Where does the Solar System really end?

 We could say that this is where the Sun's gravity ceases to be strong enough to prevent objects from escaping.  In this case, the boundary would be the edge of the Oort Cloud, a loosely connected sphere, extending nearly 3 light-years from the Sun, that contains chunks of rock and ice left over from the formation of the Solar System.  We could alternatively say that the boundary is where the energy particles from the Sun (the solar wind) stop flowing away from us, blocked by the pressure of the gas and dust that lies between the stars, in what is called the interstellar medium.

 Today we're going to focus on the second option: the heliopause, the boundary where the solar wind meets the interstellar medium.  The heliopause marks the edge of the heliosphere, the region around the Sun hit by the solar wind.  Both the solar wind and the interstellar medium are made of plasma, a gas of electrically charged particles that can be considered the fourth state of matter.  In a plasma, some of the electrons have been ripped from the atoms so that charged particles (ions) circulate.  The heliosphere could be studied through direct measurements with Voyager missions, launched in the 1970s.

 After traveling far beyond the planets of the Solar System, the two Voyager missions first arrived in the region where the solar wind speed is subsonic, known as "termination shock".  The next big milestone of this trip was the heliopause.  This is the region where the Solar System interacts with the Galaxy we inhabit, so it is important and crucial to study it.  We can learn a lot about how the forces and magnetic fields in the plasma of the interstellar medium confine and influence the solar wind.  The heliopause is important even for us to understand how life can occur in solar systems, it is what protects us from dangerous cosmic rays and other high energy radiation that can be disastrous for life.

 Voyager 1 first crossed the heliopause in 2012, 122 astronomical units (AU) away, meaning that Voyager 1 was 123 times farther from the Sun than Earth.  Voyager 2 finally reached this milestone in late 2018, at 119 AU, passing through a slightly different flow of solar wind than Voyager 1, as shown in Figure 2. Although Voyager 1 gave us the first information about the heliopause , he passed through a strange place, where the solar wind seemed to flow more slowly and in ways we didn't expect.  The probe furthermore could not make all the planned measurements as its instrument for analyzing plasmas was not working.  So, since 2012, astronomers have been eagerly waiting for Voyager 2 to reach the heliopause so that new measurements could be made allowing us to understand the heliopause from another perspective, including the velocity and direction of plasma flow, temperature and density in that region.

 After all, what did Voyager 2 see there?  As it approached the heliopause, the probe entered a “boundary layer” – a region where the density and magnetic field increase as the solar wind encounters the interstellar medium.  Voyager 1 also traveled through this layer and observed something strange: the solar wind flow stagnated, showing slower speeds than expected.  Voyager 2 saw very different solar wind speeds in this border region.  Although we're not sure why these two observations are so different, the authors think this might be due to instabilities in the boundary layer.  The heliosphere is not a perfect bubble, but its edges can have irregular swirls and spots, resulting in different velocity measurements in different regions of the sphere.  It took the probe eight days to cross this border region, but the heliopause itself is such a defined region that it took just one day to cross it.

 After the heliopause, Voyager 2 reached the “very local interstellar medium”.  The interstellar medium next to us isn't perfectly smooth either.  Voyager 2 observed variations in the velocity, flow direction, density and temperature of the plasma in this region and found that the farther away from the heliopause, the denser the interstellar medium becomes.  This makes sense, because this region is cooler than near the heliopause, where the gas is heated by the collision between the solar wind and the interstellar medium.  In fact, temperatures at the edge of the heliosphere are even higher than expected, suggesting that the gas is heated beyond our predictions.  Voyager 2 also passed through an interesting region where the plasma current increased (illustrated in the data in Figure 3);  the authors think this is the effect of a shock – a sudden change in pressure and density.

 Having direct measurements of all that plasma and matter beyond the heliopause is important, because it's literally the stuff that's in between ALL the stars!  Most of the Universe (apart from dark matter, of course) is made of plasma, and after more than 30 years of traveling and waiting, we now have a chance to observe this plasma directly.

You Might Also Like :