Unraveling the Mysteries of the Bright Exoplanet LTT9779 b

After the Moon, the planet Venus is the most luminous object we can see in our night sky, thanks to its thick cloud layer that reflects approximately 75 percent of the sunlight, compared to Earth's 30 percent. Now, astronomers have found an exoplanet that mirrors Venus's brilliance: LTT9779 b. According to detailed observations from ESA's Cheops mission, this exoplanet reflects up to 80 percent of the light from its host star. This phenomenon, known as "albedo," is usually low in most planets due to the absorption of light by their atmospheres or by a dark or rough surface. The exceptions are often icy planets or ones like Venus with a reflective cloud layer.

The Cheops mission's precision measurements were a concerted effort to expand on the initial discovery and description of the planet in 2020 by NASA's TESS mission and ground-based tools like ESO's HARPS instrument in Chile.

A Universe's Mirror in Metallic Clouds

LTT9779 b, with its Neptune-like size, is currently the largest known "mirror" in the universe. Its high reflectivity comes from being enveloped in metallic clouds composed largely of silicate—the same material that forms sand and glass—mixed with metals such as titanium.

The high albedo of LTT9779 b was unexpected since the side of the planet facing its star is believed to have a temperature of around 2,000 °C. Temperatures above 100 °C are generally too high for water clouds to form, and this exoplanet's atmospheric temperature should also be too high for metal or glass clouds. 

Scientists believe that the cloud formation on LTT9779 b occurs similarly to how condensation forms in a bathroom after a hot shower. The planet's atmosphere, saturated with silicates and metal vapors, can form metallic clouds despite its high temperature, just as a steam-filled bathroom can form clouds when the air is so saturated with steam that it can't hold any more.

Surprising Features of LTT9779 b

LTT9779 b's brightness isn't its only surprise. Its size and temperature categorize it as an "ultracold Neptunian," but no other planet of this size and mass has been found orbiting so close to its star, placing it in the "hot Neptunian desert."

With a radius 4.7 times larger than Earth's and a year that lasts only 19 hours, LTT9779 b is unique. All previously discovered planets that orbit their star in less than a day are either hot Jovians—gaseous giants with a radius at least ten times larger than Earth's—or rocky planets smaller than two Earth radii.

Conventionally, planets like this should have their atmosphere stripped away by their star, leaving behind bare rock. However, LTT9779 b's existence defies this norm. Its metallic clouds help it endure the hot Neptunian desert by reflecting light, preventing the planet from overheating and evaporating. Simultaneously, the heavy metallic nature of the planet and its atmosphere makes it harder to disperse.

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