There’s growing evidence for water worlds in our galaxy, worlds even richer in water than Earth. Some distant planets might be true water worlds, completely covered in deep oceans. How can we learn more about them? In late November 2023, scientists at Johns Hopkins University in Baltimore, Maryland, said that haze in the planets’ atmospheres might help. These scientists used a computer model to simulate different kinds of haze on water worlds, with the idea of helping other scientists learn how these worlds form and evolve. They said their study might also provide clues about the habitability – the potential to sustain any form of life, even microbial life – of water worlds.
The researchers published their peer-reviewed paper on November 27 in Nature Astronomy. There is a free preprint of the paper available on arXiv as well.
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Water worlds and the search for life
Water is essential for life, at least life as we know it. Therefore, water worlds are an intriguing target for astronomers. Lead author Chao He at Johns Hopkins said:
Water is the first thing we look for when we’re trying to see if a planet is habitable, and there are already exciting observations of water in exoplanet atmospheres.
But haze can be a problem. It can hamper the view of telescopes, both on the ground and in space. He added:
But our experiments and modeling suggest these planets most likely also contain haze. This haze really complicates our observations, as it clouds our view of an exoplanet’s atmospheric chemistry and molecular features.
The paper also noted that:
Previous laboratory studies have shown that water-rich atmospheres are likely to result in organic haze formation. Opacity from organic hazes can mask spectral features from water and other gases.
How haze on water worlds affects habitability
Haze – solid particles suspended in gas – can not only interfere with astronomers’ observations, it can have an effect on the habitability of the planet itself. It can influence global temperatures and affect how much starlight reaches the planet’s surface. Both of those factors can have an impact on biological activity, either positive or negative.
To try to determine if a water world planet might be habitable, computer modelling is needed. Co-author Sarah Hörst, also at Johns Hopkins, said:
The big picture is whether there is life outside the solar system, but trying to answer that kind of question requires really detailed modeling of all different types, specifically in planets with lots of water. This has been a huge challenge because we just don’t have the lab work to do that, so we are trying to use these new lab techniques to get more out of the data that we’re taking in with all these big, fancy telescopes.
Simulating alien atmospheres
So, how did the researchers simulate the possible atmospheres of water worlds? They used two mixtures of gas containing water vapor and other compounds scientists think are common on exoplanets. Then, the researchers used ultraviolet light to simulate the ultraviolet light coming from the planets’ host stars. It is that light that can react with the a planet’s atmosphere to produce haze. Then, in turn, the researchers wanted to know how the haze particles would react with light in a planet’s atmosphere. To do that, they measured how much light the particles absorbed or reflected.
Further, the research team also compared the data to an actual known exoplanet called GJ 1214 b. It was a match!
Titan as an analog for haze on water worlds
The researchers also used Saturn’s moon Titan’s atmosphere – which is so hazy it completely obscures the surface – as an analog. The paper said:
Because the optical properties of exoplanet organic hazes are not yet known, the optical constants of the Titan haze analog (produced with 10% CH4 in N2) from Khare et al. (1984) and of soots (carbonaceous particles formed from incomplete combustion of hydrocarbons) are widely used to generate models and interpret observations.
There are some limits, however, as the paper explained:
In reality, the organic hazes formed in diverse atmospheres are expected to have a variety of compositions and therefore diverse optical properties. The optical properties of Khare’s Titan haze analogue or soots simply cannot represent the wide variety of atmospheric hazes we anticipate on exoplanets. It is therefore necessary to measure the optical properties of organic haze analogs formed over a broad range of simulated atmospheric conditions, given that JWST has begun to deliver unprecedented observations of various exoplanets. Exoplanets are expected to exhibit a wide diversity of atmospheric compositions.
Overall, the results showed how different kinds of hazes can cause astronomers to misinterpret what the atmosphere of a planet as a whole is actually like. They can “cloud the data” so to speak, leading to errors in analysis.
Further study of haze on water worlds
Now, the researchers are working to create additional haze analogs. As Hörst explained:
People will be able to use that data when they model those atmospheres to try to understand things like what the temperature is like in the atmosphere and the surface of that planet, whether there are clouds, how high they are and what they are made of, or how fast the winds go. All those kinds of things can help us really focus our attention on specific planets and make our experiments unique instead of just running generalized tests when trying to understand the big picture.
How many water worlds are there in our galaxy? Could any of them support life? The study of atmospheric hazes on these exotic worlds, and more observations, will help to answer such fascinating questions.
Bottom line: Researchers at Johns Hopkins are simulating haze on water worlds. The haze muddles observations but can also help determine if an exoplanet is habitable.
Source: Optical properties of organic haze analogues in water-rich exoplanet atmospheres observable with JWST
Source (preprint): Optical Properties of Organic Haze Analogues in Water-rich Exoplanet Atmospheres Observable with JWST
Via Johns Hopkins University
Read more: Habitable water worlds don’t have to be Earth-like
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