Quartz clouds may bend light on distant exoplanets, like ice crystals do on Earth

Anyone who has trudged across a snowy field on a sunlit January afternoon has probably looked up to see a crystal-rimmed halo or the two jewel-like “sun dogs” that hover left and right of the Sun.

Those ethereal spectacles form when minute ice plates and columns aloft all tip at nearly the same angle, bending sunlight into glowing rings and patches.

Earth’s atmosphere, it turns out, is not the only stage on which light and aligned crystals can dance.

A team at Cornell University argues that a kindred bit of optical wizardry may be playing out 1,300 light-years away on WASP-17 b – a giant exoplanet so hot its clouds are built from molten rock.

Vaporized rock becomes crystals

WASP-17 b belongs to the “hot Jupiter” class of worlds. These bloated gas giants orbit scorchingly close to their host stars. One loop takes just four Earth days, and daytime temperatures soar past 2,000 °F.

Under such conditions, common silicates – minerals we normally see as sand or quartz – can vaporize near the planet’s equator. They rise on towering plumes and condense into microscopic crystals higher in the atmosphere.

In 2023, Professor Nikole Lewis and doctoral student Elijah Mullens used the James Webb Space Telescope (JWST) to detect the unmistakable spectral fingerprints of quartz nanograins wafting in those clouds. Each crystal is only about ten nanometers long, yet elongated.

For Lewis, the discovery rang an intellectual bell from Cornell’s past. In 1952, campus astrophysicist Tommy Gold suggested that gas streaming across asymmetrical dust grains could physically torque them into alignment. He compared it to how river currents swivel tiny boats so that their bows face downstream.

Though later work showed the idea does not explain dust alignment in the ultra-thin interstellar medium, Lewis saw potential elsewhere. He realized it might excel inside the deep, roaring atmosphere of a hot Jupiter. There, winds blow at nearly 10,000 mph.

Quartz bend light far away

If the quartz grains do point the same way, they should refract and polarize light from stars in distinctive ways – exactly as terrestrial ice plates produce halos, sun dogs, and vertical pillars called crown flashes.

Mullens and Lewis show that aligned silicates on WASP-17 b could produce unusual light patterns in reflected starlight. These might appear as luminous off-center spots, pastel arcs, or rainbowed pillars when seen in reflected optical light.

JWST observes primarily in infrared wavelengths and cannot produce direct images of such distant features. However, aligned crystals leave spectral and polarization signatures that the telescope can measure, allowing scientists to infer the geometry indirectly.

“Just like the alignment of ice crystals in Earth’s atmosphere produces observable phenomena, we can observe the alignment of silicate crystals in hot Jupiter exoplanets,” Mullens said.

Detecting those telltale alignments would not merely produce pretty pictures in the mind’s eye. On Earth, the exact kind of halo you see signals specific combinations of temperature, humidity, and crystal habit.

By analogy, the pattern of quartz “sun dogs” could reveal wind shear, electric fields, or magnetic influences in a distant planet’s atmosphere. This provides vital data for improving models of exoplanet weather and energy balance.

A new life for a classic theory

Gold’s 1952 mechanical-alignment hypothesis has long lain dormant. It was overshadowed by other mechanisms such as radiative torques, where uneven stellar heating spins dust grains, and magnetic alignment.

The Cornell study revives Gold’s idea in a fresh context. Hot Jupiter combine enormous atmospheric density with hypersonic wind speeds – conditions tailor-made for the drag forces Gold envisioned.

“Now we see that the 1952 proposal doesn’t work for the interstellar medium, but it probably works for a hot Jupiter exoplanet, a very hot planetary atmosphere with high-speed winds,” Lewis said.

“When we started looking at planetary atmospheres, in particular these hot Jupiters, it occurred to me that with 10,000-mile-per-hour winds zipping around in these very dense atmospheres, surely the grains would align.”

The finding also broadens our understanding of cloud chemistry beyond water. WASP-17 b shows quartz can form high-altitude haze, unlike Earth’s ice clouds or iron and corundum on brown dwarfs.

That powerful blend of mineralogy and dynamics hints that many other hot Jupiters – and perhaps smaller, rocky exoplanets orbiting red dwarf stars – might host their own glittering atmospheric optics.

Quartz light shows hold clues

Each optical effect is, in essence, a diagnostic of crystal size, shape, and orientation. If quartz grains align horizontally with gale-force winds, that tells researchers something about atmospheric drag and turbulence. If they tilt vertically, electric or magnetic fields may dominate.

Should grains remain random, other mechanisms – perhaps collisions or chemical processes – might rule. Mullens noted that the alignment of grains could even feed back into the thermal structure.

It may change how effectively clouds reflect starlight or trap infrared heat, thereby modulating the planet’s climate.

“Other than being pretty, these effects can teach us about how crystals are interacting in the atmosphere,” Mullens said. “It’s really information-rich, just as on Earth where the atmospheric conditions need to be a certain way for them to be horizontally oriented to produce a sun dog.”

An upcoming look at alien clouds

Mullens is the principal investigator on a new JWST program slated for the coming cycle that will scrutinize WASP-17 b’s spectrum across an even broader wavelength range.

By teasing out subtle polarization angles and searching for additional mineral fingerprints, the team hopes to confirm whether horizontal alignment indeed dominates – or whether odd vertical or spiral patterns emerge.

Each dataset reveals how extreme winds, mineral clouds, and stellar radiation shape alien skies unlike those on Earth. The method could apply to exoplanets with high-altitude rain or snow of minerals like titanium oxide, iron, or sulfides.

Somewhere out there, in the glow of alien sunsets, quartz halos or ruby-red pillars may arch across the horizon, silent testimonies to the physics of light and crystal first glimpsed here on Earth.

The study is published in The Astrophysical Journal Letters.

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