With the help of ESA’s CHEOPS mission, a team led by James Jenkins of the University of Diego Portales in Santiago de Chile has tracked down the exoplanet LTT9779 b. According to the journal Astronomy & Astrophysics, it reflects 80 per cent of the light from its star, making it the strongest known “mirror” of the universe.
The exoplanet also belongs to the class of ultra-hot Neptunes. The reason for its extreme reflectivity is that it is surrounded by clouds of compounds of heavy elements, known in astronomy as metals. These reflective clouds consist largely of silicate mixed with oxides of elements such as calcium and titanium.
The extremely high albedo of LTT9779 b amazed the team, because the side of the planet facing the star is about 2000 Celsius hot. Any value above 100 degrees Celsius is too hot for the formation of water vapour clouds, but the atmospheric temperature of this planet should be too hot even for clouds of metal or glass.
LTT9779 b can form metallic clouds despite great heat because the atmosphere is heavily saturated with silicate and metal vapours. The working group calculated that the planet must contain at least 400 times as many heavy elements as the Sun for silicate clouds to condense.
In addition to the high albedo, it surprised the working group that the exoplanet exists at all. So far, no other ultra-hot Neptune of this size and mass is known to orbit so close to its star. It exists in a region also known as the “hot Neptune desert”. LTT9779 b has a radius 4.7 times that of Earth and its year lasts only 19 hours.