Blazing Exoplanet Discovered Reflecting 80% of Its Star’s Light Thanks to Metallic Clouds

Scientists have discovered the brightest exoplanet ever, namely LTT9779 b which reflects as much as 80% of its star’s light. This planet is encased by ‘metallic aura’ formed by silicates and metals such as titanium to give reflectivity of a mirror. Despite of the temperature of about 2000 degrees Celsius, through which cloud formation is thought to be impossible, metal vapors similar to that of steam in the bathroom condense. LTT9779 b does not fit into current theories since it is located in the ‘hot Neptune desert’, that is, the region in which gaseous planets of this size cannot sustain an atmosphere.

Venus is the second brightest point in night sky excluding the Moon with the albedo of 71% while the albedo for the Earth is about 30%.

For the first time, astronomers have discovered an exoplanet that rivals Venus in brightness: LTT9779 b. The new data from ESA’s Cheops mission showed that it is an 80% magnitude planet meaning that it reflects 80 percent of the light from its host star.

The Cheops observations were intended as a high precision follow-up mission to TESS and ground based observations in 2020 which first announced the existence of the planet using ESO HARPS in Chile.

LTT9779 b is approximately the size of Neptune making LTT9779 b the largest “mirror” in the Universe. It owes this high albedo to its metal clouds, formed of silicate- the same material used in sand and glass-and metal including titanium.

“If Ceres has a second rocky mantle, then it is closely circling the sun, with heavy metal clouds above, and titanium drops raining,” he said James Jenkins, an astronomer from Diego Portales University and CATA, Santiago, Chile. This discovery is detailed in a scholarly paper that Jenkins co-wrote with other researchers and is available today in the Astronomy & Astrophysics journal.

A sky full of metallic clouds

As a term “albedo” means the amount or the ratio of reflected light on an object. It is found that most of the planets possess a low albedo value; this is attributed to the dense layer of atmospheric haze or a dull, rugged surface. Among the exceptions there are: icy bodies and planets with a reflective cloud deck such as Venus.

The calculated albedo of LTT9779 b was relatively high while the temperature of the side of the planet facing its host star is said to be about 2000°C. Any water clouds cannot form at temperatures above 100°C and it was believed that the temperature of the atmosphere is too high for clouds of metal or glass to be formed.

“This was rather intriguing until it became clear that cloud formation on this planet could be explained in the similar way to how water condensation happens in the bathroom after taking hot shower,” adds Vivien Parmentier, the researcher at the Observatory of Côte d’Azur, France and one of the authors of the study. Vivien continues, “In a bathroom, one can either cool the air until it thickens and water vapor condenses or have the hot water running until clouds form because the air is already saturated with it… Likewise, LTT9779 b can produce metallic clouds even though it is hot because the air is oversaturated with silicate and metal vapor. ”

The planet that should not exist

However, it is not only the object’s reflectivity that is unexpected in LTT9779 b. It is so large and hot, it is considered an “ultra hot Neptune” but no other planet like this in terms of size or mass have been discovered at this proximity to its star putting it in what scientists are calling the “hot Neptune desert. ”

With a radius 4. Seven times that of Earth, LTT9779 b orbits its star in 19 hours only. The former identified planets with such short orbits (less than a day) are either ‘hot Jupiters’ which is a large gas giants with radius that is at least ten times that of Earth or other planets that have a size less than double that of the Earth.

‘It’s a planet that shouldn’t exist,’ Vivien Parmentier. “We would expect such a planet to have lost all of its atmosphere to its star leaving only a small dry core behind. ”

Sergio Hoyer, the first author of the study conducted at the Marseille Astrophysics Laboratory, said, “We think metallic clouds save the planet from the Neptune desert – these clouds do not absorb heat and actually shield the planet from being evaporated by the heat. Second, a high metallicity of both the planet and its atmosphere make them heavier, thus, can not be stripped by the star’s gravity. ”

Observing an exoplanet when it is concealed

To find out features of LTT9779 b, the Cheops space mission of ESA, which is dedicated to exoplanet characterization, watched the planet’s transit behind the star. The planet is the reflecting surface and hence the illumination that results from the star and planet is brighter just before the absence of the planet as observed than it is afterwards. Hence scientists can find out how much light the planet reflects by comparing the amount of visible light received before the actual planet disappears and after it does so.

It was important for this task that Cheops offers high precision and is permanently observing the target. “Measuring this small variation of the star’s signal when the planet was being occulted was only achievable with Cheops” says Sergio Hoyer.

Maximilian Günther, ESA’s Cheops project scientist, comments: “Cheops is the first space mission designed for the characterisation of known exoplanets. Unlike giant survey missions that are aimed at revealing new planetary systems, Cheops can focus on objects of interest swiftly and offer coverage and accuracy that cannot be easily surpassed.

In order to characterise an exoplanet one needs more than one instrument. “LTT9779 b is perfectly suitable for the follow-up observations with the enhanced functionality of the Hubble Space Telescope and the James Webb Space Telescope,” said Emily Rickman, an ESA scientific operations scientist. “These telescopes will help us collect data on this exo-planet in a range of wavelengths other than visible light including infrared and UV light to better understand its atmospheric structure. ”

The prospects of exoplanet as science appear quite bright in the future because Cheops is the first of three dedicated exoplanet missions. Plato, which is expected to launch in 2026, will concentrate in identifying the Earth-like planets and their positions in the zone of potentially habitable distances from their stellar bodies. Ariel, due for launch in 2029, will target the characterisation of 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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