It’s 2134, and humanity has finally embraced green technologies while ridding the Earth of harmful fossil-burning technologies, most notably gasoline, wood, coal, and oil. As a result, soot has been rendered obsolete, and all commercial products from soot, including shoes, wires, computer products, and eye products, are now produced from eco-friendly technologies. However, the uber-rich who still fancy non-eco-friendly products are willing to pay soot’s weight in gold for it. Therefore, the Exoplanet Research Corporation outfits its best ship to search for soot-enriched exoplanet atmospheres.

Back in the present day, a team of researchers recently explored how a class of exoplanets called sub-Neptunes could possess soot-like atmospheres, challenging past studies regarding the atmospheric compositions of sub-Neptunes potentially involving methane. With their findings recently published in The Astrophysical Journal Letters, the researchers aspired to ascertain the atmospheric compositions of sub-Neptunes whose equilibrium temperatures range between approximately 500-800 Kelvin (227-527 degrees Celsius/441-981 degrees Fahrenheit). In planetary science, a planet’s equilibrium temperature is the theoretical temperature a planet would be if it would heated only by its host star.

For the study, the researchers used a series of computer models to simulate the production of polycyclic aromatic hydrocarbons (PAHs) in exoplanet atmospheres, with PAHs being found inside and on the surface of soot, as the latter acts as a sponge for PAHs. Along with equilibrium temperature, the team incorporated the ratio of carbon to oxygen (C/O) and metallicities within the atmospheres.

In the end, the researchers found that sub-Neptune upper atmospheres function like massive combustion engines, referring to them as “soot factories”, which the atmosphere transports the PAHs to the upper atmosphere where NASA’s James Webb Space Telescope (JWST) could potentially use its powerful instruments to observe them. The team also notes the number of PAHs peaked at 600 K but decreased with both higher and lower equilibrium temperatures, along with PAH numbers varying with different C/O and metallicities.

“As far as I know, this is the first time anyone has applied chemical engineering to the field of exoplanet study,” said Dr. Jeehyun Yang, who is a postdoctoral scholar at the University of Chicago and lead author of the study. “I think it’s a great case study that shows why having people from all different backgrounds can help us untangle these mysteries.”

The researchers discussed how several sub-Neptune exoplanets fit within the data from their findings, with this exoplanet data being obtained by JWST and NASA’s Hubble Space Telescope. These exoplanets include GJ 436 b (32 light-years), GJ 1214 b (48 light-years), HD 97658 b (70 light-years), GJ 3090 b (73 light-years), LP 791-18 c (86 light-years), TOI-836 c (90 light-years), GJ 9827 d (97 light-years), GJ 3470 b (100 light-years), TOI-674 b (150 light-years). The researchers note in their conclusions that GJ 1214 b holds the most promise for hosting a soot-producing atmosphere given its equilibrium temperature, C/O ratio, and metallicity.

Located approximately 48 light-years away, GJ 1214 b is estimated to have a mass and radius of approximately 6.26 and 2.74 of Earth, respectively. It orbits its red dwarf star in approximately 1.58 days, with red dwarf stars being smaller and cooler than our sun. Given its extremely close distance to its star, GJ 1214 b is tidally locked, meaning it always has one side facing its star. As a result, JWST has measured significant temperature differences between the dayside and night side of the exoplanet, indicating GJ 1214 b doesn’t successfully transfer heat within its atmosphere. Finally, its equilibrium temperature is approximately 550 K, which falls in the range of the study’s parameters, along with its atmospheric composition having a high-metallicity content.

What new insights into exoplanet “soot factories” will researchers make in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!