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Warm Neptune exoplanet found to have water vapor and clouds

Astronomers describe an exoplanet with a primitive atmosphere and strong water signature.

A Neptune-sized exoplanet found orbiting close to its parent star has an atmosphere that contains both water vapor and exotic clouds, according to astronomers who observed it using both NASA’s Hubble and Spitzer space telescopes.

Located approximately 437 light years from Earth, HAT-P-26b circles its star once every 4.2 Earth days, meaning it probably is tidally locked, with the same hemisphere always facing the star and the other always facing away from it.

Although its mass and size are similar to those of Neptune, its composition is more like that of Jupiter and Saturn, largely hydrogen and helium.

A research team led by David Sing of the University of Exeter in England and Hannah Wakeford of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, studied observations of the planet transiting or passing in front of its star with both the Hubble and Spitzer telescopes.

They successfully identified the molecules in the planet’s atmosphere during these transits, when that atmosphere filtered out specific wavelengths of the star’s light.

Water molecules were especially abundant in the atmosphere.

“For this mass range, this is the strongest water-absorption feature that we have ever measured,” Wakeford reported to the website Space.com.

From data collected, the research team also determined that exotic clouds likely travel across the sky deep within the planet’s atmosphere.

Unlike clouds on Earth, which are composed of water vapor, these clouds are likely made up of disodium sulfide, giving the planet’s sky a strange appearance.

“This would be a very alien sky that you would be looking at. These clouds would cause scattering in all of the colors, so you’d get a kind of scattery, washed-out gray sky, which is interesting if you were looking through these clouds,” Wakeford explained.

Data collected during the observations also enabled the researchers to measure the atmosphere’s metallicity, meaning the degree to which it is made up of elements other than hydrogen and helium.

Surprisingly, they found HAT-P-26b to have a metallicity more like that of Jupiter and Saturn in our solar system. The latter two have much lower metallicities than Uranus and Neptune.

In our solar system, the more massive a planet is, the lower its metallicity is.

Because HAT-P-26b does not fit that pattern, the researchers believe it may have formed closer to the star and migrated outward.

“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,” Wakeford said in a NASA statement.

Findings of the study have been published in the May 11 issue of the journal Science.

Laurel Kornfeld

Laurel Kornfeld

Staff Writer
Laurel Kornfeld is a freelance writer and amateur astronomer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science in astronomy from Swinburne University’s Astronomy Online program.
About Laurel Kornfeld (1015 Articles)
Laurel Kornfeld is a freelance writer and amateur astronomer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science in astronomy from Swinburne University’s Astronomy Online program.