Clear skies on exo-Neptune

Astronomers have discovered clear skies and steamy water vapour on a gaseous planet outside our solar system. The planet, known as HAT-P-11b, is about the size of Neptune, making it the smallest-ever planet for which water vapour has been detected.

Using data from the NASA/ESA Hubble Space Telescope, the Spitzer Space Telescope, and the Kepler Space Telescope, an international team including astronomers from the University of Cambridge found that HAT-P-11b is blanketed in water vapour, hydrogen gas, and other yet-to-be-identified molecules. The results are published today (24 September) in the online version of the journal Nature.

“This discovery is milepost on the road to eventually searching for molecules in the atmospheres of smaller, rocky planets more like Earth,” said John Grunsfeld, assistant administrator of NASA’s Science Mission Directorate. “Such achievements are only possible when we combine the capabilities of these unique and powerful observatories.”

Clouds in the atmospheres of planets can block the view to underlying molecules that reveal information about the planets’ compositions and histories. Finding clear skies on a Neptune-size planet is a good sign that smaller planets might have similarly good visibility.

“When astronomers go observing at night with telescopes, they say ‘clear skies’ to mean good luck,” said Jonathan Fraine of the University of Maryland, the study’s lead author. “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 a so-called exo-Neptune — a Neptune-sized planet that orbits another star. It is located 120 light-years away in the constellation of Cygnus (The Swan). Unlike Neptune, this planet orbits closer to its star, making one lap roughly every five days. It is a warm world thought to have a rocky core, a mantle of fluid and ice, and a thick gaseous atmosphere. Not much else was known about the composition of the planet, or other exo-Neptunes like it, until now.

Part of the challenge in analysing the atmospheres of planets like this is their size. Larger Jupiter-like planets are easier to observe and researchers have already been able to detect water vapour in the atmospheres of some of these giant planets. Smaller planets are more difficult to probe — and all the smaller ones observed to date have appeared to be cloudy.

The team used Hubble’s Wide Field Camera 3 and a technique called transmission spectroscopy, in which a planet is observed as it crosses in front of its parent star. Starlight filters through the rim of the planet’s atmosphere and into a telescope. If molecules like water vapour are present, they absorb some of the starlight, leaving distinct signatures in the light that reaches our telescopes.

“We set out to look at the atmosphere of HAT-P-11b without knowing if its weather would be cloudy or not,” said Dr Nikku Madhusudhan, from Cambridge’s Institute of Astronomy, who was part of the study team. “By using transmission spectroscopy, we could use Hubble to detect water vapour in the planet. This told us that the planet didn’t have thick clouds blocking the view and is a very hopeful sign that we can find and analyse more cloudless, smaller, planets in the future. It is ground-breaking!”

Before the team could celebrate they had to be sure that the water vapour was from the planet and not from cool starspots — “freckles” on the face of stars — on the parent star. Luckily, Kepler had been observing the patch of sky in which HAT-P-11b happens to lie for years. Those visible-light data were combined with targeted infrared Spitzer observations. By comparing the datasets the astronomers could confirm that the starspots were too hot to contain any water vapour, and so it must belong to the planet.

The results from all three telescopes demonstrate that HAT-P-11b is blanketed in water vapour, hydrogen gas, and other yet-to-be-identified molecules. So in fact it is not only the smallest planet to have water vapour found in its atmosphere but is also the smallest planet for which molecules of any kind have been directly detected using spectroscopy. Theorists will be drawing up new models to explain the planet’s makeup and origins.

Although HAT-P-11b is dubbed as an exo-Neptune it is actually quite unlike any planet in our Solar System. It is thought that exo-Neptunes may have diverse compositions that reflect their formation histories. New findings such as this can help 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 Drake Deming, a co-author of the study also from University of Maryland. “We want to expand our knowledge to a diverse range of exoplanets.”

The astronomers plan to examine more exo-Neptunes in the future, and hope to apply the same method to smaller super-Earths — massive, rocky cousins to our home world with up to ten times the mass of Earth. Our solar system does not contain a super-Earth, but NASA’s Kepler mission is finding them around other stars in droves, and the NASA/ESA James Webb Space Telescope, scheduled to launch in 2018, will search super-Earths for signs of water vapour and other molecules. However, finding signs of oceans and potentially habitable worlds is likely a way off.

This work is important for future studies of super-Earths and even smaller planets. It could allow astronomers to pick out in advance the planets with atmospheres clear enough for molecules to be detected. Once again, astronomers will be crossing their fingers for clear skies.