ESA's Cheops captures shiniest exoplanet in space, and it's the size of Neptune

Incredibly hot exoplanet orbits around its host star in less than a day, and is covered with reflective metal clouds, causing it to gleam like a mirrorball.
Shubhangi Dua
The shiniest known exoplanet: LTT9779b
The shiniest known exoplanet: LTT9779b

European Space Agency / YouTube 

The European Space Agency's (ESA) Cheops (CHaracterising ExOPlanet Satellite) assignment is the first space mission dedicated to studying bright, nearby stars, known to host exoplanets.

Since 2019, the mission’s goal has been to observe alien worlds and conduct high-precision observations of the planet's size as it passes in front of its host star. 

Now the satellite has proven effective in capturing images of the largest known “mirror” in the Universe, surpassing the current most luminous planet Venus, whose thick cloud layer reflects around 75 percent of the sun’s light.

The newly discovered ultra-hot Neptune-sized exoplanet – LTT9779b accounts for 80 percent of the light emitted by its host star.

The planet, which orbits around its star in just 19 hours, was first discovered in 2020 under NASA’s TESS mission. 

Watch the shiniest planet orbiting around its host below.

Largest mirror exoplanet

ESA says that the exoplanet is around the size of Neptune, making it the largest “mirror” in the Universe that we know of today.

James Jenkins, an astronomer at Diego Portales University and CATA (Santiago, Chile) and co-author of the study said, “Imagine a burning world, close to its star, with heavy clouds of metals floating aloft, raining down titanium droplets.”

The astronomers were able to analyze data using multiple instruments, including the Hubble and James Webb space telescopes, which explored the exoplanet’s atmosphere. 

Scientists studied the planet’s metallic clouds to uncover the mechanisms behind its ability to maintain such high levels of reflectivity. 

Research indicates the side of the planet that is closest to the star, reaches a scorching temperature of 3632 degrees Fahrenheit (2,000 degrees Celsius). Such high temperatures are considered far too hot for clouds to form.

Additionally, research indicates the side of the planet which is close to the star reaches a scorching temperature of 2,000 degrees Celsius. Such high temperatures are considered far too hot for clouds to form.

Vivien Parmentier, a researcher at the Observatory of Côte d’Azur and co-author of this research notes, “It was really a puzzle until we realized we should think about this cloud formation in the same way as condensation forming in a bathroom after a hot shower,”

She further explains: “To steam up a bathroom you can either cool the air until water vapor condenses, or you can keep the hot water running until clouds form because the air is so saturated with vapor that it simply can’t hold anymore. Similarly, LTT9779 b can form metallic clouds despite being so hot because the atmosphere is oversaturated with silicate and metal vapors.”

Hot Neptune dessert

The Neptune-like planet has a radius 4.7 times as big as Earth’s while other planets orbiting for less than 24 hours are either gas giant planets – ‘hot Jupiters’ with their radius ten times larger than Earth's, or rocky planets smaller than two Earth radii, the study describes. 

Scientists said that no planet of this size and mass has been found to orbit so close to their star. “This means it lives in what’s known as the ‘hot Neptune desert’".

The study’s author from the Marseille Astrophysics Laboratory, Sergio Hoyer addresses the theory says that the metal clouds could help the planet survive in hot Neptune desert. 

“The clouds reflect light and stop the planet from getting too hot and evaporating. Meanwhile, being highly metallic makes the planet and its atmosphere heavy and harder to blow away,” he said.

Cheops mission is the first of the three satellites dedicated to exoplanet projects. Plato is another spacecraft set to join the mission in 2026. It will focus on Earth-like planets orbiting at a possibly life-supporting distance from their star. While the third, Ariel will launch in 2029 and specialize in studying exoplanet atmospheres.

The research paper was published earlier today in the journal Astronomy & Astrophysics.


Optical secondary eclipse measurements of small planets can provide a wealth of information about the reflective properties of these worlds, but the measurements are particularly challenging to attain because of their relatively shallow depth. If such signals can be detected and modeled, however, they can provide planetary albedos, thermal characteristics, and information on absorbers in the upper atmosphere.

Aims. We aim to detect and characterize the optical secondary eclipse of the planet LTT 9779 b using the CHaracterising ExOPlanet Satellite (CHEOPS) to measure the planetary albedo and search for the signature of atmospheric condensates.

Methods. We observed ten secondary eclipses of the planet with CHEOPS. We carefully analyzed and detrended the light curves using three independent methods to perform the final astrophysical detrending and eclipse model fitting of the individual and combined light curves.

Results. Each of our analysis methods yielded statistically similar results, providing a robust detection of the eclipse of LTT 9779 b with a depth of 115±24 ppm. This surprisingly large depth provides a geometric albedo for the planet of 0.80−0.17+0.10, consistent with estimates of radiative-convective models. This value is similar to that of Venus in our own Solar System. When combining the eclipse from CHEOPS with the measurements from TESS and Spitzer, our global climate models indicate that LTT 9779 b likely has a super metal-rich atmosphere, with a lower limit of 400× solar being found, and the presence of silicate clouds. The observations also reveal hints of optical eclipse depth variability, but these have yet to be confirmed.

Conclusions. The results found here in the optical when combined with those in the near-infrared provide the first steps toward understanding the atmospheric structure and physical processes of ultrahot Neptune worlds that inhabit the Neptune desert.

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