In astronomy, there is a concept called “degeneracy”. It has nothing to do with delinquent people, but instead is used to describe data that could be interpreted multiple ways. In some cases, that interpretation is translated into exciting new possibilities. But many times, when that happens, other, more mundane explanations are ignored for the publicity that the more interesting possibilities provide. That seems to have been the case for many “sub-Neptune” exoplanets discovered recently. Some theories have described them as Hycean worlds - worlds that are filled with water oceans or ice. But a new paper from Robb Calder of the University of Cambridge and his co-authors, available in pre-print on arXiv, shows that, most likely, these planets are almost all made of molten lava instead.
Much of this debate has to do with the chemical signatures found surrounding these planets. Perhaps most famously, planet K2-18 was proposed as a Hycean world due to its atmospheric chemistry. In close observational studies, prior research had found methane and carbon dioxide in the planet’s atmosphere, but not much ammonia. According to that research, this was a “smoking gun” for a liquid water ocean under a hydrogen atmosphere as water naturally dissolves ammonia.
But something else dissolves ammonia just as well - molten rock. So the lack of ammonia could equally be a telltale sign of liquid magma or liquid water. To prove their point, the researchers in the most recent paper attempt to model the thermal evolution of “gas dwarfs” - an alternative planetary composition that could fit the data collected about Hycean worlds just as well. The key question was whether or not these unique exoplanets, which don’t have an analogue in our own solar system, maintained the magma oceans during their evolutionary process, thereby having a distinct impact on the atmospheres as they mature.
Fraser showcases the problem with the idea of "Hycean" worlds.The simple answer to that question is yes - almost all of the sub-Neptune exoplanets we have discovered so far would have kept their magma oceans during their lifetime. According to the paper, 98% of them are just as easily explained by being magma worlds rather than Hycean ones.
To prove their point, the authors developed a metric they call the Solidification Shoreline. It maps the effective temperature of the star the planet is orbiting with the instellation flux, the amount of energy the atmosphere receives, of that same star. Instellation flux has a direct impact on the temperature of the atmosphere of an exoplanet, while the effective temperature of the star was simply something easily available for most of the stars they were analyzing the exoplanets of. A more detailed explanation would have included the envelope mass fraction - i.e. how much of the weight of the planet is contained in its atmosphere - but that data wasn’t available for many planets in the dataset.
The Solidification Shoreline provides a clear delineation to determine whether or not the planet receives enough heat to maintain a magma ocean. If a planet is “above” the shoreline, it does - if it's below it, its mantle will cool over time, making it no longer a lava world.
Fraser discusses a strange gap in the exoplanet population that we're still struggling to understand.To calculate where a planet fell on that line, the authors use the PROTEUS model that models a planet’s interior climate. They found that 98% of the thousands of sub-Neptunes they modeled, which are the most common type of exoplanet found so far, fell above the Shoreline, meaning they are likely magma worlds rather than Hycean ones.
While this may dash the hopes of many an astrobiologist who were looking to these worlds for potential signs of life, its a useful model to inform our understanding of the dynamics of planetary formation. There are complex interacting forces in the story of every exoplanet’s evolution, but sub-Neptunes in particular are difficult for us to grasp given that we can only see them from a distance. As our technology gets better, and we collect more data about these abundant exoplanets, perhaps we’ll be able to definitively find a solution to the degeneracy they pose. Until then, maybe it’s better not to hope for too many water worlds.
Learn More:
R. Calder et al - Most Rocky Sub-Neptunes are Molten: Mapping the Solidification Shoreline for Gas Dwarf Exoplanets
UT - Bad News And Good News: Hycean Worlds Aren't Real, But Earth's Water Isn't Unusual
UT - Another Hycean Planet Found? TOI-270 d
UT - If Hycean Worlds Really Exist, What are Their Oceans Like?
