A team of astronomers from the United States, Canada and the Universities of Bristol and Oxford in the UK have now come up with a model that predicts which of the many types of proposed clouds, from sapphire to smoggy methane haze, to expect on hot Jupiters of different temperatures, up to thousands of degrees Kelvin.

Surprisingly, the most common type of cloud, expected over a large range of temperatures, should consist of liquid or solid droplets of silicon and oxygen, like melted quartz or molten sand. On cooler hot Jupiters, below about 950 Kelvin (675 degrees Celsius), skies are dominated by a hydrocarbon haze, essentially smog.

The model will help astronomers studying the gases in the atmospheres of these strange and distant worlds, since clouds interfere with measurements of the atmospheric composition. It could also help planetary scientists understand the atmospheres of cooler giant planets and their moons, such as Jupiter and Saturn’s moon Titan in our own solar system.

Peter Gao, a postdoctoral fellow at the University of California, Berkeley, who is first author of a paper describing the model in the journal Nature Astronomy, said: “The kinds of clouds that can exist in these hot atmospheres are things that we don’t really think of as clouds in the solar system.

“There have been models that predict various compositions, but the point of this study was to assess which of these compositions actually matter and compare the model to the available data that we have.”

The study takes advantage of a boom in the study of exoplanet atmospheres over the past decade.

Though exoplanets are too distant and dim to be visible, many telescopes, in particular the Hubble Space Telescope, are able to focus on stars and capture starlight passing through the atmospheres of planets as they pass in front of their star.

The wavelengths of light that are absorbed, revealed by spectroscopic measurements, tell astronomers which elements make up the atmosphere. To date, this technique and others have found or inferred the presence of water, methane, carbon monoxide and carbon dioxide, potassium and sodium gases and, in the hottest of the planets, vaporised aluminum oxide, iron and titanium.

Dr Hannah Wakeford, astrophysicist and co-author from the University of Bristol’s School of Physics, said: “Over the last decade measurements using the Hubble Space Telescope have been leading the way in terms of detecting what is in the atmospheres of exoplanets.

“The most common measurement we have made is of water vapor in the atmospheres of these hot giant planets. But these measurements have also revealed that clouds in their atmospheres are absorbing and blocking a lot of the light from the star. Here we are using those pioneering observations with our models to help understand more about those exotic clouds.”

But while some planets seem to have clear atmospheres and clear spectroscopic features, many have clouds that completely block the starlight filtering through, preventing the study of gases below the upper cloud layers. The composition of the gas can tell astronomers how exoplanets form and whether the building blocks of life are present around other stars.

Peter Gao added: “We have found a lot of clouds: some kinds of particles — not molecules, but small droplets — that are hanging out in these atmospheres.

“We don't really know what they are made of, but they are contaminating our observations, essentially making it more difficult for us to assess the composition and abundances of important molecules like water and methane.”

Astronomers have proposed many strange types of clouds to explain these observations, composed of aluminum oxides, such as corundum, the stuff of rubies and sapphires; molten salt, such as potassium chloride; silicon oxides, or silicates, like quartz, the main component of sand; sulfides of manganese or zinc that exist as rocks on Earth; and organic hydrocarbon compounds. They could be liquid or solid aerosols.

The research team adapted computer models initially created for Earth’s water clouds, and subsequently extended to the cloudy atmospheres of planets like Jupiter, which has ammonia and methane clouds. He expanded the model even further to the much higher temperatures seen on hot gas giant planets — up to 2,800 Kelvin, or 4,600 degrees Fahrenheit (2,500 degrees Celsius) — and the elements likely to condense into clouds at these temperatures.

The model takes into account how gases of various atoms or molecules condense into a droplet, how the droplets grow or evaporate and whether they are likely to be transported in the atmosphere by winds or updrafts or sink because of gravity.

Peter Gao, who has also modeled sulfuric acid clouds on Venus, said: “The idea is that the same physical principles guide the formation of all types of clouds. What I have done is to take this model and bring it out to the rest of the galaxy, making it able to simulate silicate clouds and iron clouds and salt clouds.”

He then compared his predictions to available data on 30 exoplanets, out of a total of about 70 transiting exoplanets with recorded transmission spectra to date.

The model revealed that many of the exotic clouds proposed over the years are difficult to form because the energy required to condense is too high. Silicate clouds condense easily, however, and dominate over a 1,200-degree Kelvin range of temperatures: from about 900 to 2,000 Kelvin - a range of about 1,000 degrees Celsius.

According to the model, in the hottest atmospheres, aluminum oxides and titanium oxides condense into high-level clouds. In exoplanets with cooler atmospheres, those clouds form deeper in the planet and are obscured by higher silicate clouds. On even cooler exoplanets, these silicate clouds also form deeper, leaving clear upper atmospheres. At even cooler temperatures, ultraviolet light from the star converts organic molecules like methane into extremely long hydrocarbon chains that form a high-level haze akin to smog. This smog can obscure lower-lying salt clouds of potassium or sodium chloride.

For those astronomers seeking a cloudless planet to more easily study the gases in the atmosphere, Gao suggested focusing on planets between about 900 and 1,400 Kelvin, or those hotter than about 2,200 Kelvin.

Dr Wakeford added: “The presence of clouds has been measured in a number of exoplanet atmospheres before, but it is when we look collectively at a large sample that we can pick apart the physics and chemistry in the atmospheres of these worlds.

“The dominant cloud species is as common as sand — it is essentially sand — and it will be really exciting to be able to measure the spectral signatures of the clouds themselves for the first time with the upcoming James Webb Space Telescope.”

Future observations, such as those by NASA’s JWST, scheduled for launch within a few years, should be able to confirm these predictions, and perhaps shed light on the hidden cloud layers of planets closer to home.

Peter Gao said that similar exotic clouds may exist at depths within Jupiter or Saturn where the temperatures are close to those found on hot Jupiters.

He added: “Because there are thousands of exoplanets versus just one Jupiter, we can study a bunch of them and see what the average is and how that compares to Jupiter.”

He and his colleagues plan to test the model against observational data from other exoplanets and also brown dwarfs, which are basically gas giant planets so massive they’re almost a star. They, too, have clouds.

Jonathan Fortney of UC Santa Cruz, another co-author, said: “In studying planetary atmospheres in the solar system, we typically have the context of images. We have no such luck with exoplanets. They are just dots or shadows.

“That's a huge loss in information. But what we do have to make up for that is a much larger sample size. And that allows us to look for trends — here, a trend in cloudiness — with planetary temperature, something that we just don't have the luxury of in our solar system.”

Peter Gao was supported by NASA’s postdoctoral program and a 51 Pegasi Fellowship from the Heising-Simons Foundation.