A US team discovered the barrier, some 7,200 miles above the Earth's surface, that blocks high energy electrons threatening astronauts and satellites.
Scientists identified an "extremely sharp" boundary within the Van Allen radiation belts, two large doughnut-shaped rings held in place by the Earth's magnetic field that are filled with fast-moving particles.
Lead researcher Professor Daniel Baker, from the University of Colorado at Boulder, said: "It's almost like these electrons are running into a glass wall in space.
"Somewhat like the shields created by force fields on Star Trek that were used to repel alien weapons, we are seeing an invisible shield blocking these electrons. It's an extremely puzzling phenomenon."
The team originally thought the highly charged electrons, which loop around the Earth at more than 100,000 miles per second, would slowly drift downward into the upper atmosphere. But a pair of probes launched in 2012 to investigate the Van Allen belts showed that the electrons are stopped in their tracks before they get that far.
The nature of the force field remains an unsolved mystery. It does not appear to be linked to magnetic field lines or human-generated radio signals, and scientists are not convinced that a cloud of cold electrically charged gas called the plasmasphere that stretches thousands of miles into the outer Van Allen belt can fully explain the phenomenon either.
The research is reported in the journal Nature.