Studying the atmospheres of exoplanets is a process that is fairly well along, especially when it comes to hot Jupiters. Here we have a massive target so close to its star that, when a transit occurs, we can look at the star’s light filtering through the atmosphere of the planet. Even so, clouds are a problem because they prevent accurate readings of atmospheric composition below the upper cloud layers. Aerosols — suspended solid particles or droplets in a gas — are common, range widely in composition, and make studying a planet’s atmosphere harder.
We’d like to learn more about which aerosols are where and in what kind of conditions, for we have a useful database of planets to work with. Over 70 exoplanets currently have transmission spectra available. A wide range of cloud types, many of them exotic indeed, have been proposed by astronomers to explain what they are seeing.
Imagine clouds of sapphire, or rubies, which is essentially what we get with aerosols of aluminum oxides like corundum. Potassium chloride can produce a molten salt. Sulfides of manganese or zinc can be components, as well as organic hydrocarbon compounds. Which of these are most likely to form and affect our observations? And what about silicates?
A new model, produced by an international team of astronomers, bodes well for future work. The model predicts that the most common type of hot Jupiter cloud consists not of the most exotic of these ingredients but of liquid or solid droplets of silicon and oxygen — think melted quartz.
But much depends on the temperature, with the cooler hot Jupiters (below about 950 Kelvin) marked by hydrocarbon hazes. Peter Gao (UC-Berkeley) is first author of a paper describing the model that pulls all these and more possibilities together:
“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 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.”
Some planets have clear atmospheres, making spectroscopy easier, but all too frequently high clouds block observations of the gases below them. Gao considers such clouds a kind of contamination in the data, making it hard to trace atmospheric elements like water and methane. The new model examines how gases of various atoms or molecules condense into cloud droplets, their patterns of growth or evaporation, and their transport by local winds.
Image: Predicted cloud altitudes and compositions for a range of temperatures common on hot Jupiter planets. The range, in Kelvin, corresponds to about 800-3,500 degrees Fahrenheit, or 427-1,927 degrees Celsius. Credit: UC Berkeley. Image by Peter Gao.
The team worked with computer models of Earth’s clouds and extended them to planets like Jupiter, where we find ammonia and methane clouds, before moving on to hot Jupiter temperatures up to 2,800 K (2,500 degrees Celsius) and the kind of elements that could condense into clouds under these conditions. The scientists simulated the distribution of aerosol particles, studying cloud formation through thermochemical reactions and haze formation through methane photochemistry. This is intricate stuff, modeling condensation from one gas to another, so that we can simulate the emergence of unusual clouds, but it draws on 30 of the exoplanets with recorded transmission spectra as a check on the model’s accuracy.
Using the model, we can move through layers of atmosphere as mediated by temperature, with the hottest atmospheres showing condensation of aluminum oxides and titanium oxides, producing high-level clouds, while lowering the temperature allows such clouds to form deeper in the planet’s atmosphere, leaving them obscured by bands of higher silicate clouds. Lower the temperatures further and the upper atmosphere becomes clear as the silicate clouds form further down. High-level hazes can form at lower temperatures still.
Looking for a clear sky to study the atmosphere without hindrance? Planets in the range of 950 to 1,400 K are the most likely to produce a cloudless sky, but planets hotter than 2,200 K also fit the bill, says Gao. Hannah Wakeford (University of Bristol, UK) is a co-author on the paper:
“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 (JWST).”
The key finding here is that only one type of cloud made of silicates dominates cloud opacity over a wide range of temperatures, and thus has the greatest implications for observation. Silicates dominate above planetary equilibrium temperatures of 950 K and extend out to 2,000 K, while hydrocarbon hazes dominate below 950 K. Many of the most exotic cloud types proposed in the literature simply require too much energy to condense.
Too bad. I liked the idea of sapphire clouds. But as the paper notes: “The observed trends in warm giant exoplanet cloudiness is a natural consequence of the dominance of only two types of aerosol.” And it continues:
Even though we do not consider the day- and nightside cloud opacity of warm giant exoplanets explicitly in our modelling, our finding that only one type of cloud—silicates—dominates exoplanet cloud opacity over a wide range of temperatures has important implications for exoplanet emission and reflected light observations. For example, the brightness temperature of an atmosphere with an optically thick silicate cloud deck would be fixed to a value slightly below the condensation temperature of silicates where the cloud deck becomes optically thin, resulting in minimal variations in the atmospheric brightness temperature for 950 K < Teq < 2,100 K. This is indeed what is observed for the nightsides of warm giant exoplanets, which all have brightness temperatures of ~1,100 K… Meanwhile, the relatively high albedo of certain warm giant exoplanets such as Kepler-7b could also be explained by the dominance of silicate clouds, which are highly reflective at optical wavelengths.
The paper is Gao et al., “Aerosol composition of hot giant exoplanets dominated by silicates and hydrocarbon hazes,” Nature Astronomy 25 May 2020 (abstract).