Data from ESA’s Cheops exoplanet mission led to the surprising discovery that 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 discovered .
Besides the Moon, the brightest object in our night sky is the planet Venus, whose thick cloud cover reflects about 75% of sunlight. By comparison, Earth reflects only about 30% of incoming sunlight.
Now, for the first time, astronomers have discovered an exoplanet that can match the brightness of Venus: the planet LTT9779 b. New detailed measurements from ESA’s Cheops mission reveal that this planet reflects as much as 80% of the light from its host star.
Cheops’ high-precision measurements were a purposeful extension of planet iinitial discovery and characterization in 2020 by NASA TPP mission and ground instruments such as ESO HARPS instrument in Chile.
The exoplanet is about the size of Neptune, making it the largest “mirror” in the universe we know today. The reason for its high reflectivity is that it is covered by metallic clouds. They are made mostly of silicate – the same stuff sand and glass are made of – mixed with metals like titanium.
“Imagine a burning world, close to its star, with heavy clouds of metals drifting up, raining down titanium droplets,” says James Jenkins, an astronomer at Diego Portales University and CATA (Santiago, Chile). James co-authored a scientific paper describing the new research published today in the journal Astronomy & Astrophysics.
A sky full of clouds of metal
The fraction of light that an object reflects is called its “albedo.” Most planets have a low albedo either because they have an atmosphere that absorbs a lot of light or because their surface is dark or rough. Exceptions are usually frozen ice worlds or planets like Venus that have a reflective cloud layer.
The high albedo of LTT9779 b came as a surprise, as the side of the planet facing its star is estimated to be about 2000 °C. Any temperature above 100 °C is too hot for water clouds to form, but the temperature of this planet’s atmosphere must even be too hot for clouds made of metal or glass.
“It was really a puzzle until we realized that we had to think of this cloud formation in the same way as condensation in the bathroom after a hot shower,” notes Vivienne Parmentier, a researcher at Cote d’Azur Observatory (France) and co-author of this study. Vivienne explains: “To steam a bath, you can either cool the air until the water vapor condenses, or you can keep the hot water flowing until clouds form because the air is so saturated with steam that it just can’t hold any more. Similarly, LTT9779 b can form metal clouds even though it is so hot because the atmosphere is saturated with silicate and metal vapors.
The planet that shouldn’t exist
Being brilliant isn’t the only surprising thing about LTT9779 b. Its size and temperature make it a so-called “ultra-hot Neptune,” but no other planets of this size and mass have been found to orbit so close to their star. This means it lives in what is known as “Neptune’s hot desert”.
The planet has a radius 4.7 times that of Earth, and one year on LTT9779 b takes only 19 hours. All planets discovered so far 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 smaller than two Earth radii .
“This is a planet that shouldn’t exist,” says Vivienne. “We expect the atmospheres of planets like this to be blown away from their star, leaving behind bare rock.”
First author Sergio Heuer of Astrophysics Laboratory in Marseille comments: “We believe these metal clouds help the planet survive in the hot desert of Neptune. Clouds reflect light and stop the planet from getting too hot and evaporating. Meanwhile, being heavily metalized makes the planet and its atmosphere heavy and harder to blow away.”
The brightest known exoplanet: LTT9779 b
Studying an exoplanet by looking when it is hidden
To determine the properties of LTT9779 b, ESA’s exoplanet characterizing mission Cheops looked at when the planet moves behind its host star. Because the planet reflects light, the star and planet together send more light to the space telescope just before the planet goes out of view than right after. The difference in visible light received just before and after the planet is occulted tells you how much light the planet reflects.
This project relies on Cheops’ precision and 24/7 coverage. “Precisely measuring the small change in signal from the star eclipsing the planet was only possible with Cheops,” says Sergio.
ESA’s Cheops Project Scientist Maximilian Gunther added: “Cheops is the first space mission dedicated to tracking and characterizing already known exoplanets. Unlike large research missions focused on discovering new exoplanetary systems, Cheops has enough flexibility to quickly focus on interesting targets and can reach coverage and precision that we often simply can’t achieve otherwise.
By looking at the same exoplanet with different instruments, we get the full picture. “The LTT9779 b is an ideal tracking target with the outstanding capabilities of both The Hubble and James Webb Space Telescopes,” notes Emily Rickman, ESA Science Operations Scientist. “They will allow us to probe this exoplanet with a wider range of wavelengths, including infrared and ultraviolet light, to better understand the composition of its atmosphere.”
The future of exoplanet research is bright as Cheops is the first of a trio of dedicated exoplanet missions. It will be joined by Plato in 2026, which will focus on Earth-like planets orbiting at a potentially life-supporting distance from their star. Ariel is due to join the fleet in 2029 and will specialize in studying exoplanet atmospheres.