It Rains Gemstones on this Sweltering Exoplanet

You might think our local gas giants are extreme — Jupiter with its gaggle of moons, and Saturn with the iconic rings, but they’re nothing compared to WASP-121b. This exoplanet caught the attention of astronomers a few years back when they realized its atmosphere was hot enough to bleed metal. That’s on the day side. On the cooler night side, it rains gemstones because that’s the kind of weather you get on a truly extreme world like WASP-121b.

This is what’s known as a hot Jupiter because it’s a gas giant… and it’s extremely hot. WASP-121b was the first exoplanet to have its atmosphere characterized by the Hubble Space Telescope. This solar system is about 900 light years away, a distance that would make most exoplanets difficult or impossible to scan directly. WASP-121b, however, is 20 percent larger than Jupiter, and it orbits so close to its star that it gives off a lot of energy — it’s 10 times hotter than any previously discovered exoplanet.

The day side of WASP-121b reaches temperatures of 4,600 degrees Fahrenheit (2,500 Celsius), causing heavy metals to boil off in the upper layers of the atmosphere. It’s tidally locked, so the same side is always facing the star. New analysis of WASP-121b shows that the night side is a mere 2,200 degrees Fahrenheit, positively frosty compared to the day side. The team calculated wind speeds on the planet at more than 11,000 miles per hour (17,702 kilometers per hour).

This is the first time we’ve been able to gather data from a super-hot exoplanet like this, and astronomers have made some guesses about how the temperature gradient between the halves works. The model suggests that the night side of WASP-121b has clouds with metallic contents including iron, titanium, and corundum. The latter is the primary constituent of rubies and sapphires. The coloration is dependent on trace metals like titanium (sapphire) and chromium (ruby).

So what happens when the gemstone clouds get blown across the terminator from night to day? The hostile, super-heated environment most likely causes the minerals to precipitate out like rain. But instead of water, this is a rain of gemstones. Yes, molten gemstones, but everything on WASP-121b is molten. It’s a fascinating world, and as such, this planet will be one of the first observed by the James Webb Space Telescope. The recently launched spacecraft has a larger mirror and better infrared sensitivity, which is ideal for analyzing the composition of exoplanet atmospheres. NASA is still calibrating Webb, but the first observation should begin this summer.