(ORDO NEWS) — The giant planets in our solar system and flying around other stars have exotic clouds that are unlike anything on Earth, and the gas giants orbiting their stars – the so-called hot Jupiters – boast the most extreme.
A team of astronomers from the United States, Canada and England came up with a model that predicts which of the many types of clouds offered, from sapphire to smoky methane fog, should be expected on hot Jupiters with different temperatures, up to thousands of Kelvin.
Surprisingly, the most common type of cloud expected over a wide temperature range should consist of liquid or solid droplets of silicon and oxygen, such as molten quartz or molten sand. On colder hot Jupiters, with temperatures below about 950 ° K (1250 ° Fahrenheit), hydrocarbon haze prevails in the sky, essentially smog.
This model will help astronomers study gases in the atmospheres of these strange and distant worlds, as clouds interfere with measurements of atmospheric composition. It can also help planetologists understand the atmosphere of colder giant planets and their moons, such as Jupiter and Saturn’s satellite, Titan, in our own solar system.
“The types of clouds that can exist in these hot atmospheres are things that we don’t really think are clouds in the solar system,” said Peter Gao, a graduate student at the University of California at Berkeley, who is the first author of an article describing the model that appeared May 25th in the journal Nature Astronomy. “There were models that predict different compounds, but the point of this study was to evaluate which of these compounds really matter and compare the model with the data we have.”
This study takes advantage of the boom over the past decade in studying the atmosphere of exoplanets. Although exoplanets are too distant and dim objects to be visible, many telescopes – in particular, the Hubble Space Telescope – are able to focus on stars and capture starlight passing through the atmosphere of planets when they pass in front of their stars. Absorbed waves of light detected by spectroscopic measurements tell astronomers what elements make up the atmosphere. To date, this method and other methods have discovered or suggested the presence of water, methane, carbon monoxide and carbon dioxide, potassium and sodium gases, as well as, on the hottest of the planets, vaporized oxides of aluminum, iron and titanium.
But while some planets seem to have a transparent atmosphere and clear spectroscopic characteristics, many have clouds that completely block the seeping starlight, preventing the study of gases under the upper layers of the clouds. The composition of the gases can tell astronomers how exoplanets are formed and whether building blocks of life are present around other stars.
“We found a lot of clouds: some types of particles – not molecules, but small droplets – that hang in these atmospheres,” Gao said. “We really don’t know what they are made of, but they pollute our observations, making it difficult for us to evaluate the composition and content of important molecules such as water and methane.”
To explain these observations, astronomers have proposed many strange types of clouds composed of aluminum oxides such as corundum, the substance of rubies and sapphires; molten salt such as potassium chloride; silicon oxides or silicates such as quartz, the main component of sand; manganese or zinc sulfides that exist in the form of rocks on Earth; and organic hydrocarbon compounds. The clouds can be liquid or solid aerosols, Gao said.
Gao adapted computer models originally created for the Earth’s water clouds, and then extended them to the cloud atmospheres of such planets as Jupiter, where there are clouds of ammonia and methane. He expanded the model even further, to much higher temperatures observed on hot gas giant planets – up to 2800 ° K or 2500 ° C, where the elements are likely to condense into clouds at these temperatures.
The model takes into account how gases of various atoms or molecules condense into droplets, how these droplets grow or evaporate, and whether they can be transported in the atmosphere by winds or ascending flows or fall under the influence of gravity.
“The idea is that the same physical principles govern the formation of all types of clouds,” said Gao, who also modeled sulfuric clouds on Venus. What I did was take this model and bring it to the rest of the galaxy, making it capable of simulating silicate, iron and salt clouds. ”
Then he compared his predictions with available data on 30 exoplanets out of a total of about 70 exoplanets recorded on Today, transmission spectra.The
model showed that many of the exotic clouds proposed over the years are difficult to form because the energy needed to condense the gases is too high, however silicate clouds condense easily and dominate in the temperature range from about 900 ° to 2000 ° Kelvin.
According to the model, in the hottest atmospheres, aluminum and titanium oxides condense into high-level clouds. On exoplanets with a cooler atmosphere, these clouds form deeper and are obscured by higher silicate clouds. On even colder exoplanets, these silicate clouds also form deeper in the atmosphere, leaving transparent upper layers of the atmosphere. At even lower temperatures, ultraviolet light from an exoplanet star turns organic molecules, such as methane, into extremely long hydrocarbon chains that form a high-level smog-like haze. This smog can overshadow the salt clouds of potassium or sodium chloride below.
For those astronomers looking for a cloudless planet to make it easier to study gases in the atmosphere, Gao suggested focusing on planets between temperatures between 900 ° C and 1400 ° K or those hotter than 2200 ° K.
“The presence of clouds was discovered in in a number of exoplanet atmospheres before, but just when we look at a large array of data, we can distinguish physics and chemistry in the atmospheres of these worlds, says co-author Hannah Wakeford, an astrophysicist at the University of Bristol in the UK. It will be really exciting when we can first measure the spectral signatures of the clouds themselves with the new James Webb Space Telescope (JWST). ”
Future observations, such as those made by NASA’s JWST, are scheduled to be launched in a few years. He should be able to confirm these predictions and possibly shed light on the hidden cloud coverings of the planets closer to our home. Gao said that such exotic clouds could exist deep inside Jupiter or Saturn, where the temperature is close to that observed on hot Jupiters.
“Since there are thousands of exoplanets against just one Jupiter, we can study a bunch of them and see what the average is and how it relates to Jupiter,” Gao said.
He and his colleagues plan to test the model based on observational data from other exoplanets, as well as from brown dwarfs, which are mainly gas giants, so massive that they are almost stars. They also have clouds.
“When studying the atmospheres of the planets of the solar system, we usually deal with the context of images. With exoplanets we are so unlucky. It’s just dots or shadows, ”said Jonathan Fortney of the University of California, Santa Cruz. “This is a huge loss of information. But we must make up for this with a much larger sample size. And this allows us to look for trends, something that we do not have in our solar system.”
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