Astrophile: A scorched world with snow black and smoky

Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Object: Titanium oxide snow
Location: The hot-Jupiter planet HD 209458b

There is something magical about waking up to discover it has snowed during the night. But there's no powdery white blanket when it snows on exoplanet HD 209458b. Snow there is black, smoky and hot as hell ? resembling a forest fire more than a winter wonderland. Put it this way: you won't be needing mittens.

HD 209458b belongs to a family called hot Jupiters, gas-giant planets that are constantly being roasted due to their closeness to their sun. By contrast, the gas giants in our immediate neighbourhood, including Jupiter, are frigid, lying at the solar system's far reaches.

HD 209458b is also noteworthy because it is tidally locked, so one side is permanently facing towards its star while the other is in perpetual night. On the face of it, these conditions wouldn't seem to invite snow: temperatures on the day side come close to 2000??C, while the night side is comparatively chilly at around 500??C.

Snow made of water is, of course, impossible on this scorched world, but the drastic temperature differential sets up atmospheric currents that swirl material from the day side to night and vice versa. That means that any substances with the right combination of properties might be gaseous on the day side and then condense into a solid on the night side, and fall as precipitation. Say hello to titanium oxide snow.

Stuck on the surface

Although oxides of titanium make up only a small component of a hot Jupiter's atmosphere, these compounds have the right properties to fall as snow. But there was a snag that could have put a stop to any blizzards. Older computer models of hot Jupiters suggested that titanium oxides condensing in the air on the night side would snow ? and remain on the relatively cool surface forever. "Imagine on Earth if you had no mechanism to evaporate water, it would never rain," says Vivien Parmentier of the C?te d'Azur Observatory in Nice, France.

Now he and colleagues have created a more detailed 3D computer model that shows that the snow can become a gas again as it falls and the temperature and pressure increase. Strong updraughts can then blow the titanium oxides back to the upper atmosphere. "The gas can come back on the top layers and snow again and again," says Parmentier.

Snowfall on HD 209458b would be like none you've ever seen. Though titanium dioxide is white and shiny, for example, the snowflakes would also contain silica oxides from the atmosphere, making them black. Since the atmosphere is also dark, snowstorms on the planet would be a smoky affair, the opposite of the white-outs we get on Earth. "It would be like being in the middle of a forest fire," says Parmentier.

Although the team studied a particular hot Jupiter, their model should apply equally to other planets of this type, suggesting hot snow is a common occurrence. Parmentier says we may have already spotted snow clouds on another hot Jupiter, HD 189733b, as spectral analysis of the planet suggests the presence of microscopic particles in its atmosphere.

David Sing of the University of Exeter, UK, who helped identify such particles on HD 189733b, says the team's new model goes a long way to explaining how titanium oxides behave on hot Jupiters. "We're pretty used to water condensing on Earth; there it is titanium because the temperatures are so much hotter."

Hot, black snow ? now that would be something to wake up to.

Reference: http://arxiv.org/abs/1301.4522

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