Watery Atmosphere Discovered on Neptune-Like Exoplanet

The finding could provide an important breakthrough in understanding how planets form, and how atmospheres can vary between exoplanets with different masses.

While the number of known exoplanets continues to soar, uncovering specific details about these distant worlds has been more challenging. The delicate task of finding out the specifics of an exoplanet’s atmosphere can provide data about the planet’s potential habitability, as well as clues to how the exoplanet formed.

A new study of a Neptune-size world has revealed a surprisingly “primitive atmosphere” composed almost entirely of hydrogen and helium, with water vapor and possible clouds. Scientists say this finding could provide an important breakthrough in understanding how planets form, and how atmospheres can vary between exoplanets with different masses.

“Astronomers have just begun to investigate the atmospheres of these distant Neptune-mass planets, and almost right away we found an example that goes against the trend in our solar system,” said Hannah Wakeford from NASA’s Goddard Space Flight Center. “This kind of unexpected result is why I really love exploring the atmospheres of alien planets.”

Wakeford, along with David Sing from the University of Exeter and a team of other researchers, looked at the exoplanet HAT-P-26b, which was discovered in 2010 and is located about 430 light years from Earth. It is a “warm” world that orbits relatively close to its star, circling it every 4.23 days.

Using observations from the Hubble Space Telescope and the Spitzer Space Telescope, the researchers found strong indications of water as well as a possible cloud layer with a unique makeup in the atmosphere of HAT-P-26b.  

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“What we have detected is a strong water absorption feature with some evidence of a cloud layer deep in the atmosphere at the base altitude of the measurements we have made,” Wakeford told Seeker. “This cloud would not be made of water due to the high temperature of the atmosphere but would instead be more exotic and composed most likely of Na2S (di-sodium sulfide). At the altitudes we probed with these observations and the strong water vapor absorption signature, this is a relatively cloudless portion of the atmosphere.” 

Measuring the abundance of atmospheric water provides information about the proportion of elements in the atmosphere that are heavier than hydrogen and helium, a value astronomers refer to as the metallicity. Surprisingly, it was lower than expected, only about 4.8 times that of the Sun — less than that of Neptune and Uranus, for example, and closer to the value for Jupiter.

“HAT-P-26b is the first to buck the trend seen with our solar system and other exoplanets, which suggest that lower mass planets have higher metallicities,” Wakeford said via email. “As the first to buck this trend, HAT-P-26b will be an important marker in our understanding of planetary formation scenarios.”

When a distant planet transits, or passes in front its host star, starlight gets filtered through the planet’s atmosphere. Using a spectrometer, astronomers can measure the light to see what elements are present in the planet’s atmosphere.

“This is the strongest water absorption feature that has measured for an exoplanet of this size,” Wakeford said. “Additionally, what is most important is that we have been able to use this detection to approximate the overall metallicity of the atmosphere based on the water abundance, and compared to similar sized planets in our solar system we find that HAT-P-26b has a much lower metallicity.”

This suggests that it formed differently — perhaps closer to the star or late in the lifetime of the disk around the star — compared to Neptune.

In our own solar system, the metallicity in Jupiter, which is five times greater than the Sun, and Saturn (10 times), suggest that these “gas giants” are made almost entirely of hydrogen and helium. Neptune and Uranus, however, are richer in the heavier elements, with metallicities of about 100 times that of the sun.

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Scientists think this happened because, as the solar system was taking shape, Neptune and Uranus formed in a region toward the outskirts of the enormous disk of dust, gas, and debris that swirled around the immature sun.

As a result, they would have been bombarded with a lot of icy debris that was rich in heavier elements, while Jupiter and Saturn formed in a warmer part of the disk where there would be less icy debris.

The researchers say that this finding could have wide implications for how scientists think about the birth and development of planetary systems in distant galaxies.

“Measurements like this are important to provide insights into how planetary systems can form and evolve different from our own,” Wakeford said.

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