The Week in Space and Physics: Watching the Kaon Decay

On rare kaon decays, a planet for Barnard's Star, Voyager 2 and the Earth's lost ring.

Over the last thirteen years the VVVX survey has mapped our galaxy in infrared light. Shown here are some of the highlights of what it has seen. Image credit: ESO/VVVX survey.

The future truths of physics, Albert Michelson remarked in 1894, lie in ever more accurate measurement. He was wrong: the future of physics lay in the still undiscovered theories of relativity and quantum physics. Today, however, with those two theories firmly established, the statement might at last have found its place.

Take the example of the kaon, a subatomic particle made of two quarks, rather than the three found in protons or neutrons. In September, physicists announced the detection of an ultra-rare decay of this particle; one that takes place in less than one out of every billion observations. Even more accurate measurements of this process, they hope, might one day lead to the fabled truths of future physics.

Modern particle physics is based on something called the Standard Model, a description of each of the fundamental forces and particles of nature. There are problems, of course. The model has no space for gravity, speaks nothing of dark matter or dark energy, and there are some puzzles over neutrinos and antimatter. But on the whole the model works well. There has been, indeed, no experimental measurement that has ever defied its predictions.

This model says that kaon decays will very occasionally produce a set of three other particles: a pion, a neutrino, and an antineutrino. The model also says this takes place extremely rarely, mostly because it requires an unlikely group of particles to turn up at exactly the right time and place. In particular, the Standard Model predicts that only eight out of every one hundred billion kaons will decay in this way.

Why does this matter? Well physicists think this decay could be one of the best chances for spotting “new” physics at work. Should the decay happen slightly more or less often than the Standard Model predicts, then it could be a sign of unknown particles or forces getting involved. That would then allow physicists to go beyond the Standard Model, and uncover new truths about the fundamentals of nature.

Of course, studying such a rare decay is difficult. But over the past few years, researchers have been running an experiment named NA62 at the European Centre for Nuclear Research, or CERN, in Geneva. It has tracked the decays of vast numbers of kaons created inside particle colliders and measured, with extraordinary accuracy, how often they decay in various ways.

Not only did they thus manage to see kaons decaying into pions and neutrinos, but they also managed to count how often this comes to pass. It happens, they say, roughly thirteen times for every hundred billion kaons - so around sixty percent more often than the Standard Model predicts. If this discrepancy holds true, then the kaon really might point the way to new laws of physics.

Sadly, however, the evidence is still weak. The figures presented come with a large margin of error, and more observations - and so more time - will be needed to reduce it. When that happens the discrepancy could also disappear, and the results may fall back in line with the Standard Model. If so, physicists will need to turn elsewhere to find their future truths.

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Barnard’s Planet

After the Alpha Centauri system, the closest star to our sun is Barnard’s Star, a faint red dwarf lying about six light years away. It is old - the star has been burning for at least seven billion years - and alone - like the Sun it travels through space unaccompanied by any other star.

The question, for a long time, has been whether it has any planets. Indeed, plenty of planets have been found around other red dwarfs, most notably the seven orbiting TRAPPIST-1, but none were confirmed around Barnard’s Star.

Back in the 1970s, however, the Dutch astronomer Peter van de Kamp thought he’d spotted one. After looking at the way the star wobbled, he calculated it should be accompanied by a large gas giant, a bit bigger than Jupiter. Even better, another study concluded, we might be able to visit it: a spacecraft equipped with nuclear fusion engines could, it found, reach the star within half a century of flight.

Sadly the planet turned out to be imaginary. Van de Kamp had measured the wobble incorrectly, and corrected measurements showed no sign of a Jupiter-sized planet. A later claim of an Earth-sized planet, made in 2018, also turned out to be false. It was more likely, a later study found, that astronomers had instead seen some kind of activity on the star’s surface instead of a planet.

Now astronomers are once more claiming to have found a planet. As before, they are basing their claims on the wobble of the star, which seems to point to the presence of at least one small world in orbit. But unlike last time, these claims seem more certain and have been confirmed by multiple telescopes.

There is, they say, certainly one planet around Barnard’s Star. It is small and rocky, somewhere in size between Mars and Venus, and - since it orbits close to the star - extremely hot. Possibly there are also three other planets, although the evidence for them is weaker. All of them, if they exist, are both smaller than the Earth and too hot for water to exist on their surfaces.

Still, the discovery seems to settle the long speculation about planets around Barnard’s Star. And, since they are only six light years away, it is not inconceivable that we might one day send a spacecraft to check them out.

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Voyager 2: A Slow Shutdown Begins

Voyager 2 owes its remarkable longevity in part to its nuclear generators. Three of them are onboard, each fueled by two dozen plutonium spheres, and each outputting a few hundred watts of power. That means the spacecraft can keep running, even though it is, as of writing, more than twelve billion miles from the Sun.

Yet plutonium decays over time, and so, after close to half a century in space, Voyager’s generators are slowly losing their power. As they do, operators have taken steps to keep the probe running, first by shutting down non-essential systems, and then by making its onboard electronics run more efficiently. Now, however, they have been forced to shut down the first of its scientific instruments.

That is the plasma science instrument, a device which measures the flow of plasma around the spacecraft. This proved vital in detecting exactly when the spacecraft had moved beyond the Sun’s magnetic field, or heliosphere, back in 2018. Since then, however, the flow of plasma has fallen dramatically, and so the amount of useful data it has returned has dropped.

Three other science instruments remain active on Voyage . 2 But as the generators continue their decline, operators will eventually be forced to turn them off as well. Still, the probe has a few years left at least - NASA reckons it has enough plutonium left to operate its radios until the early 2030s. 

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A Ring Around the Earth

Saturn is famous for its rings, but it is not the only planet to have them. Rings stretch around both Uranus and Neptune, though they are dark and hard to see. Jupiter, too, has one - though it is faint and dusty and was only discovered when Voyager 1 flew past the planet in 1979. Now some researchers think the Earth might once have had a ring too. 

Beginning around 460 million years ago, they say, was an unusual string of meteor impacts. Over the following forty million years the number of impacts spiked, and, oddly, most of them seem to have hit our planet near the equator. Such a thing is statistically unlikely, and so probably not caused by random chance.

Instead they speculate that a large asteroid broke up around our planet and formed a temporary ring lasting millions of years. Afterwards, they say, chunks of the ring would have rained down on the planet, forming craters at an unusually fast rate and explaining the odd spike in meteor impacts.