In a study published recently in the Astrophysical Journal Letters, a team of astronomers analyzed spectroscopic atmospheric data from 19 exoplanets, ranging in size from mini-Neptunes to super-Jupiters and in temperature from nearly 68 to over 3,632 degrees Fahrenheit (20-2,000 degrees Celsius). They found that while water vapor is common on these exoplanets, the amounts were surprisingly lower than expected, while the amounts of other elements were consistent with expectations.

In our Solar System, the amount of carbon relative to hydrogen in the atmospheres of giant planets is significantly higher than that of the Sun.

This ‘super-solar’ abundance is thought to have originated when the planets were being formed, and 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 — especially oxygen, which is the most abundant element in the Universe after hydrogen and helium. This means that water, a dominant carrier of oxygen, is also expected to be overabundant in such atmospheres.

In the new study, Dr. Nikku Madhusudhan from the Institute of Astronomy at the University of Cambridge and colleagues used extensive spectroscopic data from space-based and ground-based telescopes, including the NASA/ESA Hubble Space Telescope, NASA’s Spitzer Space Telescope, ESO’s Very Large Telescope, and the Gran Telescopio Canarias.

The range of available observations, along with detailed computational models, statistical methods, and atomic properties of sodium and potassium, allowed the astronomers to obtain estimates of the chemical abundances in the exoplanet atmospheres across the sample.

They reported the abundance of water vapor in 14 of the 19 planets, and the abundance of sodium and potassium in six planets each.

Their results suggest a depletion of oxygen relative to other elements and provide chemical clues into how these exoplanets may have formed without 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,” Dr. Madhusudhan said.

“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,” said Luis Welbanks, a Ph.D. student at the University of Cambridge.

The findings show that different chemical elements can no longer be assumed to be equally abundant in planetary atmospheres, challenging assumptions in several theoretical models.

“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,” Dr. Madhusudhan said.

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