particles streaming out of the sun—inflates a bubble in the surrounding interstellar material. This bubble is known as the "heliosphere" and it represents the extent of our star's magnetic influence.

"Amongst many things, it acts as a magnetic barrier which protects us from the radiation beyond, known as 'cosmic rays,'" Jamie Rankin, lead author of the research from Princeton University, told Newsweek.

"The heliosphere consists of four main regions: 1) the supersonic solar wind, which extends out to roughly 8.5 billion miles away, 2) the termination shock, a transition region where the solar wind abruptly slows, 3) the heliosheath, a hot region where the subsonic wind turbulently mixes with interstellar particles, and 4) the heliopause, where the solar wind plasma and magnetic field finally meets the interstellar wind," Rankin said.

In Earth's atmosphere, air pressure is generated by gas molecules being dragged down towards the center of the planet by gravity. In the near-vacuum of space, however, pressure is is created by the interactions and collisions between magnetic fields, plasma—gas which has had its electrons removed—and various particles.

"Unlike air pressure on earth, where gas particles collide like bouncing billiard balls, the pressure in the solar wind comes as a result of its highly energized, electromagnetic behavior," Rankin said.

"This pressure is produced by the sum of many different outward-flowing, energizing effects, including the plasma and magnetic fields that make up the solar wind, and particles that get energized throughout the solar system."

In the heliosheath, the pressure of particles from other stars and interstellar space starts to become dominant. But this region is incredibly far away so it is difficult to study using Earth-based observatories. Fortunately, scientists were able to conduct observations with the Voyager spacecraft—which were launched in 1977 and have been travelling away from our planet ever since.

While Voyager 1 and 2 are both now located beyond the solar system in interstellar space, at the time of the observations, the latter was still in the heliosheath, while the former was just within reach of it.

However, the measurements of pressure in the region were only made possible as a result of a fortuitously timed stellar event known as a "global merged interaction region." This phenomenon occurs when the sun shoots out huge bursts of particles into space, which then merge together, creating a wave of plasma that travels through the solar system.

One of these waves reached the heliosheath in 2012—just as Voyager 2 happened to be in the region. Observations from the spacecraft showed that the wave led to a decrease in galactic cosmic rays—a process that was also picked up by Voyager 1 four months later, just beyond the edge of the solar system. The positions of the two spacecraft coupled with the solar event enabled the scientists to calculate the pressure in this region for the first time.

"There was really unique timing for this event because we saw it right after Voyager 1 crossed into the local interstellar space," Rankin said in a statement. "And while this is the first event that Voyager saw, there are more in the data that we can continue to look at to see how things in the heliosheath and interstellar space are changing over time.

"In adding up the pieces known from previous studies, we found our new [pressure] value is still larger than what's been measured so far. It says that there are some other parts to the pressure that aren't being considered right now that could contribute."

The scientists say that studying the the pressure at the edge of the solar system can help us to better understand the influence of the sun on interstellar space, casting light on the processes at work in out own planetary system, as well as others.

Illustration depicting the layers of the heliosphere. NASA/IBEX/Adler Planetarium