Researchers from NASA, in collaboration with the University of Maryland, detected an unusual "warm Neptune" that has a primordial atmosphere consisting mostly of hydrogen and helium.

The exoplanet, dubbed as HAT-P-26b, is about 437 light-years away from Earth and orbits a star about twice as old as our Sun. NASA's observation, described in a paper published in the journal Science, showed that the warm atmosphere has a strong water atmosphere and is relatively clear of clouds.

"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 annah Wakeford, a postdoctoral researcher at NASA's Goddard Space Flight Center and lead author of the study, in a press release. "This kind of unexpected result is why I really love exploring the atmospheres of alien planets."

For the study, the researchers combined the observations from four transits of the planet measured by NASA's Hubble and Spitzer Telescope. Although HAT-P-26b is not considered as a water world, the observations indicate a strong water signature in its atmosphere. Planet HAT-P-26b is a warm Neptune, which means it has the same size as Neptune but orbits close to its parent star.

Unlike Uranus and Neptune, HAT-P-26b most likely formed either closer to its parent star, got closer during its development of its planetary system or a combination of both. Surprisingly, the metallicity of HAT-P-26b is only about 4.8 times that of the sun. Metallicity is an indication of how rich the planet is in all elements heavier than hydrogen and helium. It is measured by astronomers to provide some clues how the planet was formed.

Smaller gas giants located in the outer region of planetary system, such as Uranus and Neptune, have higher metallicity because they would have been bombarded with a lot of icy debris during planetary formation. On the other hand, inner gas giants like Jupiter and Saturn have lower metallicity because they formed in the warmer part of the accretion disk, making them less likely to encounter icy debris.