Purple Haze: Alien Atmospheres Recreated In the Lab

By combining various gases with plasma, researchers are learning about the haze around distant planets

Saturn's hazy moon Titan NASA

Since the first exoplanets—planets outside our own Solar System—were discovered in 1992, astronomers have catalogued over 3,700 of them from stars all over the galaxy. In the last decade we’ve actually begun to “see” some exoplanets through various imaging technologies, revealing colorful clouds and hazes. The problem is, our experience with alien atmospheres is pitifully small and we don’t know what those hazes represent. That’s why in a new study, researchers recreated the atmospheres of alien worlds in the laboratory, giving them a model for understanding these hazy worlds, reports Marty Halton at the BBC.

According to a press release, our current telescopes are able to get a decent enough glimpse of some planets that we can use spectrometry to determine what the main elements in their atmospheres are. But when it comes to hazy-looking atmospheres, our instruments fail. That’s why researchers at Johns Hopkins University decided to try and simulate those atmospheres to understand them better.

The team first created computer models of various atmospheres that might be possible on two common classes of planets called super-Earths and mini-Neptunes, neither of which are found in our home solar system. By combining various ratios of carbon dioxide, hydrogen and gaseous water with helium, carbon monoxide, methane and nitrogen and modeling what happens to those combos at three sets of temperatures, they simulated the possible atmospheres of 9 hazy planets.

The team then created those atmospheres in the lab by flowing those gases into a plasma chamber to simulate interactions with solar wind, which reacts with gases in the atmosphere to create haze particles. Halton reports that some of the reactions were quite colorful, burning olive green and purple. The researchers collected the atmospheric particles deposited on quartz plates over the course of three days. The research appears in the journal Nature Astronomy.

Unlike clouds, which continually dissipate and reform, Sarah Hörst, lead author of the study, explains that haze is more of a one-way process. Both haze and clouds are made up of particles suspended in an atmosphere, she wrote in 2016, but haze particles build up in an atmosphere, where they can scatter light and affect temperature.

The next step is to analyze the haze particles created in the chamber to understand how they might interact with light and impact the temperature of a planet. The experiment doesn’t just apply to exoplanets. It could also give us some insight into hazy neighbors like Titan, Saturn’s moon, which is a candidate for supporting life. A 2013 study based on data from the Cassini spacecraft showed that Titan’s haze was produced by polycyclic aromatic hydrocarbons, the same substances that create haze from car exhaust (as well as burning coal or even wood) here on Earth. The study could help researchers understand how Titan’s haze impacts the moon and influences the possiblity of life on the hazy world.

“We’re really excited to figure out where particles form, what they’re made out of, and what that means for organic inventories for the origin of life,” Hörst tells Halton. “I think we are going to learn a lot about [our] Solar System from doing these experiments. We don't want to learn about just one planet; we want to learn how planets work.”

While the imaging of exoplanets is still relatively rare, that won’t be the case for long, and having some insight into the composition of hazy atmospheres will be useful. In 2019, the James Webb Space Telescope is scheduled to launch and will offer the best glimpses of exoplanets yet, and in the 2020s a new generation of ground-based telescopes, like the Giant Magellan Telescope, will also come online.

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