Physicists say they are closer than ever to understanding a vast mysterious ribbon of particles at the edge of the Solar System.
Nasa's Interstellar Boundary Explorer (IBEX) produced the first-ever all-sky map of the edge of the Solar System back in 2009.
The effort mapped interactions between charged particles from the Sun and outer space at the solar system's boundary, also known as the heliosphere.
Above: the ribbon at the edge of our Solar System
The interactions are normally invisible to our eyes, but Nasa's IBEX instrument can discern patterns made by the particles which stream in from the galaxy.
What it found in 2009 was a strange ribbon "dancing across the boundary", Nasa said, formed by many more neutral atoms streaming in from outside than in the surrounding areas.
Many theories have been developed, but none have been proven to explain why this ribbon exists.
But now, in a paper published in the Astrophysical Journal, researchers have proposed a theory that it says is similar to boats being "trapped" in a harbour.
It builds on an idea proposed in 2009 that the ribbon exists in a place where neutral hydrogen atoms from the solar wind cross the galactic magnetic field. The idea is that they have electrons stripped away and become charged, just as they cross the magnetic region - forcing them to gyrate around the field lines. When they're fired back to the Sun, they pick up electrons and form the ribbon of neutral atoms.
The new theory adds a process in which the rotation of charged ions creates waves in the magnetic field, which traps them in the ribbon.
"Think of the ribbon as a harbor and the solar wind particles it contains as boats," says Nathan Schwadron, the first author on the paper and scientist at The University of New Hampshire, Durham.
"The boats can be trapped in the harbor if the ocean waves outside it are powerful enough. This is the nature of the new ribbon model. The ribbon is a region where particles, originally from the solar wind, become trapped or retained due to intense waves and vibrations in the magnetic field."
If proven correct, the theory could tell us more about how the galactic magnetic field works and how our Solar System interacts with it.