Reveal hidden alien oceans, with chemistry

Sub-Neptunes are planets smaller than Neptune but larger than Earth. They are generally between 1.7 and 3.5 times the diameter of the Earth. A new study from NASA says astronomers can detect the oceans on some of these worlds by analyzing the chemistry of their atmospheres. Image via NASA / JPL-Caltech.

Our planet Earth is the only world in our solar system with liquid water on its surface. In this solar system, the oceans of Earth are unique. But scientists believe there are many more ocean worlds elsewhere in our Milky Way galaxy. At the end of October 2021, NASA published a new study suggesting that scientists can find alien oceans hidden on distant exoplanets through the use of chemistry. The study showed that, on worlds that have oceans, the chemical composition of the atmosphere is markedly different from that of worlds without oceans on their surface.

The new peer-reviewed research paper has been published in the Letters from the astrophysical journal October 28. It is also available as a free preprint on arXiv.

Use chemistry to find hidden alien oceans

The new study proposes that astronomers can detect oceans on exoplanets by analyzing the chemistry of their atmospheres. Generally, this could apply to Earth-sized worlds, super-Earths, or even some sub-Neptunes (any planet with a radius smaller than Neptune but larger than Earth). The article focuses on planets with diameters between 1.7 and 3.5 times the Earth. Telescopes equipped with spectrometers, including the future James Webb Space Telescope (JWST), can identify the chemical makeup of the atmosphere of some of these planets. They can find gases such as oxygen, carbon dioxide or methane, which could be clues to life.

It’s exciting, but such chemical analyzes can reveal other things about these planets as well. He could find evidence for the oceans, which also has big implications for habitability, of course.

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Too hot for liquid water or just enough?

At least in some cases, these telescopes can help determine if there is liquid water on the surface of any of these planets. Specifically, by analyzing chemicals in the atmosphere, scientists can estimate whether the surface temperature is too high for liquid water.

So what chemicals could indicate an ocean under the clouds? One discovery would be carbon dioxide and nitrogen in the atmosphere, where nitrogen molecules are made up of two nitrogen atoms. Why is this important? This would be proof that the planet’s atmosphere is cooler and thinner, that is to say. more like those of terrestrial planets like Earth. This would indicate that thermochemical equilibrium (chemical equilibrium) has not occurred on this planet.

In thermochemical equilibrium, the chemistry of the atmosphere is altered. This happens when the planet’s atmosphere is mostly hydrogen, which is common for sub-Neptune worlds. In these cases, the carbon and nitrogen are in the form of methane and ammonia, and the atmosphere is significantly thicker.

In these scenarios, the thick atmosphere, like that of gas and ice giants in our solar system, traps heat. Thermochemical equilibrium will occur when the temperature reaches 1430 degrees F (770 degrees C). It’s too hot to support oceans of liquid water.

Smiling man with glasses and suit on plain background.
The new study of the oceans of exoplanets was led by Renyu Hu at NASA’s Jet Propulsion Laboratory. Image via JPL-Caltech.

Lack of ammonia

Another key indicator of possible oceans is that something is faded away in the atmosphere: ammonia. Because ammonia is very soluble in water, it is said to be almost nonexistent on oceanic planets. So planets with massive oceans should hardly have ammonia in their atmosphere. The pH (acidity) level of the ocean, however, would also affect how much ammonia is still present, if any.

In addition, there should be more carbon dioxide than carbon monoxide on the ocean worlds. If there was both a lack of ammonia and an excess of carbon dioxide in a planet’s atmosphere, that would be convincing proof of an oceanic world. As Renyu Hu of NASA’s Jet Propulsion Laboratory (JPL), who led the new study, said:

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 dissolved gases in an ocean of liquid water, we would regard these as a strong indication of habitability.

Hu continued:

We do not have direct observational evidence to tell us what the common physical characteristics of sub-Neptunes are. Many of them may have massive hydrogen atmospheres, but quite a few could still be “ocean planets”. Hope this article motivates many more observations in the near future to find out.

Alien oceans hidden on giant planets with a big sun.
Artist’s concept of a sub-Neptune planet, with a global ocean. Image via Amanda Smith / University of Cambridge.

Future observations

NASA’s James Webb Space Telescope (JWST), scheduled to launch on December 18, will have a spectrometer capable of analyzing the atmospheres of some of these worlds. As stated in the document:

These gases lead to distinctive features in the planet’s transmission spectrum, and a moderate number of repeated transit observations with the James Webb Space Telescope should distinguish a small atmosphere from a massive atmosphere on planets like K2-18. b. This method thus provides a means of using short-term facilities to constrain the atmospheric mass and habitability of temperate sub-Neptune exoplanets.

In other words, JWST will be able to identify signs in the atmosphere that may reveal planets supporting the oceans. It will be exciting to see what JWST finds in the months and years to come!

Bottom line: NASA scientists say that to find alien oceans hidden on distant exoplanets, use chemistry. Oceanic worlds will have markedly different atmospheric compositions than planets that do not have an ocean, including a lack of ammonia.

Source: Unveiling of enveloped oceans on temperate sub-Neptunes via transit signatures of solubility equilibria with respect to thermochemistry of gases, preprint


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