Precipitation on Earth can be a mild inconvenience, but it’s part of the ecosystem that makes our planet habitable. Some much less pleasant substances fall from the sky on other planets and moons. For example, it rains sulfuric acid on Venus and liquid methane on Titan. NASA has just detected a planet with an entirely new type of weather. On Kepler-13Ab, it snows metal.
NASA made this discovery with the aging Hubble Space Telescope–it’s more than a quarter-century old, but it’s still the most capable telescope we have. Kepler-13Ab is a colossal exoplanet with a mass of about six Jupiters located 1,730 light years away. It’s also a member of a common class of gas giants known as a “hot Jupiter.” That means it orbits very close to its host star.
Kepler-13Ab might be big, but even Hubble can’t image an exoplanet directly. It can, however, gather a lot of data about a planet’s atmosphere under certain circumstances. In the case of Kepler-13Ab, its orbit passes in front of and behind its host star from our perspective (that’s how the planet was originally discovered by the Kepler probe). The team was able to perform spectroscopic analysis in near-infrared as the planet passed behind the star, which is known as a secondary eclipse.
Hubble revealed several details about Kepler-13Ab’s composition during this observation. First, Kepler-13Ab is tidally locked to its host star–it’s always day on one side and always night on the other. This isn’t unusual in the universe. For instance, Earth rotates, but the moon is tidally locked to Earth. The temperature of Kepler-13Ab’s atmosphere also gets colder at increasing altitude, which is the opposite of what should happen on a hot Jupiter. NASA says this unusual property is a consequence of how titanium oxide circulates in the clouds of Kepler-13Ab.
Normally, titanium oxide (which is the active ingredient in sunscreen) in the clouds of a hot Jupiter absorbs heat and radiates it out into space. That makes the upper atmosphere warmer. The titanium oxide precipitates out of the clouds as solid “snow” on the night side of the planet. Astronomers believe this process happens in all hot Jupiters, but a combination of being tidally locked and extremely massive prevents the titanium oxide on Kepler-13Ab from mixing back into the dayside atmosphere. Thus, the heat-radiating titanium oxide is continuously removed from planet’s atmosphere, leading to a so-called “cold trap.”
NASA expects to have even better tools to study exoplanets when the Webb Space Telescope is online in several years. Data gathered from planets like Kepler-13Ab are instrumental in refining the techniques we will one day use on smaller Earth-like planets.
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