A mass equivalent to 1500 Earths has vanished from the sun. Tracking it down could transform how we see the stars
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.
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.