The jokes came easily for when he began studying the properties of the empty space beyond our solar system. 鈥淚 remember the look on my father鈥檚 face when I was in graduate school,鈥 he says. 鈥淚 told him I was an expert in nothing.鈥
Most of us can see why Bania鈥檚 dad hesitated over his son鈥檚 choice of specialism. The exciting bits of the universe are the shining stars, the exotic planets, the icy comets. Isn鈥檛 interstellar space just a featureless void 鈥 and so far away as聽to be tough to study anyway?
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Not so. All the atoms in the universe鈥檚 stars and planets account for only a piffling 4 per cent of its regular matter. The rest is spread out聽thinly in the gaps between the stars, in interstellar and intergalactic space. By that measure, what we normally think of as 鈥渆mpty space鈥 isn鈥檛 nothing 鈥 it is almost everything.
Over the past decade or so, researchers like Bania have been showing that interstellar space is deeply fascinating. This so-called nothingness is brimming with exotic molecules, pulsating with radio waves and聽divided into gigantic bubbles, each with their own character. Now, as we are beginning to map out our place within the void more keenly, we are coming to see that this variety matters immensely 鈥 and that as the solar system heads towards a new region of interstellar space, there could be important ramifications for life on Earth.
We are used to living in a thick soup of atmosphere. In a cubic centimetre of air, a volume the size of a six-sided dice, there are trillions of atoms. Gas, dust, water vapour, viruses, pollen and more all waft around. Just聽beyond our atmosphere, however, in interplanetary space, the conditions are close to a perfect vacuum. 鈥淪pace is huge, and it鈥檚 mostly empty,鈥 says at Wesleyan University in Connecticut. Out there, the same volume contains, on average, just five atoms.
This matter mostly consists of charged particles streaming out from the sun as the聽solar wind. We have known for decades about this flow of material and how it creates a聽protective zone around the solar system called the heliosphere. It cocoons us from high鈥慹nergy cosmic rays shooting at us from deep space聽鈥 and a good thing too, because those rays can damage the cells and DNA in聽living things. Radiation levels are eight to 10聽times higher outside this zone. 鈥淲ithout our heliosphere, would life even exist?鈥 asks at Princeton University. Much about this crucial zone remained a mystery for聽a long time, though, not least where it ends聽and where interstellar space begins.
The Voyager probes
That changed thanks to two space probes, Voyager 1 and 2. They both launched in 1977 and were sent on different trajectories, with the principal aim of exploring the outer planets of the solar system. But once they had flown past them, they kept on going towards the inky blackness of interstellar space.
In 2012, Voyager 1 recorded a huge drop in the strength of the solar wind and a simultaneous rise in the聽number of incoming cosmic rays. This occurred at 122 astronomical units (AU) from the sun (one AU is the distance between Earth and the sun, about 150 million kilometres). This, scientists later declared, was the heliopause, the edge of the heliosphere. Voyager 2 had taken a longer route heading out聽at a slightly different angle to its twin, but,聽in 2018, it also detected the heliopause at聽a聽similar distance from the sun.
At the heliopause, the interaction of the outflowing solar wind and the incoming interstellar medium results in temperatures of聽tens of thousands of degrees Celsius, causing particles to move at high velocity. But the sparseness of matter here 鈥 barely a single atom per cubic centimetre in the interstellar medium 鈥 means you would still freeze to death.
That is because in order to heat anything up, particles would have to collide with it, and聽with so few particles, that would take an聽awfully long time. The density of matter out聽here is equivalent to 鈥渁n orange inside the聽volume of Earth鈥, says Bania. 鈥淭he very best聽laboratory vacuums we can produce are聽a聽million times denser.鈥
There are still many mysteries about the heliopause, though. For instance, when researchers analysed data sent from Voyager 2 in 2019, they found it appeared to have a smoother passage through a thinner section of heliopause than Voyager 1. We don鈥檛 know why.
The shape of the heliosphere
Then there is the more fundamental question of the shape of the heliopause. Our聽solar system is moving through the surrounding interstellar medium, which pushes against the heliosphere and distorts it.聽For this reason, the leading nose of our heliosphere is widely agreed to be rounded. But聽the shape of its 鈥渢ail鈥 remains controversial. Many favour a simple teardrop form.
However, astronomer at Boston University in Massachusetts and her colleagues have been聽working on a NASA-funded project called Shield, using data from observations of the heliosphere to build computer models of it. This work led Opher to argue in 2021 that , with聽a two-pronged tail. With more probes designed to observe the heliosphere set to launch in the coming years, Opher reckons the聽question could finally be settled.
As well as finding out where interstellar space starts, we are discovering more and more聽about what it contains. We know there is聽a scattering of atoms and dust and that the density of this medium can vary considerably throughout our galaxy. 鈥淚f interstellar space were uniform and smooth, it鈥檇 be much less interesting,鈥 says at Princeton University. It was always thought, however, that complex molecules couldn鈥檛 exist in interstellar space, as they would surely be ripped apart by the barrage of powerful cosmic rays.
But in 1970, astronomers Robert Wilson and Arno Penzias 鈥 famed for their accidental discovery of the cosmic microwave background radiation, the afterglow of the big bang 鈥 in the Orion nebula, a gas鈥憆ich region of our galaxy.
The discovery kick-started a new field, the study of molecules drifting in the interstellar medium. Bania is one of those who led the charge. 鈥淭he big breakthrough in my lifetime was the whole discovery of molecules in interstellar space,鈥 he says. As of 2022, , mostly identified from the way they absorb specific wavelengths of passing radio waves. 鈥淲e found this rich molecular chemistry in space,鈥 says Bania.
Some of these molecules are complex hydrocarbons. Even simple amino acids, the building blocks of proteins, have been spotted. That led astronomers to explore whether the ingredients of life could have been delivered to聽planets like Earth across interstellar space, either floating freely or on comets or asteroids, something that remains unanswered today. 鈥淚t鈥檚 a very interesting question,鈥 says at Leiden University in the聽Netherlands. 鈥淒id life start somewhere in聽the universe and spread? Or do we just have conditions everywhere that are amenable to the formation of life?鈥
Just as interstellar space isn鈥檛 empty, it isn鈥檛 a聽still, tranquil zone either. Instead, it is like an聽ocean full of waves. We learned this, again, from the Voyager probes. In 2021, when at Cornell University in New York and her colleagues analysed data sent by Voyager 1 from beyond the heliopause, they were surprised to find . These were caused by events from the sun seeping through the heliopause and interacting with聽the interstellar medium. 鈥淚t鈥檚 really exciting,鈥 says Rankin.
Interstellar space may have waves like an聽ocean, but it isn鈥檛 an unbroken expanse. Instead, it is divided into many bubbles, each with its own character. This was originally recognised in 1992 by astronomer at the Paris Sciences et Lettres University in France.
Local Interstellar Cloud
By studying the motion of sodium gas in our corner of the galaxy, she聽found that the solar system was , now known as the Local Interstellar Cloud. She also realised we were heading out of that bubble and towards another one called the G-cloud (see 鈥淎 new bubble鈥, above). 鈥淚t was a big moment in my career,鈥 says Lallement. Within a year, by NASA鈥檚 Ulysses spacecraft would聽support the findings.
Later work by Lallement and others using the Hubble Space Telescope has helped pinpoint our position more accurately by聽measuring the motion of our sun with respect聽to our neighbouring star system Alpha聽Centauri. This showed that our solar system entered the Local Interstellar Cloud about 60,000 years ago and will pass into the聽G-cloud聽in about 2000 years. In cosmic terms, we are right on the edge.
What happens when we enter this new bubble? The good news is that the G-cloud appears to have a similar density to our Local Interstellar Cloud, meaning few changes. The bad news is that the character of the boundary between the clouds is uncertain. It isn鈥檛 clear if they are touching or if there is an intermediary region of different density between.
If we encounter a higher density region, that could push more heavily on our heliosphere, causing it to shrink and allowing harmful cosmic rays to penetrate deeper in towards the solar system鈥檚 rocky planets like Earth. That would be unwelcome.
A higher flux of cosmic rays might increase Earth鈥檚 cloud cover and cool our climate, and it could also cause more genetic mutations in cells as high-energy particles enter our bodies. 鈥淓arth would see the聽effects [of cosmic rays] much more than at聽present,鈥 says Jeffrey Linsky at the University of Boulder, Colorado. On the other hand, if we enter a region of lower density, the heliosphere could expand, increasing the volume of space that is shielded from cosmic rays and possibly boosting the habitability of areas at the distant edges of the solar system.
On a grander scale, our 10-light-year-wide Local Interstellar Cloud resides in a much larger, irregularly shaped structure called the Local Bubble, which is 1000 light years across. This is a giant shell of expanding gas formed by聽more than a dozen stars exploding as supernovae, with a density around a tenth that聽of the space outside the Local Bubble. Recent estimates have suggested that our solar system entered this bubble about 5 million years ago, and we are now roughly at its centre.
In another 8 million years, it is predicted we will reach its edge. In 2022, at the Space Telescope Science Institute in Maryland and her colleagues used the European Space Agency鈥檚 Gaia telescope to track the motions and positions of stars in our vicinity. This showed that the centre of the Local Bubble is relatively empty, but that . 鈥淲e鈥檙e accidentally in a great position,鈥 says Zucker.
Yet that could change as we near the edge. Ongoing work by Opher suggests the higher pressure we would experience as we near the edge of the Local Bubble would shrink the heliopause to the wrong side of Earth鈥檚 orbit, exposing us to far more cosmic rays. Earth would be 鈥渋n interstellar space鈥, says Opher. It聽is聽reasonably well known that the sun will get聽much hotter over the next billion years, hot聽enough to boil Earth鈥檚 oceans away. But our聽traverse of interstellar space could have its聽own serious consequences for life on Earth聽far sooner. 鈥淲here we were in the past and where we are going to be in the future is聽critical,鈥 says Opher. 鈥淚 think it will have a聽direct聽effect on habitability.鈥
We can only learn so much about interstellar聽space while studying it from within our solar system. That is why scientists are now proposing new missions that would surpass the work of the Voyager craft and be equipped with instruments specifically designed to study the great beyond.
Missions to interstellar space
In the US, a proposal called is being considered by聽NASA, with a decision as to whether it gets funding expected by the end of 2024. This 50鈥憏ear mission would launch in the 2030s and聽reach the heliopause in 15 years, before travelling perhaps 10 times further than the Voyager craft have. 鈥淭he whole idea is to get far聽beyond the heliopause into the pristine interstellar medium,鈥 says Linsky, who is part聽of the mission鈥檚 science team.
One of the key goals would be to take an image of our heliosphere 鈥 you might call it the聽ultimate selfie 鈥 and so deduce its shape once and for all. This would be done using an instrument designed to detect atoms from our sun hitting the interstellar medium, revealing whether it is a croissant shape like Opher expects or something else.
Another proposed mission, from China, called Interstellar Express and comprising two聽craft intended to launch later this decade, would carry similar instruments, providing views from different positions outside the solar system, giving us a better understanding of the heliosphere鈥檚 shape. These missions could also directly study remnants of the supernovae that fashioned our Local Bubble, perhaps telling us when and where some of these explosions took place. 鈥淲e鈥檙e beginning to piece that together,鈥 says Jesse Miller at the University of Illinois Urbana-Champaign.
The interstellar medium is much more than聽just the gap between the stars. It is the ocean in which our sun and all the other stars聽in our galaxy sail. More than ever, we know where we are in this ocean. 鈥淚t鈥檚 kind of extraordinary,鈥 says Rankin. 鈥淲e鈥檙e starting to聽look back on ourselves from the outside now聽for the first time.鈥
