How to find hidden oceans on distant worlds? Use chemistry

Sub-Neptunes are planets with diameters between 1.7 and 3.0 times Earth's. However, scientists believe that many sub-Neptunes may have thick atmospheres that could cloak rocky surfaces or liquid seas. Credit: NASA/JPL-Caltech
A new study has shown that the chemical composition of an exoplanet’s atmosphere can sometimes reveal whether the surface temperature is too high for liquid water.

Our solar system has two types of planets: small and rocky like Earth and large and gaseous like Neptune. Astronomers have discovered planets around other stars that are slightly smaller than Earth, but larger than Neptune. These planets might have rocky surfaces, liquid-water oceans or both. However, most likely they will be covered with thicker and more opaque atmospheres than Earth's.

The Astrophysical Journal Letters has accepted the new study. It shows how the chemistry in these atmospheres can reveal clues about the underside. For example, which planets are too warm to support liquid water oceans. This technique could be used to help scientists narrow down their search for potential habitable exoplanets. There are more than 4,500 confirmed exoplanets in our galaxy. Scientists estimate that there are hundreds of billions of other exoplanets.

NASA space telescopes with spectrometers are capable of revealing the chemical composition of exoplanets' atmospheres. Although a chemical profile of Earth would not be able reveal photos of humans or cows on its surface, it would show methane and carbon dioxide produced by mammals and oxygen produced from trees. These chemicals are not enough to indicate life. However, they can be combined to show that the planet may be habitable.

Scientists sometimes use terms such as hot Jupiter or sub-Neptune to describe the similarities and differences between exoplanets (planets outside our solar system) compared to planets within our solar system. This helps them understand the vast array of exoplanets. Credit: NASA/JPL-Caltech

This paper reveals which chemicals could point to hidden oceans in exoplanets with diameters between 1.7-3.5 times Earth's. These planets are often called "sub-Neptunes" because Neptune's diameter is four times that of Earth.

An atmosphere thickened on a sub-Neptune world would trap heat and raise temperature. The atmosphere will change its chemical composition if it reaches a certain temperature, typically around 1,430 degrees Fahrenheit (770 Celsius). The thermochemical equilibrium is achieved. Assuming that the planet's atmosphere consists mostly of hydrogen (which is common for gaseous exoplanets), methane, ammonia and carbon will dominate.

These chemicals would be absent in a cooler and thinner atmosphere, where thermochemical equilibrium is not possible. In such a case, the dominant forms carbon and nitrogen would be carbon dioxide plus molecules with two nitrogen atoms.

According to the study, a liquid-water ocean beneath the atmosphere would produce additional signs. This would include the absence of almost all stray ammonia which would be dissolved by the ocean. The pH (or acidity) of an ocean's water will determine how easily ammonia can be dissolved in water. Researchers found that the atmosphere should not contain ammonia if there is an ocean beneath.

Additionally, carbon dioxide would be higher than carbon monoxide in atmosphere. In contrast, carbon dioxide should be lower than carbon monoxide after thermochemical equilibrium.

Renyu Hu, a NASA researcher who conducted the study, said that if we saw the signatures for thermochemical equilibrium, it would be conclusive that the planet is too warm to be habitable. "Vice versa: If we don't see the signatures of thermochemical equilibrium, but also see signatures from gas dissolved in liquid-water oceans, then we would consider those to be strong indicators of habitability.

NASA's James Webb Space Telescope will launch Dec. 18. It will be equipped with a spectrometer that can study exoplanet atmospherics. Hu and other scientists are trying to predict what chemical profiles Webb will observe in these atmospheres, and what they might reveal about distant worlds. The observatory can detect signs of thermochemical equilibrium within sub-Neptune atmospherics, i.e. signs of hidden oceanas as described in the paper.

Webb will be discovering new planets and conducting more detailed studies on existing planets. This information could aid scientists in deciding which planets are worth further observations, particularly if they want to find life-supporting planets.

Hu stated that there is no direct observational evidence that would indicate the physical characteristics of sub-Neptunes. Many of them could have huge hydrogen atmospheres, while a number of others could still be "ocean planets." This paper should inspire many more observations to discover the truth.

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More information: Unveiling shrouded oceans on temperate sub-Neptunes via transit signatures of solubility equilibria vs. gas thermochemistry, arXiv:2108.04745 [astro-ph.EP] Journal information: Astrophysical Journal Letters Unveiling shrouded oceans on temperate sub-Neptunes via transit signatures of solubility equilibria vs. gas thermochemistry, arXiv:2108.04745 [astro-ph.EP]