We can’t see the heliosphere. We know where it starts but not really where it ends. And we are pretty certain that most stars, and therefore most planetary systems, are bounded by heliospheres, or “astropheres,” as well.
It has a measurable physical presence, but it is always changing. And although it is hardly well known, it plays a substantial role in the dynamics of our solar system and our lives.
As it is studied further and deeper, it has become apparent that the heliosphere might be important — maybe even essential – for the existence of life on Earth and anywhere else it may exist. Often likened to an enormous bubble or cocoon, it is the protected space in which our solar system and more exists.
Despite the fact that it is the largest physical system in the entire solar system, the heliosphere was only discovered at the dawn of the space age in the late 1950’s, when it was theorized by University of Chicago physicist Eugene Parker as being the result of what he termed the solar wind.
It took another decade for satellite measurements to confirm its existence and to determine some of its properties — that it is made up of an endless supply of charged particles that are shot off the sun — too hot to form into atoms. Together these particles, which are superimposed with the interplanetary magnetic field, constitute the ingredients of he heliosphere.
Just as the Earth’s magnetic fields protect us from some of the effects of the Sun’s hazardous emanations, the heliosphere protects everything inside its bubble from many, though not all, of the incoming and more hazardous high-energy cosmic rays headed our way.
As measurable proof that the heliosphere does offer significant protection, when the Voyager 1 spacecraft left the heliosphere in 2012 and entered the intersellar medium, instruments onboard detected a tripling of amount of cosmic radiation suddenly hitting the spacecraft.
With the discovery of thousands of exoplanets beyond our solar system in the past two decades and as the search for potentially habitable planets has picked up to great speed, interest in what might be variations among atmospheres around other distant stars has increased as well. Solar physics strongly suggests that most stars have solar winds and resulting heliospheres as well, so they have become part of the equation in thinking about what would make a habitable planet.
Until less than a decade ago, the consensus view was that the heliosphere is shaped like a gigantic comet. It has a nose pushing into the interstellar medium, then comes the whole solar system and more with the sun in the middle, and then a long tail following along. This visualization of the heliosphere made sense because the Sun is orbiting the Milky Way and as a result that solar motion would send much of the material in the solar wind into a following tail.
For astrobiology, the properties and shapes of the heliosphere, and parallel astropheres around other stars, are important. As heliophysicist Merav Opher of Boston University explain it, “If we want to find an Earth-like analog… we need to understand the properties of the astrosphere. We have to have an understanding of the shielding properties.”
That conclusion led Opher to study the characteristics of the heliosphere, which in turn led to the detection of some anomalies that didn’t fit in the traditional model.
In 2015, Opher and her team came up with a theory that the heliosphere is not shaped like a comet, as it is usually shown in graphics, but rather like a croissant. This comes primarily from theory and modeling but Opher believes it can be demonstrated to be true through observations as well.
Her logic is that the comet model — where a vast tail of solar wind material follows the nose of the heliosphere and the sun and planets as the Sun travels through the interstellar medium — misses an essential component to the dimensions of the heliosphere. That missing piece is the interplanetary magnetic field of the sun that is present throughout the heliosphere, but is far less powerful than the hypersonic blast of solar wind particles coming off the corona.
“These magnetic fields were considered insignificant because they are so small, so weak in relation to the thermal component of the wind,” said Opher. “But our calculations showed that the magnetic components make the heliosphere into what we describe as a croissant — a crescent with columnated tubes on each end.”
While initially critiqued as implausible, the hypothesis of the croissant-shaped heliosphere has caught on with some others in the field. After all, the father of heliophysics who first theorized the existence of a solar wind, Eugene N. Parker of the University of Chicago, did produce back in the 1970s two possible models of the shape of the heliosphere — one comet-shaped and another rather like a croissant with two distinct columns of material flowing off from the nose back into the tail.
As a further sign that Opher’s theories — which go well beyond the crescent-shaped heliosphere — are entering the mainstream, NASA recently awarded her SHIELD laboratory (Solar wind with Hydrogen Ion Exchange and Large-scale Dynamics) at BU a five-year, $1 million per year grant to study the heliosphere from their pioneering perspective.
Opher’s team, which includes astrophysicists from 12 universities, was one of the nine centers initially chosen for NASA-funding and that group was just culled down to three — Boston University, Stanford University and Johns Hopkins University.
NASA calls the centers innovation hubs for solar and space sciences, which have become increasingly important as well as potential radiation hazards to astronauts, satellites and power grids need to be identified and mitigated. “These high-performing teams address cutting-edge science questions supporting NASA’s mission, and advancing solar and geospace science,” said Nicola Fox, heliophysics division director,in a release.
And that includes Opher’s new approach to the basic shape of the heliosphere.
Reflecting the growing interest in the subject and field, Opher and her colleagues are hardly alone in coming up with novel conclusion to the basic shape of heliosphere. Data from NASA’s Cassini space mission, for instance, supported the theory that the heliosphere is indeed round.
As explained by Arik Posner, a program scientist in the NASA Heliophysics Division, the increase in heliophysics research came after years when frustrated sun scientists — who study the heliosphere, space weather, the structure of the Sun, solar and planetary magnetic fields and more — had their research proposals rejected at a very high rate. NASA now has 20 solar missions flying and 14 more in the works, and Posner said that “lots of great science has been left on the table” because of the limited research money for more long-lived missions.
In its most recent decadal survey for heliophysics, the National Academy of Sciences made additional funding for research a top priority, and in recent years Congress and then NASA have responded. Posner said that while only 15 percent of research proposals were being funded before 2017, that number is now up to 30 percent.
The border of the heliosphere — rather like the heliosphere itself — does not get much public attention. One of the few times that it did is when the Voyager 1 spacecraft, and then Voyager 2, approached the region where a much weakened solar wind of the heliosphere hit the opposing force of the interstellar medium, the heliopause or boundary. Voyager 1 and 2 passed into “outer space” in 2012 and 2018 respectively.
Remarkably, the two spacecraft, launched in 1977, continue their travels into deep space. While the pull of the Sun’s gravity remains, the material influence of the Sun is greatly reduced and below that of the interstellar medium, with its thin broth of dust and gas from other dead and living stars and its own interstellar magnetic field. Voyager 1 is 14.5 billion miles from the Sun and Voyager 2 is 12 billion miles. Though both were launched as planetary missions, they have both been part of the NASA heliophysics program for some time.
it is actually occupied by a thin broth of dust and gas from other stars—living stars, dead stars, and stars not yet born. Averaged across the whole galaxy, every sugar-cube-sized volume of space holds just a single atom, and the area around our solar system is even less dense.
Posner was one of the NASA scientists following the Voyager approaches to the border with the interstellar medium, and he said the spacecraft sent back data that resolved some questions about the heliosphere but left many unresolved. And one of the main unresolved questions is that of the shape of the xxx.
“This is an issue that is controversially discussed in all meetings,” he said. “When Gene Parker first theorized the existence of a heliosphere, he proposed two shapes — one with the comet tail dominated by effects of the interstellar medium and other with outflowing jets arranged more by magnetic fields.”
Versions of those models are what are now being debated but Posner expects no resolution anytime soon. The computing power needed to settle the issue remains well beyond reach and no NASA mission to test the theories has been approved.
So is the heliosphere comet-shaped or like a croissant? Posner said that “the truth may be somewhere in between. Personally, I’m split in my own views.”
One crucial conclusion in the heliophysics field that is not at all controversial is that solar winds and consequently astropheres — solar atmospheres similar to the heliosphere — can be found around most stars. As Gene Parker theorized fifty years ago, the extraordinary heat of stars will almost always require a physical release of particles, magnetic fields and heat — stellar, as opposed to solar, winds.
Because of the boundary characteristics of an astrophere, they will keep out many of the high-energy cosmic rays coming from distant supernovae, supermassive black holes and other cosmic phenomena that are so destructive of DNA and life.
So from an astrobiology perspective, the fundamental nature of astropheres around most stars is a positive for the potential emergence of life.