Wednesday, March 20, 2013

Humanity has NOT gone interstellar

As I'm sure many of you saw, today the American Geophysical Union published a press release stating that, for the first time in our history as a species, humanity has left the solar system. That's right. The space probe Voyager 1, launched on September 5, 1977, has moved beyond the edge of the solar system into interstellar space. Obviously this is very exciting news an...

Wait a minute, what's this? Oh... huh. It appears that NASA's Voyager team has quickly released a press release of their own. According to the Voyager science team, Voyager 1 has NOT left the solar system or entered interstellar space. Of course, this part of the story is getting pretty widely ignored in favor of the more sensational idea that something we made has left the solar system. At this point, even the AGU has submitted a correction. Check the press release link again and you'll see
Please note that the headline on this release has been changed to better represent the findings reported in the study "

right at the top of the page.

Of course, none of this changes how far the Voyager 1 probe has travelled. It is still absolutely remarkable what the Voyager missions have done for us from an astronomical perspective, and I don't think we should ever forget that regardless of what boundaries it has or has not crossed. As of the time of this writing, Voyager 1 is 123.6886 times farther away from the Sun than Earth is, and moving at something in the neighborhood of 8 to 9 kilometers per second.

Also as of this writing, you may notice that that website actually has two different counters: one keeping track of Voyager 1's distance from the Sun, and another tracking its distance from Earth. You may also notice that while the distance from the Sun is increasing, the distance from Earth is actually decreasing. Don't worry; this isn't wrong. It just so happens that Earth is at a point in its orbit around the Sun where we happen to be moving faster than Voyager 1 in the same direction as Voyager 1 is travelling, so we're temporarily catching up with it. But I digress.

We can use this latest communication disaster as a starting point for a very interesting question though: where does our solar system end? This is actually a really tricky question, and I can't say we have a definitive answer. I sure as heck don't know, but I'll try to summarize where current thinking is on the issue.

Normally one thinks about the solar system as a collection of large bodies orbiting the Sun, in which case, you have the eight planets of the solar system and their moons. But we also have the dwarf planets and their moons (Pluto has at least 5 now). And we can't forget the asteroid belt. That's it, right?


Beyond the orbit of Neptune, we have a collection of small, icy/rocky objects that appear to resemble the hypothesized composition of Pluto. This is the Kuiper Belt. The Kuiper Belt runs from about 30 AU to 50 AU (where 1 AU is the average distance between Earth and the Sun), and is most likely made of the leftovers from the formation of the solar system. One of the telling hints is that the Kupier Belt, rather than being a spherical cloud, is more of a disc that appears to share a plane with the rest of the solar system. But at that distance from the Sun, the leftovers just weren't close enough together to attract one another through their mutual gravity, so they never formed any planets. Is the Kupier Belt the edge of the solar system?

Still no.

The way that astronomers seem to define the edge of the solar system these days is the edge of the heliosphere. The heliosphere is region surrounding the Sun that is dominated by the solar wind, the constant stream of high-energy particles emitted by the Sun. You can even see evidence for the solar wind in Earth's aurorae. The lights are caused by the charged particles (electrons and protons, mostly) interacting with Earth's magnetic field, which causes them to move toward the magnetic poles and emit radiation. If we go far enough away from the Sun, the solar wind will weaken, but it will also begin to run into gas that is not associated with the Sun at all. The key is that there is no reason to believe that this will happen at a single, easily defined, distance from the Sun. More likely, it will be a transition region.

How will we be able to tell? Well, both Voyager probes actually have detectors made for detecting these types of particles. So not only will they see a decrease in the particles coming from the Sun, but they should see an increase in cosmic rays from elsewhere in our galaxy. Interestingly enough, this is what we see from Voyager 1's detectors.

Data from Voyager 1's cosmic ray detectors showing the relative detection rates of solar wind particles (shown in blue) and galactic cosmic rays (red and black). Image credit: Webber & McDonald (2013)
Wait, isn't that exactly what I just described? Yes. But there's way more going on here. From an update released by NASA's Jet Propulsion Laboratory in December of 2012, the cosmic rays are only one indicator that the Voyager team is studying. The Sun also provides another detectable signal in the form of its magnetic field. In the region through with Voyager 1 is currently travelling, it appears that the Sun's magnetic field is currently getting stronger. While this is evidence that Voyager 1 is getting really close to the beginning of true interstellar space, it doesn't seem to be there quite yet.


  1. So, bear with me, I might have gotten out of my E&M final 10 hours ago, so all my knowledge of electromagnetism has left the building. Is there a particular reason the magnetic field is going up? The only thing I can possibly think of right now is Voyager is travelling at some angle to the plane and the B field is stronger at the poles... but I'm pretty tired.

    1. This is mostly supposition on my part, so don't quote me, but I would (at least naively) expect that the stronger magnetic fields are the result of the Sun's overall magnetic field interacting with the interstellar medium. Perhaps the interacting is causing the field to be compressed, so the "field lines" are being squeezed closer to one another near the termination of the heliosphere. This would have the effect of strengthening magnetic fields, but I don't know if that would/could also explain the changing particle fluxes as well. Plus, I also don't know if that's legit or not.

      We do see that where the Earth's magnetic field meets the solar wind, it gets compressed, so that's part of my reasoning. But you know how we astronomers are with magnetic fields...

  2. I know this is as silly as the debate over the definition of "planet", but....

    Why not count the Oort cloud as part of the solar system? Why not measure the end of the solar system as anything outside the orbit of the farthest object found so far to be orbiting the sun? Or, if theory trumps observation, why not measure the end of the solar system as outside the farthest conceivable orbit, just closer than a distance where Alpha Centauri would dominate?

    And say, where does Alpha Centauri's solar system end? I understand there is some debate about whether Proxima Centauri is really orbiting Alpha Cen A & B, but assuming it is indeed part of the Alpha Centauri system, using the heliosphere definition, the Alpha Centauri system would have two disconnected regions!

    Actually, here's a serious question which isn't just quibbling over the definitions of words:

    I wonder if there are even star systems where the components orbit such sometimes they are far enough away from each other that their heliospheres don't touch, and other times they are close enough that their heliospheres intermingle? For such star systems with, say, six or more components, the shape of the intermingling heliospheres might get pretty hairy!