On This Exoplanet, It Rains Precious Gems

A study published this week indicates that an exoplanet (a planet outside our solar system) some 855 light-years away has a metallic atmosphere and liquid, gem-like rain. Researchers believe that vast temperature differences cause this phenomenal weather. One side of the exoplanet is cold enough for iron and corundum — a mineral common to rubies and sapphires — to form clouds. As we’ll see, the ruby-sapphire mineral then rains in a weather cycle similar to, but much more violent than Earth’s.

Never a calm day on WASP-121b

Exoplanet WASP-121b is a gas giant that astronomers discovered in 2016. It is larger in diameter and mass, and hotter than any planets within our solar system. It’s akin to a giant, ultra-hot Jupiter. The Hubble Space Telescope has provided what we know about its weird atmosphere and weather. The study, published in Nature Astronomy journal, follows a recent examination of the exoplanet’s dark side.

Much like Earth’s moon, WASP-121b is tidally locked, meaning that one side permanently faces the star around which it orbits. The exoplanet’s star-facing side thus experiences constant daylight, while the space-facing side remains in continual darkness.

Study coauthor Tansu Daylan of MIT explained, “Hot Jupiters are famous for having very bright day sides, but the night side is a different beast. WASP-121b’s night side is about ten times fainter than its day side.”

The two-faced conditions on the gas giant come from its tidal lock, while its nearness to its sun whips up bizarre weather. Here’s how experts think it works.

Rendering of WASP-121b's phases. Image: NASA, JPL-Caltech, Aix-Marseille University (AMU), Vivien Parmentier

Rendering of WASP-121b’s phases. Image: NASA, JPL-Caltech, Aix-Marseille University (AMU), Vivien Parmentier


Heat, wind, and gemstone rain

First, let’s note the two key influences on WASP-121b’s atmosphere: temperature and wind speed.

  • Temperatures – “Extreme” seems like an understatement about the planet’s temps: Its solar side is a more-than-balmy 2,227°C at its deepest layer and 3,227°C at its uppermost. On the dark side, its deepest layer is its warmest, at a still-scorching 1,527°C, while the upper layer cools off — if one can call it that — to around 1,227°C.
  • Wind speed – Estimates suggest blustery winds of up to 17,703kph.

“These winds are much faster than our jet stream and can probably move clouds across the entire planet in about 20 hours,” said Daylan.

That wind hellaciously and turbulently influences WASP-121b’s cloud/rain cycle.

The immense heat of WASP-121b’s sunny side means that water molecules that reach it do not evaporate, condense, and fall in the form of rain as on Earth. Instead, the heat violently breaks those molecules into atoms. The wind then violently sweeps those atoms back to the night side, where they once again merge into water.

Like the water particles, the iron and corundum clouds likely blow to the planet’s sunny side every 20 to 30 hours. As they approach, the clouds get hotter until they finally rain liquid gems. Eventually, the remaining metal compounds evaporate into gases. Then the cycle repeats.

The top of WASP-121b's atmosphere is heated to a blazing 2,500 degrees Celsius, hot enough to boil some metals. Image: NASA, ESA, and G. Bacon (STSci)

Rendering of WASP-121b and its F-type star, which lie 855 light-years from Earth. Image: NASA, ESA, and G. Bacon (STSci)


Future research

“With this observation, we’re really getting a global view of an exoplanet’s meteorology,” said lead author Thomas Mikal-Evans of the Max Planck Institute for Astronomy. “We’re now moving beyond taking isolated snapshots of specific regions of exoplanet atmospheres to study them as the 3D systems they truly are.”

Later this year, astronomers will deploy the new James Webb Space Telescope to further investigate WASP-121b.