ESA locates the largest known “mirror” in the Universe

An ultra-hot exoplanet that orbits its host star in less than a day is covered in reflective clouds of metal, making it the brightest exoplanet ever found.

Other than the Moon, the brightest object in our night sky is the planet Venus, whose thick cloud cover reflects about 75% of the Sun’s light. By comparison, Earth only reflects about 30% of incoming sunlight. . Now, for the first time, astronomers have found an exoplanet that can match the brightness of Venus: the planet LTT9779 b. New detailed measurements by ESA’s Cheops mission reveal that this planet reflects a whopping 80% of the light thrown at it by its host star.

Cheops’ high-precision measurements were a specific follow-up to the initial discovery and characterization of the planet in 2020 by NASA’s TESS mission and ground-based instruments such as ESO’s HARPS instrument in Chile. The exoplanet is about the size of Neptune, making it the largest “mirror” of the Universe we know of today. The reason for its high reflectivity is that it is covered by metallic clouds. They are mostly made of silicate, the same material that sand and glass are made of, mixed with metals like titanium.

“Imagine a world on fire, close to its star, with heavy metal clouds floating overhead, raining droplets of titanium,” says James Jenkins, an astronomer at Universidad Diego Portales and CATA (Santiago, Chile). James is co-author of a scientific paper describing the new research, published in the journal Astronomy & Astrophysics. The fraction of light that an object reflects is called its ‘albedo’. Most planets have a low albedo, either because they have a light-absorbing atmosphere or because their surfaces are dark or rough. The exceptions tend to be icy worlds or planets like Venus that have reflective cloud cover.

The high albedo of LTT9779 b was a surprise because the side of the planet facing its star is estimated to be around 2,000 °C. Anything above 100°C is too hot for water clouds to form, but the temperature of this planet’s atmosphere should be even too hot for clouds made of metal or glass. “It was really a puzzle, until we realized that we should think of this cloud formation in the same way as the condensation that forms in a bathroom after a hot shower,” says Vivien Parmentier, a researcher at the Côte d’Observatoire. ‘Azur (France) and co-author of this research. Vivien explains: “To fog up a bathroom, you can cool the air until the water vapor condenses, or you can let the hot water run until clouds form because the air is so saturated with steam that it simply can’t hold any more. De Similarly, LTT9779 b can form metallic clouds despite being so hot because the atmosphere is supersaturated with silicate and metallic vapors.”

Being brilliant isn’t the only amazing thing about LTT9779 b. Its size and temperature make it the so-called ‘ultrahot Neptune’, but no other planets of this size and mass have been found orbiting so close to its star. This means that it lives in what is known as the “hot desert of Neptune”. The planet has a radius 4.7 times that of Earth, and a year at LTT9779 b takes just 19 hours. All previously discovered planets that orbit their star in less than a day are either “hot Jupiters” (gas giants with a radius at least ten times that of Earth) or rocky planets of less than two Earth radii.