Clear Skies Reveal Water Vapor on Distant Neptune-Sized Planet

It's clear skies on the planet HAT-P-11b, making it easy for astronomers to spot steamy water vapor on this Neptune-sized exoplanet, new research describes.

This breakthrough discovery makes HAT-P-11b the smallest exoplanet ever on which water vapor has been found, and is a step towards eventually finding molecules in the atmospheres of smaller, rocky planets more akin to Earth.

Normally clouds in the atmospheres of planets outside our solar system block the view of what lies beneath them, which can reveal important information about the composition and history of a planet. But astronomers using data from the NASA and European Space Agency (ESA) Hubble Space Telescope, the Spitzer Space Telescope, and the Kepler Space Telescope have, to their delight, discovered clear skies on HAT-P-11b. This is a positive indicator that other small exoplanets might also have good visibility, giving new meaning to the mantra "clear skies."

"When astronomers go observing at night with telescopes, they say 'clear skies' to mean good luck," lead author Jonathan Fraine said in a statement. "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."

HAT-P-11b is located 20 light-years away and is thought to be a warm world with a rocky core, a mantle of fluid and ice, and a thick gaseous atmosphere. Until now, scientists knew very little about the composition of such Neptune-sized exoplanets because their small size makes it difficult to detect water vapor, and they often appear cloudy.

Researchers relied on a technique called transmission spectroscopy, in which a planet is observed as it crosses in front of its parent star. If molecules like water vapor are present, they absorb some of the starlight, leaving distinct signatures in the light that reaches telescopes on Earth. (Scroll to read on...)

The research team views this groundbreaking discovery as a hopeful sign that other smaller planets with not a cloud in the sky are just waiting to be discovered, helping astronomers to piece together a theory for the origin of these distant worlds.

"We are working our way down the line, from hot Jupiters to exo-Neptunes," said co-author Drake Deming. "We want to expand our knowledge to a diverse range of exoplanets."

The findings were published in the journal Nature.