'Warm Neptune' Has Unexpectedly Primitive Atmosphere

A study combining observations from NASA's Hubble and Spitzer space telescopes reveals that the distant planet HAT-P-26b has a primitive atmosphere composed almost entirely of hydrogen and helium. Located about 437 light-years away, HAT-P-26b orbits a star roughly twice as old as the sun.

The atmosphere of the distant "warm Neptune" HAT-P-26b, illustrated here, is unexpectedly primitive, composed primarily of hydrogen and helium. Image credit: NASA/GSFC

The analysis is one of the most detailed studies to date of a "warm Neptune," or a planet that is Neptune-sized and close to its star. The researchers determined that HAT-P-26b's atmosphere is relatively clear of clouds and has a strong water signature, although the planet is not a water world.

The discovery of an atmosphere with this composition on this exoplanet has implications for how scientists think about the birth and development of planetary systems.

Compared to Neptune and Uranus, the planets in our solar system with about the same mass, HAT-P-26b likely formed either closer to its host star or later in the development of its planetary system, or both.

Because the study provided a precise measurement of water, the researchers were able to use the water signature to estimate HAT-P-26b's metallicity.

Astronomers calculate the metallicity, an indication of how rich the planet is in all elements heavier than hydrogen and helium, because it gives them clues about how a planet formed.

To compare planets by their metallicities, scientists use the sun as a point of reference, almost like describing how much caffeine beverages have by comparing them to a cup of coffee.

Jupiter has a metallicity about 2 to 5 times that of the sun. For Saturn, it's about 10 times as much as the sun. These relatively low values mean that the two gas giants are made almost entirely of hydrogen and helium. (NASA)

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