Warm Neptune has 'primitive' atmosphere

The study of a warm Neptune-like planet is raising questions as to our understanding of how planetary systems form.

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An artist’s illustration of exoplanet HAT-P-26b. The exoplanet has relatively low metallicity, an indication of the how rich the planet is in all elements heavier than hydrogen and helium.
Credit: NASA/GSFC

The study of a distant ‘warm Neptune’ has revealed it boasts a primitive atmosphere of hydrogen and helium, and will contribute to astronomers’ understanding of how planetary systems form.

HAT-P-26b is a ‘warm Neptune’ because it is Neptune-sized but orbits close to its star. The star is about twice as old as our Sun and located about 437 lightyears away.

Astronomers used the Hubble and Spitzer space telescopes to observe the planet as it crossed in front of the star. This is known as a transit, and by analysing a portion of the stellar light that has passed through the exoplanet’s atmosphere, astronomers on Earth can tell a lot about the planet’s physical properties.

They determined that HAT-P-26b is relatively clear of clouds and has a strong water signature, the latter of which enabled scientists to estimate the planet’s metallicity. This refers to how rich a planet is in elements heavier than hydrogen and helium, and provides clues as to how the planet formed.

 


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In our Solar System, ice giants Neptune and Uranus are smaller than the gas giants Jupiter and Saturn, but richer in heavier elements. The trend appears to be that metallicities are lower for bigger planets.

Scientists believe this could be because when the Solar System was forming, Neptune and Uranus formed on the outskirts of the dusty disc surrounding our young Sun.

As a result, Neptune and Uranus would have been bombarded with icy debris rich in heavier elements, while Jupiter and Saturn would have formed in a warmer region and therefore received less icy debris.

HAT-P-26b does not follow this trend, raising questions about our formation model of planetary systems. Its metallicity is about 4.8 times that of the Sun, making it more akin to the metallicity of Jupiter, rather than Neptune.

“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,” says co-author of the study David K. Sing of the University of Exeter. “I would say that has been a theme in the studies of exoplanets: Researchers keep finding surprising diversity.”

“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,” says lead author Hannah Wakeford of NASA’s Goddard Space Flight Center. “This kind of unexpected result is why I really love exploring the atmospheres of alien planets.”


 

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