In a rare, detailed study of a far-off world known as a ‘warm Neptune,’ an international team of researchers has revealed a planet with a primitive atmosphere and a strong water signature. The study will be published in the May 12, 2017, issue of the journal Science.
A ‘warm Neptune’ is a Neptune-sized planet that orbits more closely to its host star.
Using combined observations from NASA’s Hubble and Spitzer space telescopes, astronomers find that planet HAT-P-26b, located some 437 light years from Earth, has a relatively cloudless atmosphere made up almost entirely of hydrogen and helium. Its star is about twice an old as the sun, according to a statement issued by the space agency.
“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 co-team leader and lead author Hannah Wakeford, a postdoctoral researcher at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, in the statement. “This kind of unexpected result is why I really love exploring the atmospheres of alien planets.”
Astronomers were able to determine the chemical composition of the planet’s atmosphere by using data from planetary transits, or when the planet passes in front of its star. By observing which wavelengths of light are filtered through the planet’s atmosphere during transit, they can ‘read’ its chemical signatures.
Having obtained the water signature, the researchers then used it to estimate the planet’s metallicity, which provides information about the way the world formed. They were surprised to find that planet HAT-P-26b has a metallicity closer to Jupiter than to Neptune — only about 4.8 times that of the sun.
“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,” said co-author 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.”