Voyager over the “heliocliff,” but Solar System transition mysterious

NASA

Where does our Solar System end? If you define it in terms of the Sun's gravitational influence, then it's the edge of the Oort cloud, a collection icy bodies that stretches over two light years from the Sun. You could also place it at the orbit of the last dwarf planet that roams the Kuiper belt. But if you want to define it where the Sun's energy directly affects the environment, then you'd place it at the edge of the heliosheath, where the solar wind and the Sun's magnetic field fall off, and the environment is dominated by the energetic particles of the interstellar medium.

That boundary, called the heliopause, was approached by the most distant man-made object around: Voyager 1. And, in December, researchers held a press conference to announce that, rather than crossing a clean boundary, the probe had entered a region near this edge that nobody had predicted. Today, the paper describing these results was released and, thanks to some public relations confusion, many outlets reported that the probe had left the Solar System entirely.

The evidence of a boundary is very clear in the chart shown below, which maps the particles that Voyager has recorded in its environment over the last year. In blue are low-energy protons emitted by the Sun itself. These fluctuated for a few weeks before dropping off a cliff by over 99 percent. The authors of the paper have termed this transition the "heliocliff."