Has Voyager 1 left the Solar System at last? The data have been ambiguous to say the least, with the number of announcements on the topic rivaling Spinal Tap drummers. The problem is that the region of transition between the Solar System and interstellar space didn't behave exactly as predicted. Some measurements showed the expected behavior if the probe had departed the Solar System, while others were ambiguous, and some were consistent with Voyager still being stuck within its boundaries.

However, one measurement was still missing: the density of plasma, which is much higher beyond the Solar System than inside it. That situation changed when D. A. Gurnett, W. S. Kurth, L. F. Burlaga, and N. F. Ness analyzed data collected from Voyager's instruments in April and May of 2013. They found abrupt changes in plasma density consistent with interaction between material streaming out from the Sun and matter in interstellar space, followed by a drop. Nevertheless, the magnetic field data are still inconsistent with the simplest model of the Solar System's boundary. So if Voyager 1 has indeed left the building, the shape of the door by which it exited isn't quite what we expected.

The Solar System is dominated by the Sun, both gravitationally and electromagnetically. Electrically charged particles from the Sun, known as the solar wind, stream outward and mix with material in space beyond, called the interstellar medium. The solar wind moves faster than the speed of sound in the interstellar medium, and the boundary of the Solar System is marked by a shock wave called the termination shock (the same phemonenon that produces sonic booms around fast aircraft in Earth's atmosphere).

The solar wind and the interstellar medium are both plasma—mixtures of free electrons and positively charged ions. However, the solar plasma is hotter and less dense than its interstellar counterpart, so the transitional region beyond the termination shock is marked by an increase in density and drop in temperature. That transition is known as the heliopause, and its outer edge is one of the main boundaries defining the edge of the Solar System. (The Oort Cloud, a collection of icy bodies loosely bound to the Sun via gravity, provides another boundary lying far beyond the heliopause; Voyager 1 will not reach it for a long time.)

Voyager 1, launched in 1977, reached the termination shock in 2004, when it was roughly 94 astronomical units (AU) from the Sun. (Earth is 1 AU from the Sun on average, while Neptune is about 30 AU.) Since 2012, Voyager scientists tallied a variety of measurements indicating the probe may have crossed the edge of the heliopause, including cosmic ray tallies and solar wind fluxes.

However, some problems remained. Voyager's plasma instruments didn't measure the increase in plasma density that must exist if it had entered the interstellar medium, and the magnetic fields didn't curve back as they should according to theory. The interstellar medium measurement in many ways was the larger problem, since many independent observations have determined its temperature and density. By contrast, the magnetic field anomaly could potentially be explained by a new model in which the Sun's field interacts with the galaxy's magnetic field in a novel way.

The plasma situation changed radically beginning on April 9, 2013, when Voyager's Plasma Wave Sensor picked up oscillations in the electron density. The rate of oscillation is correlated with the plasma density; prior to April, there had been no measurable oscillations. However, between April and May, the researchers found a gradual increase in the plasma density until it reached about 80,000 electrons per cubic meter—a number very close to what is measured in the ISM. (The theoretical density within the heliopause is around 2,000 electrons per cubic meter.)

So, has Voyager 1 left the Solar System? The only anomalous measurement remaining is the magnetic field direction, which doesn't correspond to the standard theories of the heliopause. With the new plasma density numbers, the case for Voyager's departure is stronger than before, leaving the magnetic problem in the hands of the theoreticians.

Science, 2013. DOI: 10.1126/science.1241681 (About DOIs).