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Cloud Atlas created by Astronomers for hot, Jupiter like exoplanets

This model will help the astronomers in studying the gases in the atmospheres of distant and strange worlds as clouds interfere in getting the measurement of the atmospheric composition.

May 27, 2020 15:14 IST
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In our solar system, giant planets and circling other stars have exotic clouds that are unlike anything on Earth, and these gas giants that have been orbiting close to their stars-so called hot Jupiters- boast the most extreme.

A team of astronomers from Canada, United States, and the United Kingdom have now come up with a model that will be able to predict which of the many types of clouds, from smoggy methane haze to sapphire, can be expected on hot Jupiters of different temperatures, which can be up to thousands of degrees kelvin.

This model will help the astronomers in studying the gases in the atmospheres of distant and strange worlds as clouds interfere in getting the measurement of the atmospheric composition. It will also be helpful in understanding the atmospheres of cooler giant planets as well as their moons, such as Saturn and Jupiter’s moon Titan.

Study on the exotic clouds:

The most common type of cloud that can be expected over a large range of temperatures must consist of solid or liquid droplets of Oxygen and Silicon, like melted quartz or molten sand. Skies on the cooler hot Jupiters that are below 950 kelvin are being dominated by a hydrocarbon haze, essentially smog. The model developed by the astronomers will help them in studying gases in the atmospheres of these distant and strange worlds.

As per Peter Gao, A post-doctoral fellow at the University of California and the first author of the paper that describes the model that appeared on May 25 in the Journal Nature Astronomy, mentions that the kind of clouds that can be existing in these hot atmospheres cannot be really considered as clouds in the solar system.

He further states that there have been models that were able to predict various compositions but the main point of the study was basically to assess which of the compositions actually matters and then to compare the model to the available data that we have.

How this study takes advantage of the previous studies of exoplanet atmospheres?

The study has taken advantage of a boom over the past decade in the study of exoplanets atmospheres. Even though exoplanets are too distant and dim to be visible, many telescopes and in particular, Hubble Space Telescope has been able to focus on the stars and captured the starlight passing through the atmospheres of planets as they pass in front of their stars.

The wavelength of lights that have been absorbed, revealed by spectroscopic measurements, informs astronomers which of the elements make up the atmosphere. This technique and others, to date, have found the presence of water, carbon monoxide and carbon dioxide, methane, potassium, and sodium gases, and in the hottest planets, vaporized aluminium oxide, titanium, and iron.

It has been observed that while some planets seem to have clear spectroscopic features and clear atmospheres, many of them have clouds that completely blocks the starlight filtering through, it prevents the study of gases below the upper cloud layers. The compositions of the gases will be able to tell astronomers how exoplanets form and whether the building blocks of life have been present around the stars.

As per Gao, many clouds have been founded: some kind of particles- that are not molecules but small droplets- have been hanging out in these atmospheres. He further added that it is not really known what they are made of but they have been contaminating the observations and makes it more difficult to assess the composition and abundances of important molecules, such as methane and water.

Proposal for many strange types of clouds:

In order to explain these observations, astronomers proposed many strange types of clouds, which are composed of aluminium oxides, molten salt such as potassium chloride; such as corundum, the stuff of sapphires and rubies; sulfides of manganese or zinc that exist as a rock on earth; silicon oxides or silicates like quartz, the main component of sand; and organic hydrocarbon compounds. As per Gao, clouds can be solid or liquid aerosols.

What the model reveals:

Gao had adapted computer models that were initially had been created Earth’s water clouds and subsequently extended to the cloudy atmospheres of planets such as Jupiter that has methane and ammonia clouds.

He further expanded the model to the much higher temperatures that have been seen on hot gas giant planets- up to 2,800 kelvin or 4,600 degrees Fahrenheit- and the elements that have been likely to condense into clouds at these temperatures.

The model then takes into account how gases of various atoms or molecules condense into the droplets, in what way these droplets grow or evaporate and whether they sink because of gravity or they have been likely to be transported in the atmosphere by winds or updrafts.

Gao states the idea is that the same physical principles guide the formation of all forms of clouds. He further mentions that what he has done is to take this model and bring it out to the rest of the galaxy, so that it is able to simulate iron clouds, silicate clouds, and salt clouds. He further compared his predictions from the data available on 30 exoplanets. They are out of a total of about 70 transiting exoplanets with recorded transmission spectra to date.

The model has revealed that many of the exotic clouds proposed over the years are difficult to form as the energy required to condense the gases is too high.

As per the model, aluminium oxides and titanium oxides condense into high-level clouds in the hottest temperatures. In exoplanets of cooler temperature, the clouds form deeper in the planet and are obscured by higher silicate clouds. On much cooler exoplanets, these silicate clouds also form deeper in the atmosphere that leaves a clear upper atmosphere. At even cooler temperatures, ultraviolet light coming from exoplanet’s star converts organic molecules like methane into extremely long hydrocarbon chains that form a high-level haze akin the smog. This smog can be obscuring lower-lying salt clouds of sodium chloride or potassium.

Gao has suggested focusing on the planets between 900 and 1400 kelvin, or those hotter than about 2,200 kelvin, for those astronomers who seek a cloudless planet for more easily studying the gases in the atmosphere.

Future observations can confirm the predictions:

Co-Author of the study, Hannah Wakeford, an astrophysicist at the University of Bristol, UK mentions that the presence of clouds has been measured in a number of exoplanets atmospheres before but when we look collectively at a large sample then we can pick apart the physics and chemistry in the atmospheres of these worlds. She further states, that the dominant cloud species is as common as sand and it will be really exciting to measure the spectral signatures of the clouds themselves for the first time with the help of the upcoming James Webb Space Telescope (JWST).

Future observations such as by NASA’s JWST that have been scheduled for launch within a few years will be able to confirm these predictions and perhaps even shed light on the hidden cloud layers of planets that are closer to home. Gao had mentioned that similar exotic clouds may exist at depths within Jupiter or Saturn where the temperatures are close as found on hot Jupiters.

Gao explains that since there are thousands of exoplanets versus one Jupiter, we can easily study a bunch of them and observe what the average is and how that is compared to Jupiter. Gao and his colleagues also plan to test the model against the observational data from other exoplanets as well as from brown dwarfs, which are gas giant planets that are so massive that they are almost stars and they too have clouds.

Jonathan Fortney of UC Santa Cruz states that in studying planetary atmospheres in the solar system, the researchers typically have the context of images but we have no such luck with exoplanets. They are just shadows or dots which is a huge loss of information. He further mentioned that to make up for that we have a much larger sample size. It allows to look for trends- a trend in cloudiness-with planetary temperature which is something that we don’t have a luxury of in our solar system.

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