Astronomers have unexpectedly discovered a primitive atmosphere around a ‘warm Neptune’.
Comparable in mass to gas giants in our own solar system but much closer to its host star, planet HAT-P-26b has an atmosphere composed almost entirely of hydrogen and helium. Located around 437 light years away, it orbits a star roughly twice as old as the sun.
Dr Hannah Wakeford, a postdoctoral NASA fellow and lead author of the study published in Science, said: “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.”
Using the Hubble and Spitzer telescopes, the research team determined HAPT-P-26b’s atmosphere is relatively clear of clouds and has a strong water signature although the planet is not a water world. This has implications for how scientists think about the birth and development of planetary systems. Compared to Neptune and Uranus, HAT-P-26b likely formed either closer to its host star or later in the development of its planetary system, or both.
Using data from the transit of the planet as it passed in front of its host star and studying how the starlight signatures change, the researchers could discover the atmosphere’s chemical composition.
This approach revealed such precise water content measurements the team were also able to estimate metallicity – an indication of how rich the planet is in elements heavier than hydrogen and helium. It was this which provided the researchers with clues about the planet’s formation.
Neptune and Uranus have metallicities about 100 times that of our Sun, with Saturn and Jupiter having far lower values as they are closer to the Sun. This is believed to be a result of planetary formation in the Solar System, with Neptune and Uranus forming in a region further from the sun, exposing them to icy debris rich in heavier elements. But HAT-P-26b bucks this trend as its metallicity is only about 4.8 times that of the Sun yet it is closer in size to a gas giant.
Professor David Sing, from the University of Exeter and second author of the paper, said: “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. I would say that has been a theme in the studies of exoplanets: Researchers keep finding surprising diversity.”