Every second of every day, our sun spits out a stream of tiny high-energy particles, known as the solar wind. This wind blows throughout the solar system, extending far beyond the orbits of the planets and out into interstellar space.
But the farther from the sun the wind gets, the more slowly it streams, changing from the raging torrent that the inner planets experience (strong enough to cause the aurora) into nothing more than an annoying drizzle. And far enough away – about twice the orbit of Neptune – it meets and mingles with all the random bits of energetic junk just floating around amongst the stars.
This boundary forms a bubble, where the solar wind gives way to the interstellar medium, and is known as the heliosphere.
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If life were completely simple and boring, the heliosphere would be…a sphere. The solar wind would slow down at the same radius all around the system, totally equally.
But life is neither simple nor boring.
For a long time researchers thought that the heliosphere might form a comet-like teardrop shape. Our system is moving through the galaxy, and so our heliosphere should be a bit blunter on one side and a bit long and tapered on the other.
Naturally, it’s a bit more complicated than that.
Researchers across the world have been using a slew of new data from the outer solar system to put together the puzzle of our heliosphere. First we have the twin Voyager probes, which are currently pushing past the heliosphere and into interstellar space. We also have New Horizons, which will soon follow them into the void. And lastly we have the late Cassini mission, which collected a wealth of outer-system information in its years orbiting Saturn.
Recently, a group of researchers led by Merav Opher at Boston University put together a model of our heliosphere, incorporating all the known data.
And the result? A weird-looking, lumpy, puffy croissant.
The odd shape comes from two sources of high-energy particles in the outer solar system. The first is the solar wind itself, generated near the surface of the sun and sent blasting. The second is a population of neutral particles that slip and sneak their way into the solar system, only for one of their electrons to get ripped off, turning them into their own version of the solar wind.
The interaction between these two groups is, needless to say, complex, and their electromagnetic dance weaves together the surprising shape.
That shape matters, because the sun isn’t the only source of high-energy particles in the cosmos. Sources across the universe spew out radiation, and the solar wind does a great job at deflecting a good fraction of it, preventing it from harming our fragile DNA. But the details of the shape can tell us how well the heliosphere works as a force field – and how life around other planets might fare.