The study of a warm, Neptune-sized exoplanet with a water-laden atmosphere has revealed a few surprises for astronomers. 

According to a report published Friday in the journal Science, astronomers using the Hubble and Spitzer telescopes say exoplanet HAT-P-26b possesses water signatures but far less than expected when compared to the gas giants in our own solar system. This suggests, the scientists say, that the composition of the planet's atmosphere is the original hydrogen-helium mixture from which it was formed.  

Discovered by the Hungarian-made Automated Telescope (HAT) Network in 2011, HAT-P-26b is a large planet orbiting close to its host star that is twice as old as our own sun. The planet is 437 light years away and is a good subject for study because it only takes a little more than four days for the planet to complete an orbit.

The researchers, noting that the study is the best measurement of water to date on an exoplanet of this size, determined that HAT-P-26b’s atmosphere is relatively clear of clouds and has a strong water signature.

What makes the study of HAT-P-26b a unique opportunity for astronomers is the difference between our solar system's cold exoplanets compared to the warm atmosphere found on HAT-P-26b. 

“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,” lead author Hannah Wakeford, a postdoctoral researcher at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said in a press release. “This kind of unexpected result is why I really love exploring the atmospheres of alien planets.”

Astronomers used four transits seen by NASA’s Hubble Space Telescope and two seen by NASA’s Spitzer Space Telescope to study the exoplanet's atmosphere and determine what metals are present through a process called transmission spectroscopy. 

The study provided a precise measurement of water, the scientists note, which enabled them to analyze what other metals comprise HAT-P-26b's atmosphere.

For all studies of metallicities of stars and planets, which determines how rich a planet is in all elements heavier than hydrogen and helium, astronomers compare findings to the known composition of the sun. Jupiter has a metallicity about 2 to 5 times that of the sun, while Saturn has a metallicity of 10 times that of the sun. 

Further out, Neptune and Uranus' metallicities are 100 times that of the sun, which indicated to scientists that lower metallicities occur on larger planets in our solar system. Other exoplanets outside our solar system, notably planet HAT-P-11b and WASP-43b, confirmed that trend.

However, in comparison, the large HAT-P-26b was found to have a metallicity just 4.8 times that of the sun, which goes against that trend.

“This analysis shows that there is a lot more diversity in the atmospheres of these exoplanets than we were expecting, which is providing insight into how planets can form and evolve differently than in our solar system,” said David K. Sing of the University of Exeter and the second author of the paper. “I would say that has been a theme in the studies of exoplanets: researchers keep finding surprising diversity.”