Neutral hydrogen gives solar system's heliosphere its croissant-shape - study

Created by the Sun, the heliosphere is a protective shield that encompasses the entire solar system in a bubble of the ionized gas magnetic field.

 Artist's interpretation depicting the heliosphere from 2011 in light of data from the Voyager spacecraft. (photo credit: Wikimedia Commons)
Artist's interpretation depicting the heliosphere from 2011 in light of data from the Voyager spacecraft.
(photo credit: Wikimedia Commons)

Earth is protected from explosions of radiation by a protective shield shaped like a delicious breakfast treat, and it gets this distinct shape from neutral hydrogen particles coming in from outside our solar system, a new study suggests.

The shield in question is called the heliosphere. Created by the Sun, this protective shield encompasses the entire solar system in a bubble of ionized gas magnetic field filling in the space between the galaxy's solar systems, protecting us from violent solar flares and it is shaped like a deflated croissant.

This was discovered in a peer-reviewed academic study in 2020 led by Merav Opher, an Israeli-born astronomy professor at Boston University, who also leads a NASA DRIVE (Diversity, Realize, Integrate, Venture, Educate) Science Center that investigates and develops predictive models of the heliosphere called SHIELD (Solar-wind with Hydrogen Ion Exchange and Large-scale Dynamics). 

In her team's latest study, published in the peer-reviewed academic periodical The Astrophysical Journal, it was found that hydrogen particles from outside our solar system play some sort of role in shaping the heliosphere.

The study itself sought to answer why the heliospheric jets become unstable.

 Artist's depiction of the heliospheric current sheet results from the influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium (solar wind). (credit: Wikimedia Commons) Artist's depiction of the heliospheric current sheet results from the influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium (solar wind). (credit: Wikimedia Commons)

“Why do stars and black holes—and our own sun—eject unstable jets?” Opher said in a statement. “We see these jets projecting as irregular columns, and [astrophysicists] have been wondering for years why these shapes present instabilities.”

Most other heliosphere models suggest it has a comet-like shape with a heliospheric jet at the end as a "tail" of sorts. 

Why does Opher's model rather resemble a certain French pastry?

The reason for this is neutral hydrogen particles - with neutral indicating an equal amount of positive and negative charges, thus leaving it neutral, with no net charge whatsoever. 

In a computational model of the "ingredients" of the pastry that is the heliosphere, Opher explained that she “took one ingredient out of the cake—the neutrals—and noticed that the jets coming from the sun, shaping the heliosphere, become super stable. When I put them back in, things start bending, the center axis starts wiggling, and that means that something inside the heliospheric jets is becoming very unstable.”

This, in turn, would be the reason for the croissant shape. Instability should, in theory, cause the Sun's jets and solar winds to change. As a result, the heliosphere itself would split its shape.

This is all still theoretical, as direct observation of the heliosphere itself isn't possible. But this idea, of neutral gases colliding with the heliosphere, does have precedence.

When materials of different densities collide, when lighter materials push against heavier materials, irregular shapes are often formed. This is what is known as the Rayleigh-Taylor instability, and it is very common. It can be seen on Earth, when oil is suspended above the water, for example. On a cosmic level, an example can be seen in the "fingers" of the Horsehead Nebula. 

What happens in this process is that gravity moves the different materials around as the two densities collide. The drag between neutral hydrogen and charged ions in the heliosphere has a similar effect.

“This finding is a really major breakthrough, it’s really set us in a direction of discovering why our model gets its distinct croissant-shaped heliosphere and why other models don’t,” Opher explained.

Why does this matter? 

The heliosphere itself is important as it acts as a shield for the solar system. It is an essential component in protecting planets – and, by extension, life – from damaging cosmic rays.

The shape of the solar system's heliosphere is what makes this protection so effective, and could potentially be one of the deciding factors on whether life can thrive. As such, finding similarly shaped heliospheres could be the key to finding new habitable worlds.

Celia Jean contributed to this report.