THERE is a hole in the sun. Right in the middle, a mass the size of 1500 Earths has simply disappeared. Much of what we know about the sun’s behaviour says it should be there – but when we interpret the data encoded in sunlight, that chunk of stuff is nowhere to be seen.
That has shaken up our understanding of how the sun works, and physicists are struggling to figure out what fills that hole. It could be a thing, like dark matter. It could be a concept, with elements such as carbon and nitrogen simply behaving in a way we didn’t expect under crushing pressure. Or perhaps we’re looking at the sun in the wrong way.
Advertisement
It’s a very hot problem, says Sunny Vagnozzi, a physicist at Stockholm University in Sweden. That’s no joke. The sun is important not just because it supplies the heat and light that sustain us. It is also our key to the wider universe, the reference against which we measure stars: their brightness, their age, how likely their solar systems are to support life. Start messing with the sun, and the consequences stretch as far as our telescopes can see. “If we get the sun wrong, we get everything wrong,” says Sarbani Basu at Yale University.
It’s not easy to figure out what’s inside the sun. “We can’t go and take a sample,” says Basu. There are two main ways to investigate. Helioseismologists such as Basu look at sound vibrations on the sun’s surface, which give outward evidence of the vast quantities of energy being unleashed within. That energy depends on the sun’s internal structure, as well as its ingredients, which Basu can identify by working backwards from observations beamed from her space-based probes.
Then there are spectroscopists, who look at the light from the sun. They pass it through high-tech prisms, decomposing it into stripes that serve as unique barcodes for its constituent elements.
For years, these two methods painted the same picture of the sun: a vast and dense ball of matter, mostly hydrogen and helium, that clumped together some 4.6 billion years ago and formed our solar system. Included in the mixture was a sprinkling of other elements carried by the explosions of larger, dying stars. For simplicity, astronomers refer to all these heavier elements, which include carbon, oxygen, nitrogen, magnesium, iron and sulphur, as metals. They can be found scattered throughout the interior of the sun, making up a little less than 2 per cent of its total mass. Despite their minority status, these heavy elements play a crucial role, shuttling energy from the core out to the boiling layer on the surface.
In the late 1990s, Martin Asplund was a young researcher in Copenhagen when he first realised this picture was not quite right. He was studying the motions of the outer layers of boiling stars, a requisite step towards performing more accurate spectroscopic calculations to unlock the light’s secrets.
At the time, the mathematical imaginings of star surfaces used by spectroscopists were simplistic. In fact, they were literally one-dimensional, concerned only with the behaviour of an idealised solar surface possessing zero width. But the surface of the sun is decidedly three-dimensional. With a departmental supercomputer at his disposal, Asplund built a model that took height and width into account.
“It could have been that it made no difference at all,” says Asplund, now at the Australian National University in Canberra. Over the years, there had been many little upgrades to these solar models, and all of them had left the heavy elements relatively untouched. But Asplund’s update was different. By 2009, he had startling results: a quarter of the metals we had counted on being there could no longer be found. They had simply vanished.
Staring at the sun
His measurements flew in the face of what researchers like Basu had observed. If you assumed that Asplund’s figures held up, helioseismology could no longer explain the behaviour of the sun. The quantities of helium on the solar surface didn’t tally; the outer layer became too thin; sound travelled through it at the wrong speed. It was clear that someone somewhere was doing something wrong. “A lot of people doubted it,” says Asplund. “I was not very popular.”
The easiest conclusion was that Asplund was wrong. In the hope of performing an independent cross-check, earlier this year a team examined the contents of the solar wind – streams of particles that continuously fly off the sun. The group found nothing to indicate that any matter was missing. Instead, they found indications of a total metallicity more or less on a par with what Basu’s work predicts.
“You might naively think this solves the problem,” says Vagnozzi, who participated in this study. “But it doesn’t.” The hole is filled, but the filling makes no sense. The proportions of various elements are all wrong – different from anything anyone else has found – offering no definitive resolution. “You essentially screw up the sun,” says Vagnozzi.
Thus far, nobody has found a way to discount Asplund’s conclusions. And as his results have become widely accepted, they have had consequences well beyond the sun. As our closest and most accessible star, the sun informs our understanding of its cousins across the cosmos. Consequently, in the decade since his figures came out, Asplund’s work on the composition of the sun has rapidly become one of the most cited papers in astronomy. More and more heavy elements get blasted out into the universe as the millennia tick past, which informs us when stars were born and helps us understand their evolution. That in turn tells us how likely they are to have Earth-like planets sailing around them, and whether any of those planets could accommodate life.
As the effects rippled through astronomy, some solar physicists relished the opportunity to question long-standing beliefs. With so many simplifications and assumptions in our calculations about the sun, something else was bound to be going on. Doron Gazit, a physicist at the Hebrew University of Jerusalem in Israel, was one of those who spotted a way through the confusion: maybe Asplund’s metal tally was spot on, but the elements were not behaving as expected. The reduced quantities of matter predicted in the 2009 paper could be made to match up with the helioseismological data if they carried energy like a larger quantity of metals would.
To Gazit, the key lay in a quality known as opacity, which dictates how much energy can pass through a given material. Heavier elements, such as those that had mysteriously disappeared from the sun’s interior, are more opaque than hydrogen and helium. For the helioseismology results to hold up, the opacity of the remaining elements inside the sun had to be increased. And that meant they must absorb more photons than had previously been thought possible.
You can think of an atom as a nucleus of protons and neutrons surrounded by electrons that orbit at precisely defined energy levels. Like a vending machine that only accepts exact change, getting anything interesting to happen involves putting in exact quantities of energy. If an incoming photon has enough energy to cause an electron to jump energy levels, it is often absorbed and contributes to the atom’s opacity. Otherwise, it passes straight through. Under the extreme temperatures and pressures of the solar core, the atoms would jiggle about more than normal. This motion would make some energy levels grow further apart, while others would come closer together. That expands the range of photons that any one atom could absorb, thereby offering a mechanism to increase the opacity.
“This is unknown physics,” says Gazit. The only way to test the opacity would be to observe atoms interacting with light under temperatures and pressures similar to those in the sun. A seemingly impossible task. But not for Jim Bailey, at Sandia National Laboratories in New Mexico. At the lab’s Z Pulsed Power Facility, or Z machine for short, matter can be exposed to some of the highest temperatures and pressures on the planet for the briefest fraction of a second.
During a run of experiments whose results were published in 2015, Bailey exposed a disc of iron barely 4 millimetres across to a flash of energy that represents five times the planet’s entire output. Conditions on the disc mimicked those inside the sun, with temperatures and pressures high enough to take the iron atoms well out of their comfort zone.
Those experiments revealed that in such situations, iron does indeed have a higher opacity than thought, although not enough to explain away the hole. It’s a pioneering measurement, says Gazit. Solar physicists agree: Bailey’s opacity measurements have the potential to solve this whole problem.
Of course, the iron in these experiments represents the conditions that exist in just one spot inside the sun. It would take Bailey forever to recreate the rest of the sun’s interior piece by piece. “There are too many elements and too many different temperatures,” he says. So now it’s up to theorists to try to figure out whether the rest of the metals could have seen their opacity increase too. But whether those refined models will push the opacity in the right direction overall is hard to say.
If not, perhaps another kind of matter is picking up the slack for the missing elements. After all, spectroscopy only detects matter capable of absorbing or emitting radiation. Dark matter, of the kind that makes up about 27 per cent of our known universe, does neither. Aldo Serenelli at the Institute of Space Sciences in Barcelona, Spain, is part of a team proposing that this property makes dark matter a plausible candidate for filling the new-found hole at the sun’s centre.
Getting several billion megatonnes of dark matter to accumulate at the centre of the sun is not as difficult as you might think. Like all other forms of matter, the dark stuff should experience the pull of gravity. On our galaxy’s slow journey through space, any dark matter we bumped into would be likely to find itself drawn to the centre of the sun. Once in place, there are all sorts of tricks it could pull to explain the helioseismology results. Perhaps the mysterious stuff is its own antiparticle, releasing energy when it collides with itself. Or if not, perhaps there’s a shell of dark matter surrounding the core of the sun. It would get pretty hot and, from time to time, particles would break away, travelling to the boiling outside and transporting energy with them.
The problem with many of these scenarios is that you wind up making things worse, not better. At best, you get a partial improvement. Still, it’s hard to rule out that there’s some unicorn version of the stuff sitting in the centre of the sun, solving all our problems. “I would say it’s a remote possibility,” says Serenelli.
“Several billion megatonnes of dark matter could lie at the centre of the sun”
Part of what makes this issue so contentious is that it questions fundamental properties of the sun we thought were well-established. Which is why some are still reluctant to accept that the metals are truly missing.
The easiest way to resolve the controversy would be to produce an independent measurement of the sun’s insides – one more conclusive than earlier attempts to test the solar wind. That knockout punch could come from neutrinos, lightweight particles produced as shrapnel in the fusion reactions taking place inside the sun.
Every second, some 65 billion of those solar neutrinos are passing through any given square centimetre on Earth, travelling at nearly light speed. The vast majority are created when hydrogen nuclei collide in the outer reaches of the sun. But one in a hundred, or thereabouts, are born during heavier fusion processes involving atoms of carbon, nitrogen and oxygen. By measuring the amount of these CNO neutrinos that reach a given spot on Earth, you can work out the exact number spilling out of the sun – and from that, how many of these heavy elements are there creating them.
Such a direct probe would allow us to bypass all the theory that underpins the work of Asplund and Basu. “We would be able to really solve this,” says Michael Wurm, a neutrino hunter working on the Borexino detector underneath the mountains of central Italy.
One little problem: neither Wurm nor anybody else has ever knowingly seen a CNO neutrino. Even by comparison with other neutrinos, which are infamously hard to detect, CNO ones are discreet. So discreet, in fact, that we may have already spotted them without realising. The trouble is that they look so much like other kinds of neutrinos – the ones produced during hydrogen fusion, for example – that working out their particular, subtle signal requires the collection of an enormous quantity of neutrinos, perhaps more than Borexino is capable of collecting.
Our best hope is in a new Canadian detector called SNO+, equipped with a massive tank of fluid primed to give off pinpricks of blue light when a neutrino passes through it. Larger and deeper underground than Borexino, it could filter out more noise and hopefully spot more particles. At present, SNO+ is still filled with water, but in January the team plans to replace that with the scintillator fluid that could serve, finally, as a glimpse into the solar core.
The neutrinos could confirm that Asplund’s numbers are correct. They could reveal that he’s wrong and that the hole never existed at all. Or they could worsen the confusion surrounding our home star. The hole in the sun could be about to get a lot bigger.
This article appeared in print under the headline “What’s wrong with the sun?”
