Back to the wide world of exoplanets. Oh, so many exoplanets. Thousands of them have already been discovered and confirmed, and yet, one of the most common types we have counted in the data is not found in our own solar system – the sub-Neptune – worlds that are 1.7 to 3.5 times larger than Earth but still smaller than Neptune. These are interesting planets that fall between the category of rocky world and ice or gas giant. They may have rocky surfaces. They may even have liquid-water oceans. And they’re probably surrounded by thick, opaque atmospheres.

Now, in a new study published in The Astrophysical Journal Letters, a team of researchers has shown that they can use the chemistry of those thick atmospheres to find out which exoplanets are too hot to support possible liquid-water oceans. That could help us narrow down the search for life beyond our solar system, since liquid water is necessary for life as we know it. I repeat: this is about life as we know it. Let’s find possible life that way before we worry about life as we don’t know it, please.

So how do we go about figuring out the composition of an atmosphere? The best way to do it from here on Earth, since we cannot sample that actual atmosphere, is to use spectroscopy. With spectroscopy, we peer at the light coming from an object, and we break that light into a spectrum. Think of a rainbow smeared out on a paper test strip, and you’ll have the right idea. Now there may be some gaps in that spectrum — black lines where color should be. These are absorption lines, and they show us what molecules are present in the material we’ve studied.

Every atom has a spectral signature — a set of absorption lines it creates based on how many electrons it has and how those electrons are excited when heated up. Molecules also have specific sets of lines based on their atoms, so when we look at a distant atmosphere that is being heated by its parent star, we can read the molecules in the spectra. Neat, huh?

This is an incredibly useful method that can be used on basically anything that radiates heat in some fashion. We’ve even used it to look at the composition of meteors as they burn up in our atmosphere and leave a trail of hot gas behind them.

Several space telescopes have spectrometers on them, including Hubble. So while we cannot get pictures of what’s on the surface of a planet, we can get a sense of the atmospheric composition. Here on Earth, we’d see carbon dioxide and methane, and oxygen. Individually, we could explain each of those molecules without involving life, but the combination of them could lead us to the possibility of life on another world.

Back to those sub-Neptunes and their thick atmospheres. Per the press release: “A thick atmosphere on a sub-Neptune planet would trap heat on the surface and raise the temperature. If the atmosphere reaches a certain threshold – typically about 770 degrees Celsius – it will undergo a process called thermochemical equilibrium that changes its chemical profile. After thermochemical equilibrium occurs – and assuming the planet’s atmosphere is composed mostly of hydrogen, which is typical for gaseous exoplanets – carbon and nitrogen will predominantly be in the form of methane and ammonia.

In a cooler, thinner atmosphere, thermochemical equilibrium would not occur, and a spectrum would reveal carbon dioxide and nitrogen gas instead. Additionally, a liquid-water ocean would change the composition by removing any ammonia, which is highly soluble in water. Lead author Renyu Hu explains: If we see the signatures of thermochemical equilibrium, we would conclude that the planet is too hot to be habitable. Vice versa, if we do not see the signature of thermochemical equilibrium and also see signatures of gas dissolved in a liquid-water ocean, we would take those as a strong indication of habitability.

More Information

NASA JPL press release

“Unveiling shrouded oceans on temperate sub-Neptunes via transit signatures of solubility equilibria vs. gas thermochemistry,” Renyu Hu et al., to be published in The Astrophysical Journal Letters (preprint on arxiv.org)