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 might potentially 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
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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 finding 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 dose 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 bends 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. Though, aligned crystals leave spectral and polarization signatures that the telescope can measure, allowing scientists to infer the geometry indirectly.
“Just like on Earth where the atmospheric conditions need to be a certain way for them to be horizontally oriented to produce a sun dog,” Mullens said. “It’s really data-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.
