Researchers Create Simulated Exoplanet Atmospheres

Using a model for exoplanets of various sizes and orbiting different types of stars, Johns Hopkins University scientists show the type — and number — of molecules that are likely to form in an atmosphere.

Illustration of the exoplanet Proxima B, orbiting Proxima Centauri, a Red Dwarf Star | Education Images/UIG via Getty Images
Illustration of the exoplanet Proxima B, orbiting Proxima Centauri, a Red Dwarf Star | Education Images/UIG via Getty Images

Scientists recently created a simulation of an exoplanet atmosphere, with an eye toward gaining a better understanding of how hazes form in extraterrestrial environments. One major goal is to help researchers using the upcoming James Webb Space Telescope, which launches in 2019, better interpret the observations from Webb's detailed exoplanet examinations.

It's an exciting direction for haze science, said principal investigator and professor at Johns Hopkins University Sarah Hörst. Most work on haze formation focuses on Titan, an orange-shrouded moon of Saturn that could host molecules key to the formation of life. A few other studies have examined Jupiter. Exoplanet hazes, however, are just recently becoming a research topic.

"We assume extrasolar planets have atmospheres that will be different from our solar system, in terms of temperature and in terms of the gasses in the atmosphere," Hörst told Seeker. "We wanted to start thinking outside the solar system in terms of trying to understand how haze formation works.

A paper based on the research was published March 5 in the journal Nature Astronomy.

Scientists have discovered thousands of exoplanets, principally using the Kepler space telescope that launched in 2009.

A handful of these planets are close enough and bright enough for other telescopes to pick up their atmospheric spectra, which shows the composition of their atmosphere. From these observations, Hörst said, scientists know that many exoplanets have some kind of particles in their atmosphere.

What's unclear is under what conditions hazes form. From Titan, scientists know that cold nitrogen and methane atmospheres have favorable conditions for haze formation, but it is difficult to extrapolate that information to different-sized planets or planets orbiting stars unlike our own, Hörst said.

"There are lots of kinds of atmospheres that have never been simulated in a lab before, and that's overwhelming," she added. "So we were trying to figure out what we could do that would be the most impactful, in terms of a starting point."

Lead author Sarah Hörst, right, and assistant research scientist Chao He examine a sample of simulated exoplanet atmosphere created in the chamber behind them. | Will Kirk/JHU

A starting point came from the Kepler data. Its surveys suggest that the most abundant type of planet is something that isn't known in our solar system — a "super-Earth" or "mini-Neptune," which refers to a class of planets approximately ranging from four to 10 times the size of Earth.

Better yet, these super-Earths or mini-Neptunes are large enough that Webb can confirm the lab measurements once it is in space. And hazes are commonly observed around these planets so far. "The handful of examples we do have [from other telescopes] so far frequently show the signatures of particles in their atmospheres. It could be a cloud layer, or it could be chemically generated haze," Hörst said.

The researchers used a chemical equilibrium model that has been used since at least the 1990s.

"If you know the temperature, and how many heavy atoms the atmosphere might have, [the] model calculates what the equilibrium atmosphere would be," Hörst said.

The model accepts as an input any kind of energy output from a star, but neglects processes in the atmosphere such as photochemistry, or the effect a star has on individual gas molecules. (There aren't any clouds in the model, either.) The model's output shows how many — and what kind — of molecules a particular planetary atmosphere would have.

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It was hard to pick a temperature range because of the large diversity of planetary systems, Hörst said, but the recent discovery of seven rocky planets near the dim, low-energy TRAPPIST-1 star influenced the team's choice. They modeled planetary atmospheres at an equilibrium temperature of 300 Kelvin (26 degrees Celsius, 80 degrees Fahrenheit), 400K (126°C, 260°F), and 600K (325°C, 620°F).

The temperatures were chosen to cover a range of heavy atoms, also referred to as metallicity, Hörst said. Between 300K and 600K, the range of possible atmospheres goes from hydrogen-rich planets like Jupiter or Saturn, to planets that are rich with water or carbon dioxide, which spans almost all the known types of atmospheres, Hörst said.

The researchers limited their models to nine planets (three temperatures per planet, with three different kinds of stars), and determined the equilibrium compositions for each of these atmospheres. They then took the gas composition and simulated them in a Johns Hopkins facility called the Planetary Haze Research chamber. The gas mixtures were exposed to a cold plasma for 72 hours to push them out of equilibrium. Then the chamber was put into an oxygen-free glove box to remove and weigh the gas samples.

The results showed particles in all of the simulated planetary atmospheres. The cooler planets (300K and 400K) that were dominated by water and methane had haze production rates that were higher than Titan's, the researchers said.

But there is more work to be done.

Hörst's team has another paper in review that simulates exoplanet chemistry using ultraviolet photons from a lamp. The team is also looking forward to the first observations from Webb.

"At that point, we'll discover that we're all wrong," Hörst joked. But she added the telescope will be a valuable contribution to the lab experiments because it will provide more data on hazes in exoplanets.