Astronomers have discovered that HAT-P-26b — a ‘warm Neptune’ orbiting a dwarf star approximately 437 light-years from Earth — has an atmosphere composed almost entirely of hydrogen and helium. A paper reporting this discovery is published in the journal Science.

While thousands of extrasolar planets have been discovered to date, little is known about their atmospheres, especially for worlds smaller than Jupiter.

However, the composition of a planet’s atmosphere can provide valuable clues as to how the planet formed.

The atmospheres of Neptune-mass planets could have arisen from many different sources, resulting in a wide range of possible atmospheric compositions.

Using observations from the NASA/ESA Hubble Space Telescope and NASA’s Spitzer Space Telescope, NASA researcher Dr. Hannah Wakeford and co-authors now report that the atmosphere of the ‘warm-Neptune’ HAT-P-26b — a planet that is similar in size to Solar System’s Neptune, but which orbits its host star more closely — has a very low metallicity, an indication of the how rich the planet is in all elements heavier than hydrogen and helium.

“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,” said Dr. Wakeford, a postdoctoral researcher at NASA’s Goddard Space Flight Center.

“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,” said co-author Dr. Tiffany Kataria, of NASA’s Jet Propulsion Laboratory.

“This exciting new discovery shows that there is a lot more diversity in the atmospheres of these exoplanets than we have previously thought,” said co-author Prof. David Sing, from the University of Exeter, UK.

“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.”

The team’s analysis provided enough detail to determine HAT-P-26b’s atmosphere is relatively clear of clouds and has a strong water signature — also the best measurement of water to date on an exoplanet of this size. The researchers then used the water signature to estimate the metallicity.

To compare planets by their metallicities, astronomers use our own Sun as a point of reference.

In the Solar System, the metallicity in Jupiter (5 times greater than the Sun) and Saturn (10 times) suggest 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.

Astronomers 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 young Sun.

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

However, this new study discovered that HAT-P-26b bucks the trend: its metallicity is only about 4.8 times that of the Sun — much closer to the value for Jupiter than for Neptune.

The findings will help provide a better understanding of how atmospheric composition varies between exoplanets with different masses, and will help constrain models of planet formation.

_____

Hannah R. Wakeford et al. 2017. HAT-P-26b: A Neptune-mass exoplanet with a well-constrained heavy element abundance. Science 356 (6338): 628-631; doi: 10.1126/science.aah4668