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Warm Neptune Has a Primitive Atmosphere of Hydrogen & Helium

‘Warm Neptune’, an exoplanet comparable in size to our Neptune, has a cloudless sky, and a primitive atmosphere, one consisting of only hydrogen and helium. The new findings are published in the journal Science.

Warm Neptune, an exoplanet named HAT-P-26b, has an atmosphere that is surprisingly primitive: it consists almost entirely of hydrogen and helium, in stark contrast with our own Neptune. Photo credits: NASA/GSFC.

‘Warm Neptune’ is an exoplanet, that is, a planet found outside of our solar system, situated 430 light years away from us; its real name is HAT-P-26b. It is similar in size to our Neptune, but one great difference is its closer distance to its sun. We have a better understanding of this heavenly body thanks to the most detailed study conducted by a team of international scientists, with co-lead researchers Hannah Wakeford from NASA, and University of Exeter’s Professor David Sing.

The main finding entails the atmosphere of Warm Neptune: it appears to consist mostly of hydrogen and helium, with a cloudless sky for ceiling. The primitive nature of its atmosphere indicates that the planetary body might have formed closer to its star and/or at a later time of the evolution of its solar system, thus differentiating it from our Neptune and Uranus.

This study is going to redefine our understanding of the formation and development of planetary systems beyond our galaxy, suggest the authors. The discovery points out at a greater diversity in the atmospheres of exoplanets than was previously assumed, says Professor Sing.

“This ‘Warm Neptune’ is a much smaller planet than those we have been able to characterize in depth, so this new discovery about its atmosphere feels like a big breakthrough in our pursuit to learn more about how solar systems are formed, and how it compares to our own,” says Prof. Sing.

The data was collected during HAT-P-26b’s transits past its host star: 4 different transits were captured by NASA’s Hubble Space Telescope, and Spitzer Space Telescope . It is possible to view the atmosphere of planets during this type of events because a portion of the light from their stars is filtered through their atmosphere which takes in some wavelengths of light only. So, observing the change in the signatures of the starlight resulting from the filtering allows scientists to determine the atmosphere composition.

Dr Sing and his colleagues found that the atmosphere of Warm Neptune has almost no cloud, and that it bears a strong water signature. The latter measurement was used to calculate metallicity, the fraction of mass of a star that is an element other than hydrogen and helium (and, therefore, heavier); this estimate provides indications about the formation of planets.

To get a better idea, consider this: the metallicity in Jupiter is 5 times greater than the sun’s, and Saturn’s is 10 times greater, suggesting that the two planets are composed mostly of hydrogen and helium; on the other hand, Neptune and Saturn are rich in heavier elements (heavier than hydrogen and helium), and thus have metallicities 100 times more than the sun’s. These figures are interpreted as follows: when the solar system was forming, Neptune and Uranus were developing at a region in the extremities of the giant disk of dust, gas, and debris surrounding the young sun. This would have resulted in icy debris rich in heavier elements falling onto Neptune and Uranus, while Jupiter and Saturn would have formed in a warmer part of the disk, and thus obtaining less of icy debris.

HAT-P-26b, on the other hand, has a metallicity only around 4.8 times greater than its sun’s, resembling Jupiter more than it does Neptune.

“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. This kind of unexpected result is why I really love exploring the atmospheres of alien planets,” says Hannah.

“To have so much information about a warm Neptune is still rare, so analyzing these data sets simultaneously is an achievement in and of itself,” adds co-author Tiffany Kataria.


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