Water Common But Scare In Exoplanets, Challenging Planet Formation Theories

For a new study published in The Astrophysical Journal Letters, researchers used extensive data from space-based and ground-based telescopes as well as a range of available observations to obtain detailed measurements of the chemical and thermal properties of 19 exoplanets.

The exoplanets they observed vary in size from “mini Neptunes” to “super Jupiters” that have the mass of 600 Earths, and they also vary in atmospheric temperatures ranging from 20 degrees to over 2,000 degrees Celsius.

Based on their observations, the researchers reported that 14 out of the 19 exoplanets are rich in water vapor, while six are abundant in sodium and potassium. However, the signs of water vapor were found in measures that were lower than they expected. By comparison, the amounts of the other elements in the exoplanets were quite consistent with their expectations.

According to project leader Dr. Nikku Madhusudhan of the University of Cambridge, the results suggest patterns in exoplanets’ chemical composition, showing how diverse they can be. In addition, the results also provide clues as to how they were formed.

In our solar system's giant planets, for example, amounts of carbon relative to amounts of hydrogen are much higher than that of the sun. It is believed that this is because of the process called accretion where ice, rocks and other particles were brought to the planet when they were being formed.

The abundance of elements in giant exoplanets is also predicted to be similarly high, particularly oxygen, as it is the most abundant element in the universe after hydrogen and helium. As such, it follows that water should then be overly abundant in giant exoplanets since it is a dominant carrier of oxygen.

However, because the findings suggest a depletion of oxygen compared to other elements, it opens up the question of how the exoplanets may have formed without the accretion of ice.

Simply put, the results of the researchers’ observations show trends and patterns that challenge current theories about planet formation.

“Measuring the abundances of these chemicals in exoplanetary atmospheres is something extraordinary, considering that we have not been able to do the same for giant planets in our solar system yet, including Jupiter, our nearest gas giant neighbor,” lead author Luis Welbanks of the Institute of Astronomy said, further stating that if the water in Jupiter ends up being abundant, unlike the exoplanets in their study, then it could mean that it formed differently than the exoplanets.

“We look forward to increasing the size of our planet sample in future studies,” Madhusudhan said. “Given that water is a key ingredient to our notion of habitability on Earth, it is important to know how much water can be found in planetary systems beyond our own.”

The new study is the most extensive survey of exoplanets’ atmospheric chemical compositions to date.