One of the most famous sayings heard when someone wants to emphasize their persistence and determination on a subject is that they will deal with it «until black snow falls». If we lived on the planet HD 209458 b this saying would not exist since French astronomers discovered that black snow actually falls on the exoplanet!
Damned
HD 209458 b is a gas giant located 150 light-years from Earth in the Pegasus constellation. It is very close to its parent star and completes a full orbit around it in just 84 hours.
As follows, the temperatures that develop on this exoplanet are very high. On the side of the planet that, during its orbital motion, sees its parent star (i.e., where it is day), the temperature reaches two thousand degrees Celsius. On the opposite side the temperature ranges around 500 degrees Celsius. Naturally, these conditions are in no way related to life, which is why the scientific community has unofficially named the planet «Osiri», the god of the dead in ancient Egypt.
HD 209458 b is at a breathing distance from its star
The phenomenon
The formation of snow requires water, which of course does not exist on HD 209458 b. However, the very large temperature difference between day and night on the planet creates various phenomena. One of these is the creation of atmospheric currents that transport turbulent material from the regions where there is day to those where there is night and vice versa. This (theoretically) means that the gaseous masses that start from the regions where there is day solidify when they enter the conditions of the night.
Eternal black snow
Scientists from the Côte d’Azur Observatory in Nice have created a three‑dimensional model of the atmospheric conditions prevailing on HD 209458 b. According to it, the hot gases as they travel toward the night‑side regions turn into titanium oxide clouds. The experts describe them as «black, hot, smoky snow» that falls toward the interior of the planet. Researchers note that the black clouds are likely the product of mixing titanium dioxide with some other element. Although titanium dioxide has a white and bright hue, the presence of another element – e.g., silicon oxides – gives the clouds a black colour.
According to the simulation, rising air currents send the flakes back into the upper layers of the planet's atmosphere. "The gases solidify, fall and then return to the upper layers, creating a continuous cycle of snowfall. Snowstorms on HD 209458 b will not be like those on Earth, they will be like being in the middle of a burning forest," says Vivienne Parmantier, a member of the research team. The research appears in the online archive of scientific preprints "arxiv.org".
