Fingernail-size discs that levitate in sunlight could someday carry sensors through some of the thinnest and coldest reaches of the atmosphere. By flying higher than commercial aircraft or weather balloons can, such swarms could reveal new insights about Earth’s changing weather and climate patterns.

The levitating devices harness a phenomenon called photophoresis. It was first discovered more than 150 years ago when chemist William Crookes invented the radiometer, a device with black and white vanes that spin when exposed to sunlight. This happens because the vanes absorb the light and give off heat, and this heat boosts the momentum of gas molecules around them. Because the black sides of the vanes are hotter than the white ones, they transfer more momentum to the gas, making the air flow in one direction with enough force to turn the vanes.

“We’ve taken this obscure piece of physics and applied it to something that could actually impact a lot of people – and help us better understand how things like weather and climate are evolving over time,” says at Harvard University.

To develop the levitating discs, Schafer and his colleagues created a 1-centimetre-wide device made of two aluminium oxide sheets full of micro-scale holes. When exposed to light, the bottom sheet – which included alternating layers of chromium with the aluminium oxide – heated up more than the top sheet, like the black sides of a radiometer’s vanes. This also created a directional airflow, but moving upward instead of sideways.

Under white LEDs and laser light – set to intensities equivalent to about 50 per cent of natural sunlight – this lifting force levitated the device. That is an improvement on other solar-powered fliers, which require light intensities several times brighter than sunlight. But the demonstration also took place in laboratory conditions with air pressure several thousand times weaker than that at Earth’s surface.

Fortunately, those low air pressure conditions are common elsewhere – like the mesosphere, an upper layer of the atmosphere that extends 50 to 85 kilometres above the planet. The researchers say scaling up their discs to 3 centimetres would let them carry 10 milligrams of payload at an altitude of 75 kilometres, bringing sensors to a region so difficult to study it has been nicknamed the “ignorosphere”. Schafer co-founded the startup Rarefied Technologies to commercialise swarms of such high-flying devices for atmospheric monitoring and telecommunications.

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After the sun sets, computer modelling suggests the discs could stay airborne by harnessing heat radiating from Earth’s surface. “If you can stay aloft at night, that’s a big change from just settling or falling,” says at the University of Pennsylvania, whose lab is doing similar research.

Aeroplanes that fly at higher altitudes can create longer-lasting vapour trails that are likely to cause more global warming. Since private jets and modern fuel-efficient jets fly higher than other passenger jets, these aircraft may be causing even more warming than previously thought.

The findings could help airlines work out which routes to fly to minimise contrails, says at Imperial College London. “If we could predict the contrail-forming regions of the atmosphere well enough, you could route aircraft around them, which would reduce this effect.”

In some conditions, the soot particles emitted by jet engines can seed the formation of ice particles in the wake of aircraft, forming clouds known as contrails that have an overall warming effect. As much as half of the warming effect of aviation is estimated to be due to contrails, rather than the carbon dioxide emissions.

How long contrails persist largely determines how much warming they cause, but their persistence is difficult to study. Gryspeerdt’s team has combined flight data and satellite observations to match specific aircraft to contrails, and see how the type of aircraft relates to persistence.

This has only been performed on a small scale before because it was done manually. But by using artificial intelligence, the team could analyse 64,000 flights. This revealed that private jets and more fuel-efficient jets, which typically cruise at around 12 kilometres (38,000 feet), a kilometre higher than other planes, are more likely to generate longer-lasting contrails. “It was not what we expected,” says Gryspeerdt.

Not all the soot particles emitted by an aircraft turn into ice particles, he says. What the team thinks is happening is that when an aircraft flies higher, a higher proportion of soot particles seed ice particles, but the overall size of ice particles is smaller.

Smaller ice particles fall more slowly, so take longer to fall to regions where the air is relatively warmer and where they sublimate back into water vapour. This means contrails persist for longer and cause more warming.

However, because the properties of these higher-altitude contrails are a bit different, the team cannot say exactly how much warming they cause. So it is not clear if the additional warming caused by longer-lasting contrails outweighs the avoided warming due to the lower fuel use of modern planes.

What is clear is that the impact of private jets is being underestimated. “They are having an even more outsized impact on the climate per passenger than we thought,” says Gryspeerdt.

Because contrails are more visible over oceans, and because the team only had data from a single geostationary satellite, they also looked only at flights over the western Atlantic, around Bermuda.

The findings may not apply to flights further north, for instance over Greenland and Iceland, says Gryspeerdt, because the air at high altitudes is drier and contrails are less likely to form.

“The study highlights the significant non-CO2 climate impact of aircraft operating at high altitudes, primarily due to the persistent contrails they produce,” says Krisztina Hencz at Transport & Environment, an environmental advocacy organisation in Europe.

High altitudes are mainly used by long-haul flights, Hencz says, but long-haul flights have been excluded from a European Union scheme that aims to reduce the non-CO2 warming. It also shows the importance of switching to fuels that generate fewer soot particles, she says.