The Super-Solar Carbon Abundance

In our solar system, the amount of carbon relative to hydrogen in the atmospheres of giant planets dwarfs the same ratio in the Sun. Referred to as the ‘super-solar’ abundance — the situation is believed to have arisen as large amounts of ice, rocks and other particles were brought into the planet in a process called accretion.

The abundances of other elements have been predicted to be similarly high in the atmospheres of giant exoplanets. This is particularly true for oxygen, the third most abundant element in the universe after hydrogen and helium. As a consequence, this means that water, a dominant carrier of oxygen, is also expected to be overabundant in such atmospheres.

The Hubble Space Telescope, shown here in orbit, was just one of the many sources of data that the team used to base their study upon. (NASA)

The team made use of extensive spectroscopic data collected by space-based and ground-based telescopes, including the Hubble Space Telescope, the Spitzer Space Telescope, the Very Large Telescope in Chile, and the Gran Telescopio Canarias in Spain. The range of available observations, along with detailed computational models, statistical methods, and atomic properties of sodium and potassium allowed the researchers to obtain estimates of the chemical abundances in the exoplanet atmospheres across the sample.

The study concluded that there was an abundance of water vapour in 14 of the 19 planets observed, and an abundance of sodium and potassium in six planets. This suggests a depletion of oxygen relative to other elements and grants researchers chemical clues as to how these exoplanets may have formed without a substantial accretion of ice.

“It is incredible to see such low water abundances in the atmospheres of a broad range of planets orbiting a variety of stars,” says Madhusudhan.

“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 neighbour,” adds Luis Welbanks, lead author of the study and PhD student at the Institute of Astronomy.

Various efforts to measure water in Jupiter’s atmosphere, including NASA’s current Juno mission, have proved challenging. “Since Jupiter is so cold any water vapour in its atmosphere would be condensed, making it difficult to measure,” explains Welbanks. “If the water abundance in Jupiter were found to be plentiful as predicted, it would imply that it formed in a different way to the exoplanets we looked at in the current study.”

The study is part of a five-year research programme designed to investigate the chemical compositions of exoplanets. As such, the team are clear about specific research goals for the future.

“We look forward to increasing the size of our planet sample in future studies,” says Madhusudhan. “Inevitably, we expect to find outliers to the current trends as well as measurements of other chemicals.

“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.”