According to astronomers, water worlds, though admittedly not those containing Kevin Costner, are one of the most common types of planets in our solar system. This is partly due to low density estimates and the abundance of water ice past the “snow line” orbit of a star. But a new paper led by Jie Li and their colleagues at the University of MIchigan, suggests there might be an alternative type of planet that fits the density data but is made up of a completely different type of material – soot.
A Soot Planet isn’t just a giant ball of black powder. “Soot” in the context of astronomy isn’t just the traditional material most people think of coming out of fire. In technical science jargon, it means “refractory organic carbon”, an organic carbon compound rich in carbon, hydrogen, oxygen, and nitrogen that commonly goes by the acronym CHON. Soot is actually common in our solar system, with some estimates putting it at 40% of the total mass of comets.
Given that comets are typically viewed as glimpses into the solar system’s past, especially during its protoplanetary days, that would imply that soot was abundant while planets were being formed. And, the researchers suggest, there might be a similar “soot line”, which is much further in than the more common “snow line”, to delineate the point past which soot would remain stable and could form large parts of any planet forming in that region.
Fraser discusses Water Worlds and how life might form on them.
In fact, according to the paper, there would be three distinct zones of protoplanetary discs, each giving birth to a unique type of planet. The inner zone would only result in rocky works, like Earth and Mars, and it would be too hot for the soot to stay together, making “soot” in this area very unlikely. Past the “soot line” but before the “snow line”, planets could form that were mainly composed of soot, but with very little water as it would still be too hot for water ice to exist in this area. These planets would look a lot like TItan, with methane atmospheres or something equivalent, and could be made up of as much as 25% soot by mass. Farther out past the “snow line”, most planets would be a combination “soot-water world”, where soot would still play a large role in the composition of the planet, but water would as well. In fact, the paper models two different types of soot-water worlds, a “dry” version that was only 25% water, and a “wet” one that contains 50% water, both of which would still contain 15-20% soot in their compositions.
Those models show a particularly interesting feature – based on the mass-radius relationships it is impossible to tell apart soot worlds and more traditional water worlds. In other words, many of the “mini-Neptunes” in the exoplanet catalog that were originally thought to be water worlds could actually be composed of carbon-rich materials rather than water. It would take looking at their actual atmospheres to determine which category they belonged to.
The James Webb Space Telescope has already started doing so for some exoplanets. It’s detected methane and carbon dioxide in the atmosphere of K2-12b and TOI-280d, two “sub-Neptunes” that, while they are currently located within the soot line for their respective stars, might have formed outside of it and migrated inward over their lifetimes. In particular, TOI-280d has a notably high carbon to oxygen ratio, indicating that it might be a soot planet a described in the paper.
These types of planets have interesting implications for habitability. They could have diamond cores, which would slow the cycling of volatiles in the planet’s mantle and have a much harder time providing a magnetic field to protect any primitive life from cosmic rays. However, they would also be flush with methane and other volatile organics, which are thought to be prerequisites for prebiotic chemistry.
Fraser discusses the discovery of methane, thought to be one of the main components of Soot Worlds, on exoplanet WASP-80b.
Ultimately understanding the fate of many of these planets will require – you guessed it – more data. Atmospheric checks as well as more detailed models of ways to differentiate between water worlds and soot worlds need to be explored and delineated. While astronomers get to work on doing that, maybe an enterprising film director can pitch a movie about a character sailing an exoplanet’s methane seas in an insulated boat. Kevin Costner’s still actively acting after all.
Learn More:
J. Li et al – Soot Planets instead of Water Worlds
UT – A New Place to Search for Habitable Planets: “The Soot Line.”
UT – Icy Comets Can Alter Exoplanet Atmospheres and Shape Habitability
UT – Why Land Detection Is Critical for Confirming Exoplanetary Life