Data from ESA’s exoplanet project Cheops has revealed the stunning discovery that an ultra-hot exoplanet that rounds its host star in less than a day is covered with reflecting metal clouds, making it the brightest exoplanet yet discovered.

Key takeaways

  • ESA’s Cheops mission discovered LTT9779 b, an ultra-hot exoplanet that reflects 80% of its star’s light, making it the brightest exoplanet yet.
  • The planet’s high reflectivity is due to metallic clouds made of silicates and metals like titanium.
  • LTT9779 b is an ‘ultra-hot Neptune’, a type of planet rarely found so close to its star, surviving in the ‘hot Neptune desert’ where others lose their atmospheres.
  • Cheops measured the planet’s brightness by observing the light change as the planet passed behind its star, showcasing Cheops’ precision.
  • LTT9779 b is a prime target for further study with the Hubble and James Webb telescopes, with more dedicated exoplanet missions, Plato and Ariel, set to launch in the coming years.

Aside from the Moon, Venus is the brightest object in our night sky, with a thick cloud cover reflecting almost 75% of the Sun’s light. In comparison, Earth only reflects around 30% of incoming sunlight.

For the first time, astronomers have discovered an exoplanet that is as bright as Venus: planet LTT9779 b. New thorough measurements by ESA’s Cheops mission indicate that this planet reflects 80% of the light emitted by its home star.

Cheops’ high-precision observations were a planned follow-up to the planet’s original discovery and description in 2020 by NASA’s TESS mission and ground-based telescopes such as the ESO HARPS instrument in Chile.

The exoplanet is around the size of Neptune, making it the biggest “mirror” in the Universe that we currently know of. The fact that it is coated in metallic clouds accounts for its great reflectivity. These are mainly formed of silicate, which is the same material as sand and glass, combined with metals such as titanium.

“Imagine a burning world, close to its star, with heavy clouds of metals floating aloft, raining down titanium droplets,” says James Jenkins, astronomer at Diego Portales University and CATA (Santiago, Chile). James co-authored a scholarly paper outlining the new discovery, which was published in the journal Astronomy & Astrophysics today.

Cheops shows scorching exoplanet acts like a mirror

A sky full of metallic clouds

The ‘albedo’ of an item refers to the proportion of light it reflects. Most planets have a low albedo, either due to an absorbent atmosphere or because their surface is dark or rough. Exceptions are cold ice worlds and planets with reflecting cloud layers, such as Venus.

The high albedo of LTT9779 b was surprising given that the planet’s side facing its star is expected to be roughly 2000 °C. Any temperature beyond 100 °C is too high for clouds of water to develop, but the temperature in this planet’s atmosphere should be too hot for clouds of metal or glass.

“It was really a puzzle, until we realised we should think about this cloud formation in the same way as condensation forming in a bathroom after a hot shower,” adds Vivien Parmentier, researcher at the Observatory of Côte d’Azur (France) and co-author of this study. Vivien explains: “To steam up a bathroom, you may either chill the air until water vapour condenses, or you can keep the hot water running until clouds develop because the air is saturated with vapour and cannot contain any more. Similarly, LTT9779 b can generate metallic clouds despite its high temperature because the environment is oversaturated with silicate and metal vapours.

The planet that should not exist

The fact that LTT9779 b is shiny isn’t the only surprise. Its size and temperature make it a ‘ultra-hot Neptune’, yet no other planets of this size or mass have been discovered to orbit so close to their star. This suggests it lives in the ‘hot Neptune desert’.

LTT9779 b has a radius 4.7 times that of Earth, and a year there lasts only 19 hours. All previously found planets that circle their star in less than a day are either ‘hot Jupiters’, which are gas giants with a radius at least 10 times that of Earth, or rocky planets with radii smaller than two Earths.

“It’s a planet that shouldn’t exist,” Vivien claims. “We expect planets like this to have their atmosphere blown away by their star, leaving behind bare rock.”

First author Sergio Hoyer of the Marseille Astrophysics Laboratory said, “We believe these metal clouds enable the planet survive in the scorching Neptune desert. The clouds reflect light, preventing the planet from becoming too hot and evaporating. Meanwhile, being extremely metallic makes the planet and its atmosphere heavier and more difficult to push away.”

Observing an exoplanet when it is concealed

To ascertain LTT9779 b’s characteristics, ESA’s exoplanet-characterising Cheops mission observed the planet as it traveled behind its home star. Because the planet reflects light, the star and planet together send more light to the space telescope just before the planet disappears than after. The difference in visible light received just before and after the planet is veiled indicates how much light the planet reflects.

Cheops’ accuracy and 24/7 coverage were critical to the success of this operation. “Precisely measuring the tiny change in signal from the star eclipsing the planet was only possible with Cheops,” Sergio explains.

ESA’s Cheops project scientist Maximilian Günther adds: “Cheops is the first space mission dedicated to the follow-up and characterization of previously identified exoplanets. Unlike massive survey missions aimed at identifying new planetary systems, Cheops has the ability to quickly focus on intriguing objects and achieve coverage and precision that we frequently cannot obtain in any other manner.”

We can gain a complete image of an exoplanet by using multiple equipment. “LTT9779 b is an ideal target for follow-up with the exceptional capabilities of both the Hubble and James Webb space telescopes,” says Emily Rickman, an ESA scientific operations scientist. “They will allow us to examine this exoplanet with a larger wavelength range, including infrared and UV light, to better understand the composition of its atmosphere.”

The future of exoplanet research is bright, since Cheops is the first of three dedicated exoplanet missions. Plato will follow in 2026, focusing for Earth-like planets circling at potentially life-supporting distances from their star. Ariel is scheduled to join the fleet in 2029 and will specialize on examining exoplanet atmospheres.

S. Hoyer et al. (2023) presented an article on the unusually high albedo of LTT 9779 b discovered by Cheops. doi: https://www.aanda.org/10.1051/0004-6361/202346117.

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