It took the power of three of NASA’s top telescopes – Hubble, Spitzer and Kepler – to spot clear skies and water vapor on a planet outside our solar system. Located 120 light-years away, HAT-P-11b is about the size of Neptune. That makes it the smallest exoplanet, discovered so far, where molecules of any kind have been detected.

“This discovery is a significant milepost on the road to eventually analyzing the atmospheric composition of smaller, rocky planets more like Earth,” said John Grunsfeld of NASA. “Such achievements are only possible today with the combined capabilities of these unique and powerful observatories.”

Astronomers needed a bit of patience and luck to spot HAT’s atmosphere. Clouds can obstruct much of the planet’s atmosphere, so astronomers needed clear skies.

“When astronomers go observing at night with telescopes, they say ‘clear skies’ to mean good luck,” said Jonathan Fraine, lead author of the study appearing in Nature. “In this case, we found clear skies on a distant planet. That’s lucky for us because it means clouds didn’t block our view of water molecules.”

In the new study, astronomers used Hubble’s Wide Field Camera 3 to observe the planet. Then they used a technique called transmission spectroscopy. Basically, they waited until the planet crosses in front of its parent star and then observed the light as it hit the planet’s atmosphere. If water vapor was there, it would absorb some of the light and leave a distinct signature when looking through the telescope. And, that’s what astronomers found.

The illustration below gives us an idea of what astronomers typically see when viewing the atmosphere of exoplanets around the size of Neptune (left), and what they found on HAT-P-11b (right).

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atmosphere illustration

Still, it wasn’t a slam dunk find for astronomers. They had to double-check, and in this case triple check, their initial findings. That’s where Kepler and Spitzer come into play. These two telescopes determined that the water vapor spotted by Hubble was not a false reading from starspots of the planet’s parent star.

“Now with data like these, we can begin to piece together a narrative for the origin of these distant worlds,” said co-author Heather Knutson.

This discovery opens up exo-Neptune size planets for more observation. Researchers can use the techniques from this discovery and apply it other discoveries. Plus, the same method could also work on super-Earths, planets up to 10 times the mass of Earth.

“The work we are doing now is important for future studies of super-Earths and even smaller planets, because we want to be able to pick out in advance the planets with clear atmospheres that will let us detect molecules,” Knutson adds.

Image credits: NASA/JPL-Caltech

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