A pot of boiling lava, sealed with a thick lid of gas. That's roughly what sub-Neptunes look like — planets that are common around other stars but absent in our own solar system. Beneath a thick hydrogen atmosphere sloshes a molten magma ocean. And it's not silent: chemicals from the interior seep into the air, leaving their 'fingerprints' behind.
Astronomers have long been able to spot these fingerprints, absorption lines in the spectrum, using telescopes like JWST. But until now, there was no way to precisely translate those lines into what's happening in the ocean itself. The new tool MELTYQ is the first to directly connect light passing through the atmosphere to magma composition. It accounts for how gases (water, carbon monoxide, carbon dioxide, ammonia) dissolve in the melt and how oxidation-reduction reactions alter their abundance in the air.
Tests on real planets K2-18 b and TOI-270 d show that the method can reproduce their observed spectra. But there are caveats. For instance, for TOI-270 d, it couldn't fully capture the CO/CO₂ absorption feature at 4.5 microns, like a translator stumbling over a complex phrase. And so far, more flexible models not tied to magma physics fit the data slightly better. So MELTYQ is a first step, not the last.
Thus, a spectrum from deep space becomes a recipe for a planetary 'pot.' Now we at least know what questions to ask.
Sub-Neptunes are the most common type of planet in the galaxy, yet none exist in our solar system. The closest analog is Neptune, but it's far larger and colder.
In Stanislaw Lem's novel 'Solaris,' the planet's ocean was alive. Our magma oceans are most likely lifeless, but their chemistry can reveal hidden conditions.