The universe hosts a vast number of exoplanets, many of which defy our terrestrial understanding of weather and climate. Astronomers observe worlds where precipitation can take forms ranging from molten metal to solid rock. The study of these exotic atmospheres has revealed environments so extreme that abundant elements are transformed into bizarre meteorological phenomena. This exploration has led to the hypothesis that on one distant world, the clouds and rain are composed of a material prized on Earth: gemstones.
Identifying the Gemstone World
The exoplanet where this gemstone precipitation is modeled to occur is HAT-P-7b, a colossal gas giant located over 1,000 light-years away. It is classified as an ultra-hot Jupiter, a massive world orbiting extremely close to its parent star. This tight orbit results in the planet being tidally locked, meaning one side perpetually faces its star in a scorching day, while the other side remains in endless night. HAT-P-7b is approximately 40 percent larger than Jupiter and is subjected to immense stellar radiation, creating the conditions for its unusual atmospheric chemistry. Scientists determined that its atmosphere contains clouds made of the mineral corundum.
The Atmospheric Chemistry of Corundum Rain
The atmospheric process on HAT-P-7b begins with the extreme heat of the day side, where temperatures can soar above 2,500 degrees Celsius (4,500 degrees Fahrenheit). At these temperatures, rock-forming elements like aluminum and oxygen are vaporized into a gaseous state. Corundum is aluminum oxide (Al2O3), and its components are abundant in the planet’s superheated atmosphere. Powerful equatorial winds transport these metal vapors from the hot day side to the cooler night side. As the gases reach the night side, the temperature drops just enough for them to condense into solid crystalline particles of corundum, forming high-altitude clouds that precipitate downward as gem-like rain.
Comparing Space Gems to Earth’s Jewels
The potential for corundum precipitation on HAT-P-7b raises the question of whether these are truly rubies and sapphires like those found in jewelry. Rubies and sapphires are both varieties of corundum, distinguished only by the trace elements that lend them color. For example, a ruby’s deep red hue comes from trace amounts of chromium, while sapphires display various colors, typically blue, due to impurities like iron and titanium. While the ultra-hot Jupiter provides the base chemical compound (Al2O3), the rapid condensation and violent weather are unlikely to produce large, clear gemstones. Instead of the perfect crystals prized on Earth, the precipitation would likely consist of micro-sized crystalline grains or amorphous solids. The speed and intensity of the atmospheric cycle prevent the slow, high-pressure formation process required for gem-quality stones.
Precipitation on Other Extreme Worlds
The existence of corundum rain on HAT-P-7b is part of a larger pattern of exotic precipitation observed across the cosmos. Extreme atmospheric conditions on other exoplanets and within our own solar system produce non-water-based rainfall.
Diamond Rain
On the ice giants Neptune and Uranus, scientists model the existence of “diamond rain.” This phenomenon occurs deep within the planets’ interiors, where immense pressure and high temperatures break down methane molecules. The resulting carbon atoms are compressed into solid diamond structures that slowly sink through the fluid layers toward the planetary core.
Glass Rain
Another example of unusual precipitation is observed on the hot Jupiter HD 189733b. Silicate particles condense in the atmosphere, resulting in rain composed of tiny shards of glass propelled sideways by winds that can reach speeds of up to 8,700 kilometers per hour.