Smallest exoplanet with water vapour found

Scientists, including one of Indian-origin, have detected clear skies and water vapour in the atmosphere of a Neptune-sized planet outside of our Solar System.
The planet, known as HAT-P-11b, is the smallest exoplanet ever on which water vapour has been detected, researchers said.
The discovery, which used data from the NASA/ESA Hubble Space Telescope, the Spitzer Space Telescope, and the Kepler Space Telescope, is a milestone on the road to eventually finding molecules in the atmospheres of smaller, rocky planets more akin to Earth, researchers said.
Finding clear skies on a Neptune-sized planet is a good sign that some smaller planets might also have similarly good visibility, they said.

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.
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 the 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 Nikku Madhusudhan, from the University of Cambridge, UK, 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 groundbreaking!" Madhusudhan said.

The study also showed 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, researchers said.