Cheops Unveils Astonishing Find: Exoplanet Behaving as a Mirror with Blazing Heat

ESA’s Cheops mission has made a fascinating discovery: an exoplanet named LTT9779 b, renowned for its remarkable shininess with an unprecedented reflectivity of 80% owing to its metallic clouds. This ultra-hot Neptune stands out as a truly unique entity due to its size and exceptionally close orbit around its host star, prompting further research to gain insights into its atmosphere and its potential for sustainability.

Data obtained from ESA’s exoplanet mission, Cheops, has brought forth a surprising revelation: an exoplanet of extreme heat, completing an orbit around its star in less than a day, is enveloped by reflective clouds composed of metal. This finding positions it as the brightest exoplanet ever documented.

Within our night sky, aside from the Moon, the most luminous celestial object is Venus, renowned for its dense cloud cover that reflects approximately 75% of the Sun’s light. In comparison, Earth reflects only around 30% of incoming sunlight.

Now, for the very first time, astronomers have encountered an exoplanet that rivals the brilliance of Venus: the exoplanet LTT9779 b. Detailed measurements conducted by ESA’s Cheops mission unveil that this remarkable planet reflects an astonishing 80% of the illuminating light emitted by its host star.

The image above showcases an artist’s depiction of exoplanet LTT9779b as it orbits its host star. With a size comparable to that of Neptune, this exoplanet possesses an extraordinary trait—it reflects a remarkable 80% of the incident light, earning it the distinction of being the largest known “mirror” in the Universe. This extraordinary shininess was unveiled through meticulous measurements conducted by ESA’s Cheops, which gauged the amount of light emanating from the planet-star system. Whenever the planet moved out of sight behind its star, the reflected starlight reached Cheops’ instruments, resulting in a slight decrease in the detected light. The detectors’ high precision enabled the measurement of this minute decline.

Cheops’ precise measurements were a deliberate follow-up to the planet’s initial discovery and characterization in 2020 by NASA’s TESS mission and ground-based instruments like the ESO HARPS instrument in Chile.

Approximately the size of Neptune, this exoplanet holds the distinction of being the largest “mirror” observed thus far in the vast expanse of the Universe. The key factor behind its exceptional reflectivity lies in its envelopment by metallic clouds. These clouds primarily consist of silicate—a material akin to sand and glass—intermixed with metals such as titanium.

James Jenkins, an astronomer from Diego Portales University and CATA in Santiago, Chile, comments, “Imagine a burning world, situated in close proximity to its star, adorned with dense clouds of metallic nature, causing titanium droplets to precipitate like rain.” James serves as a co-author of a scientific paper detailing this groundbreaking research, which has been published in the journal Astronomy & Astrophysics.

A sky filled with clouds of metal

The term “albedo” refers to the fraction of light that an object reflects. In the case of planets, their albedo is generally low due to factors such as atmospheric absorption or dark and rugged surfaces. However, exceptions are found among frozen ice worlds or planets like Venus, which possess a reflective cloud layer.

The high albedo observed in LTT9779 b came as an unexpected revelation. This is because the planet’s side facing its star reaches scorching temperatures of approximately 2000 °C. Typically, temperatures above 100 °C prevent the formation of water clouds, and in the case of this planet, even the extreme heat would inhibit the formation of clouds made of metals or glass-like substances.

Through data obtained from ESA’s exoplanet mission Cheops, an unexpected revelation has emerged. The findings indicate that an ultra-hot exoplanet, completing a rapid orbit around its host star in under a day, possesses a cover of reflective metal clouds, rendering it the most radiant exoplanet ever discovered. The credit for the accompanying image is attributed to ESA.

Vivien Parmentier, a researcher from the Observatory of Côte d’Azur in France and co-author of the study, offers an intriguing analogy to comprehend the phenomenon. Parmentier suggests viewing the formation of these clouds in a similar manner to how condensation occurs in a steam-filled bathroom after a hot shower. Parmentier elaborates, explaining that the atmosphere of LTT9779 b can generate metallic clouds despite its scorching temperatures because it becomes oversaturated with silicate and metal vapors, much like a bathroom becomes saturated with water vapor. This analogy provides insights into the unusual cloud formation on this intriguing exoplanet.

The planet that shouldn’t exist

LTT9779 b presents more surprises beyond its extraordinary reflectivity. This exoplanet, categorized as an “ultra-hot Neptune,” defies conventional expectations due to its size, temperature, and proximity to its star. It resides within a region known as the “hot Neptune desert,” as no other planets of similar size and mass have been discovered orbiting at such close distances to their stars.

With a radius 4.7 times that of Earth, LTT9779 b completes a full orbit around its star in a mere 19 hours. Previous findings have indicated that planets with orbits shorter than a day are typically either “hot Jupiters” — gas giants with radii at least ten times that of Earth — or rocky planets smaller than twice Earth’s radius.

The provided artist’s impression showcases exoplanet LTT9779 b as it orbits its host star. Roughly the size of Neptune, the planet reflects a striking 80% of the incident light, solidifying its status as the largest known “mirror” in the Universe. Detailed measurements conducted by ESA’s Cheops mission allowed for the determination of this exceptional shininess by analyzing the amount of light emitted from the planet-star system. Due to the planet’s capacity to reflect starlight back toward us, the measured light reaching Cheops’ instruments slightly decreased when the planet moved out of view behind its star. This minute decline could be accurately measured thanks to the detectors’ exceptional precision.

Vivien Parmentier emphasizes the enigmatic nature of LTT9779 b, stating, “It’s a planet that defies expectations. We anticipate that planets like this would lose their atmosphere, leaving behind only a bare rock.”

Sergio Hoyer, the lead author from the Marseille Astrophysics Laboratory, adds insights into the survival mechanisms of this extraordinary exoplanet, remarking, “We believe that these metal clouds assist the planet in enduring the scorching conditions of the hot Neptune desert. The clouds act as reflective shields, preventing excessive heating and evaporation of the planet. Furthermore, the high metallic content makes the planet and its atmosphere more massive, increasing their resilience against atmospheric loss.”

Studying an exoplanet by looking when it’s hidden

To discern the characteristics of LTT9779 b, the Cheops mission by ESA focused on observing the planet as it passed behind its host star. As the planet possesses reflective properties, the combined emissions from the star and planet produce an increased amount of light towards the space telescope just before the planet disappears from view compared to immediately after. The discrepancy in the received visible light between these two instances provides insights into the planet’s reflectivity.

The success of this endeavor hinged upon the exceptional precision and continuous coverage offered by the Cheops mission. Sergio highlights the significance of Cheops, stating, “The ability to precisely measure the subtle fluctuations in the star’s signal caused by the planet’s eclipse was achievable solely through Cheops.”

Maximilian Günther, the project scientist for ESA’s Cheops mission, emphasizes the unique nature of the mission, stating, “Cheops stands as the pioneering space endeavor solely dedicated to the characterization and follow-up of known exoplanets. Unlike large-scale survey missions that primarily focus on discovering new exoplanetary systems, Cheops possesses the necessary adaptability to swiftly target intriguing objects, allowing us to achieve coverage and precision that would otherwise be unattainable.”

To gain a comprehensive understanding of exoplanets, it is vital to employ various instruments in observing the same target. Emily Rickman, a science operations scientist at ESA, highlights the exceptional capabilities of the Hubble and James Webb space telescopes for conducting follow-up investigations on LTT9779 b. By utilizing a broader wavelength range, including infrared and ultraviolet light, these telescopes will facilitate a more comprehensive exploration of the exoplanet, enabling a deeper understanding of its atmospheric composition.

The future of exoplanet research shines brightly as Cheops serves as the precursor to a trio of dedicated exoplanet missions. Plato, scheduled for launch in 2026, will concentrate on Earth-like planets situated at potentially habitable distances from their stars. Ariel will join the mission roster in 2029, specializing in the study of exoplanet atmospheres. Together, these missions will contribute significantly to expanding our knowledge of exoplanets.

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