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‘Warm Neptune’ HAT-P-26b has primitive atmosphere

The atmosphere of the distant "warm Neptune" HAT-P-26b, illustrated here, is unexpectedly primitive, composed primarily of hydrogen and helium. By combining observations from NASA’s Hubble and Spitzer space telescopes, researchers determined that, unlike Neptune and Uranus, the exoplanet has relatively low metallicity, an indication of the how rich the planet is in all elements heavier than hydrogen and helium. Image & Caption Credit: NASA/GSFC

The atmosphere of the distant “warm Neptune” HAT-P-26b, illustrated here, is unexpectedly primitive, composed primarily of hydrogen and helium. By combining observations from NASA’s Hubble and Spitzer space telescopes, researchers determined that, unlike Neptune and Uranus, the exoplanet has relatively low metallicity, an indication of the how rich the planet is in all elements heavier than hydrogen and helium. Image & Caption Credit: NASA/GSFC

Scientists at NASA’s Jet Propulsion Laboratory in Pasadena, California, announced in a paper published on May 12, 2017, an exoplanet designated HAT-P-26b, which was confirmed in 2010, has a primitive atmosphere of hydrogen and helium.

Using the combined efforts of the Hubble and Spitzer space telescopes, astronomers were able to perform the most detailed analysis of the atmosphere of an exoplanet of this size. The Spitzer telescope, an infrared telescope, and the Hubble telescope which observes in near-infrared, visible, and ultraviolet wavelengths of light, combined to gather a wide range of wavelengths of light, and allowed a far more detailed and comprehensive observation of HAT-P-26b.

The Hubble Space Telescope. Photo Credit: NASA

The Hubble Space Telescope. Photo Credit: NASA

Using the transit method, they measured and analyzed the amount and wavelengths of light from distant stars. The team was able to gather a wide range of spectral information from these wavelengths of light spanning from yellow visible light to the near-infrared. The data allowed them to confirm the mass of the planet as well as determine the approximate metallicity (a number of elements heavier than hydrogen and helium within a celestial body), and the primary composition of the planet’s atmosphere.

Instead of the heavier elements that they expected to find in a Neptune-mass planet and which are found in both Uranus and Neptune, HAT-P26b seems to be composed primarily of hydrogen and helium, much like Jupiter and Saturn are in the Solar System. Additionally, precise measurements of the water signature of HAT-P-26b were used to estimate its metallicity. Astronomers used this information to give them a clue as to how the planet may have formed.

Using the Sun as a reference point, astronomers compared the exoplanet metallicity measurements to that of the planets in the outer Solar System. Jupiter has a metallicity of 2 to 5 times that of the Sun, and Saturn has a metallicity of 10 times that of the Sun, whereas Uranus and Neptune have metallicities approaching 100 times that of the Sun.

In stark comparison, HAT-P-26b has a metallicity of just 4.8 times that of the Sun, suggesting that not only is it closer to the gas giants in composition but also that it likely formed closer to its host star than the planets of a similar size in our own system.

The transit method. Image Credit: NASA

The transit method works by waiting for a planet to pass in front of its host star. As such, some of the star’s light passes through the planet’s atmosphere. By observing how the light changes as it goes through the atmosphere, scientists can work backward to determine the properties of the planet’s atmosphere. Image Credit: NASA

Now being referred to as a “warm Neptune”, HAT-P-26b is the approximate size of Neptune, but far warmer. The host star, HAT-P26, located some 437 light-years from Earth, orbits a star of about the same size, but twice as old as the Sun. Researchers were able to determine that HAT-P-26b’s atmosphere seems to be clear of clouds, and, despite its strong water signature, is not a water world.

An artist's concept of the Spitzer space telescope. Image Credit: NASA/JPL-Caltech

An artist’s concept of the Spitzer space telescope. Image Credit: NASA/JPL-Caltech

Hannah Wakeford, a postdoctoral researcher at NASA’s Goddard Space Flight Center in Greenbelt, Maryland said astronomers have just begun to investigate the atmospheres of distant Neptune-mass planets, and almost right away, they found an example that goes against the trend in the Solar System.

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

Wakeford is the lead author of the article which appeared in the May 12, 2017, issue of Science.

Current hypotheses about the formation of planetary systems indicate that hydrogen-rich gas giant planets, like Jupiter and Saturn, likely formed early in the life of solar systems and probably closer to their host star. Whereas ice giant planets, such as Uranus and Neptune, which are denser and composed of ices, dust, and other heavier debris, likely formed farther out in the system, as an amalgam of leftover heavier substances rich in heavier elements that accreted beyond the snow line.

“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 Tiffany Kataria of NASA’s Jet Propulsion Laboratory in Pasadena, California.

“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 David K. Sing of the University of Exeter and the second author of the paper. “I would say that has been a theme in the studies of exoplanets: Researchers keep finding surprising diversity.”

There are two other known exoplanets that seem to follow a similar planetary evolution to that of HAT-P-26b. HAT-P-11b/Kepler-3b, a planet 122 light-years away in the constellation Cygnus, and WASP-43b, a gas giant twice the size of Jupiter 261 light-years from Earth in the constellation Sextans, both fit this trend.

 

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A native of the Greater Los Angeles area, Ocean McIntyre's writing is focused primarily on science (STEM and STEAM) education and public outreach. McIntyre is a NASA/JPL Solar System Ambassador as well as holding memberships with The Planetary Society, Los Angeles Astronomical Society, and is a founding member of SafePlaceForSpace.org. McIntyre is currently studying astrophysics and planetary science with additional interests in astrobiology, cosmology and directed energy propulsion technology. With SpaceFlight Insider seeking to expand the amount of science articles it produces, McIntyre was a welcomed addition to our growing team.

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