In a recent surprising development, data from the European Space Agency’s (ESA) Cheops mission has uncovered that an incredibly hot exoplanet, which circles its parent star in less than a day, is enveloped in metallic clouds that reflect a significant amount of light, earning it the title of the most reflective exoplanet ever discovered.

In terms of brightness in our nighttime sky, Venus, second only to the Moon, is notable because its dense layer of clouds reflects roughly 75% of solar radiation, whereas Earth reflects about 30%.

However, for the first time, an exoplanet has been discovered that matches Venus’s level of reflectivity – the planet known as LTT9779 b. As per detailed measurements from ESA’s Cheops mission, the planet bounces back an astounding 80% of the light its parent star casts upon it.

The detailed readings from Cheops were a strategic follow-up to the initial discovery and characterization of the planet in 2020 by NASA’s TESS mission and terrestrial instruments like the ESO HARPS instrument in Chile.

The exoplanet, similar in size to Neptune, is the largest known reflective body in the universe. Its exceptionally high reflectivity can be attributed to the metallic clouds that cover it, primarily composed of silicate (the primary ingredient in sand and glass) combined with metals like titanium.

James Jenkins, an astronomer at Diego Portales University and CATA (Santiago, Chile) who co-authored the scientific paper, characterizes the planet as “a burning world, close to its star, with heavy clouds of metals floating aloft, raining down titanium droplets.”

Albedo, a term used to describe the proportion of light an object reflects, is usually low for most planets because they either possess an atmosphere that absorbs a lot of light or have a dark or rough surface. However, LTT9779 b was a surprising exception given that the side of the planet facing its star is estimated to reach around 2000°C. At these temperatures, not only water clouds, but even metal or glass clouds should not be able to form.

The enigma was eventually solved by considering the cloud formation in the same way as condensation forming in a bathroom after a hot shower, explains Vivien Parmentier, a researcher at the Observatory of Côte d’Azur (France). Despite the high temperatures, metallic clouds can form due to the oversaturated atmosphere filled with silicate and metal vapors.

In addition to its reflective qualities, the exoplanet LTT9779 b has other unusual characteristics. Its size and temperature classify it as an ‘ultra-hot Neptune’, but no other planets of similar size and mass have been found in such close proximity to their parent stars. Thus, it dwells in an area referred to as the ‘hot Neptune desert’.

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

According to Sergio Hoyer of the Marseille Astrophysics Laboratory, the first author of the study, the metal clouds could be aiding the planet’s survival by reflecting light and preventing the planet from overheating and evaporating. Additionally, the high metallic content adds weight to the planet and its atmosphere, making it less prone to be blown away by its star.

To determine the properties of LTT9779 b, Cheops observed the planet as it moved behind its host star. The difference in light received right before and after the planet is hidden indicates how much light the planet reflects.

Looking at the same exoplanet using different instruments gives a more comprehensive view. Both the Hubble and James Webb space telescopes will allow further exploration of this exoplanet to better understand the composition of its atmosphere.

As for the future of exoplanet research, Cheops is only the first of a trio of dedicated exoplanet missions. It will be joined by Plato in 2026, which will focus on Earth-like planets within habitable zones, and Ariel in 2029, which will specialize in studying exoplanet atmospheres.

The research paper was published in the Astronomy & Astrophysics journal (2023).