Cheops Mission Discovers Extraordinarily Reflective Exoplanet, Defying Expectations

ESA’s Cheops mission has made a remarkable discovery, uncovering an exoplanet named LTT9779 b that possesses an unprecedented albedo (reflectivity) of 80%. This finding makes LTT9779 b the most luminous exoplanet ever observed, surpassing the reflectivity of Venus, which stands at 75%, and Earth, which reflects only 30% of incoming sunlight. The striking brilliance of LTT9779 b is attributed to the presence of metallic clouds composed primarily of silicate and metals like titanium.

Cheops, ESA’s pioneering mission, has unveiled LTT9779 b, an exoplanet with an unmatched reflectivity of 80% owing to its metallic cloud cover. This peculiar Neptune-like planet exhibits an extraordinary size and orbits its star at an unusually close distance, prompting scientists to conduct further investigations into its atmosphere and potential habitability.

Cheops’ high-precision measurements were carried out as a focused follow-up study on this planet, which was initially discovered and characterized by NASA’s TESS mission in 2020, along with ground-based instruments such as the ESO HARPS instrument in Chile.

Apart from the Moon, Venus shines brightest in our night sky due to its thick cloud layer reflecting approximately 75% of the Sun’s light. In contrast, Earth reflects only around 30% of incoming sunlight. For the first time ever, astronomers have now encountered an exoplanet capable of matching Venus’ luminosity: the extraordinary LTT9779 b. Detailed measurements conducted by ESA’s Cheops mission have unveiled that this planet reflects an astonishing 80% of the light it receives from its host star.

Accompanied by an artist’s impression, the exoplanet LTT9779 b is depicted orbiting its host star. With a size comparable to Neptune, it serves as the largest known “mirror” in the Universe, reflecting 80% of the starlight it receives. This exceptional radiance was revealed through meticulous measurements performed by ESA’s Cheops, examining the amount of light emanating from the planet-star system. As the planet reflects starlight back towards us, the light reaching Cheops’ instruments slightly decreases when the planet moves out of view behind its star. Thanks to the detectors’ high precision, this slight decrease could be accurately measured. Image credit: Ricardo Ramírez Reyes (Universidad de Chile)

Cheops’ mission was focused on obtaining precise measurements of this exoplanet following its initial discovery and characterization, leveraging data from the NASA TESS mission and ground-based instruments like the ESO HARPS in Chile.

This exoplanet, equivalent in size to Neptune, reigns as the most radiant “mirror” known in the Universe today. Its extraordinary reflectivity arises from the presence of metallic clouds composed primarily of silicate, the same substance found in sand and glass, intermingled with metals such as titanium.

James Jenkins, an astronomer from Diego Portales University and CATA (Santiago, Chile), illustrates the remarkable characteristics of this planet: “Imagine a scorching world, situated near its star, adorned with dense clouds of suspended metals that precipitate as droplets of titanium.” Jenkins co-authored a scientific paper describing this breakthrough research, published in the journal Astronomy & Astrophysics.

A Sky Adorned with Metallic Clouds

The fraction of light reflected by an object is denoted as its ‘albedo.’ Most planets possess low albedo values due to factors such as atmospheric absorption or dark and rugged surfaces. Exceptions are typically found in frozen ice worlds or planets like Venus, boasting reflective cloud layers.

The high albedo exhibited by LTT9779 b comes as a surprising revelation, considering that the planet’s sunlit side reaches temperatures of approximately 2000 °C. Any temperature above 100 °C is too hot for water clouds to form, let alone clouds made of metal or glass.

Vivien Parmentier, a researcher at the Observatory of Côte d’Azur (France) and co-author of the study, sheds light on the perplexing phenomenon: “This enigma was resolved when we began comparing cloud formation to condensation after a hot shower in a bathroom. To create steam in a bathroom, one can either cool the air until water vapor condenses or keep hot water running until clouds emerge because the air becomes saturated with vapor. Similarly, LTT9779 b can produce metallic clouds despite its scorching conditions due to the atmosphere being oversaturated with silicate and metal vapors.”

The Impossible Planet

LTT9779 b’s brilliance is not its only astounding quality. Its size and temperature classify it as an “ultra-hot Neptune,” yet no other planet of this size and mass has been found orbiting so closely to its star, residing within the so-called “hot Neptune desert.”

With a radius 4.7 times that of Earth and a year lasting a mere 19 hours, LTT9779 b defies expectations. All previously identified planets with orbital periods shorter than one day are either “hot Jupiters,” massive gas giants with radii at least ten times greater than Earth’s, or rocky planets smaller than twice Earth’s radius.

“This is a planet that should not exist,” remarks Vivien. “We would typically expect planets like this to have their atmospheres stripped away by their star, leaving behind barren rock.”

Sergio Hoyer, the first author of the study from the Marseille Astrophysics Laboratory, adds, “We believe these metal clouds help the planet survive within the hot Neptune desert. By reflecting light, the clouds prevent the planet from becoming excessively hot and evaporating. Moreover, the high metallicity makes the planet and its atmosphere denser, rendering it less prone to being blown away.”

Observing an Exoplanet in Hiding

Cheops determined the properties of LTT9779 b by observing the planet as it passed behind its host star. Since the planet reflects light, the combined light from the star and the planet intensifies just before the planet is obscured, and then diminishes afterward. The disparity in visible light received just before and after the planet’s disappearance indicates the amount of light reflected by the planet.

This investigation relied on Cheops’ precision and continuous coverage. Sergio states, “Accurately measuring the slight variation in the star’s signal as it eclipses the planet was only possible with Cheops.”

Maximilian Günther, ESA’s Cheops project scientist, emphasizes, “Cheops stands as the first-ever space mission dedicated to follow-up observations and characterization of known exoplanets. Unlike large survey missions focused on discovering new exoplanetary systems, Cheops possesses the flexibility to swiftly concentrate on intriguing targets and achieve coverage and precision that would otherwise be unattainable.”

Examining an exoplanet using various instruments provides a comprehensive understanding. “LTT9779 b presents an ideal target for follow-up studies with the exceptional capabilities of both the Hubble and James Webb space telescopes,” notes Emily Rickman, ESA science operations scientist. “These telescopes will enable us to explore this exoplanet across a wider range of wavelengths, including infrared and ultraviolet light, in order to gain better insights into its atmospheric composition.”

The future of exoplanet research appears promising, with Cheops being the pioneer among a trio of dedicated exoplanet missions. Plato, slated for launch in 2026, will focus on Earth-like planets orbiting at potentially habitable distances from their stars. Ariel, set to join the mission fleet in 2029, will specialize in the study of exoplanet atmospheres.

Reference: “The extremely high albedo of LTT 9779 b revealed by CHEOPS: An ultrahot Neptune with a highly metallic atmosphere” by S. Hoyer, J. S. Jenkins, V. Parmentier, M. Deleuil, G. Scandariato, T. G. Wilson, M. R. Díaz, I. J. M. Crossfield, D. Dragomir, T. Kataria, M. Lendl, R. Ramirez, P. A. Peña Rojas, and J. I. Vinés, 10 July 2023, Astronomy & Astrophysics.
DOI: 10.1051/0004-6361/202346117

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More about exoplanet discovery

• ESA’s Cheops Mission
• NASA’s TESS Mission
• ESO’s HARPS Instrument
• Astronomy & Astrophysics Journal
• ESA’s Cheops Mission Overview