Scientists have made the first successful detection of an atmosphere with water vapour around the exoplanet K2-18 b. What’s significant about this is that K2-18 b is a non-gaseous “super-Earth” located within the habitable zone of its star. Put simply, we have detected water on a potentially habitable planet for the first time.
With more than 4000 exoplanets – planets orbiting stars other than our sun – discovered so far, it may seem like we are on the cusp of finding out whether we are alone in the universe. Sadly though, we don’t know much about these planets – in most cases just their mass and their radius.
Understanding whether a planet could host life requires a lot more information. At the moment, one extremely important piece of information that is missing is the presence, composition and structure of their atmospheres. Signs of atmospheric water, oxygen and methane would all be signs that a planet may support life.
Now we have for the first time managed to detect water vapour in the atmosphere of an exoplanet that is potentially habitable. Our results have been published in Nature Astronomy.
A planet’s atmosphere plays a vital role in shaping the conditions inside it – or on its surface, if it has one. Its composition, stability and structure all provide important clues about what it is like to be there. Through atmospheric studies, we can therefore learn about the history of the planet, investigate its habitability and, ultimately, discover signs of life.
The primary method that we use when examining exoplanets is transit spectroscopy. This involves looking at starlight as a planet passes in front of its host star. As it transits, stellar light is filtered through the planet’s atmosphere – with light being absorbed or deflected based on what compounds the atmosphere consists of.
The atmosphere therefore leaves a characteristic footprint in the stellar light that we try to observe. Further analysis can then help us match this footprint to known elements and molecules, such as water or methane.
At the moment, the study of exoplanets atmospheres is limited, as this kind of measurement requires very high precision, which current instruments were not built to deliver. But molecular signatures from water have been found in the atmospheres of gaseous planets, similar to Jupiter or Neptune. It has never before been seen in smaller planets – until now.
K2-18 b was discovered in 2015 and is one of hundreds of “super-Earths” – planets with a mass between Earth and Neptune – found by NASA’s Kepler spacecraft. It is a planet with eight times the mass of the Earth that orbits a so called “red dwarf” star, which is much cooler than the sun.
However, K2-18b is located in the “habitable zone” of its star which means it has the right temperature to support liquid water. Given its mass and radius, K2-18 b is not a gaseous planet, but has a high probability of having a rocky surface.
We developed algorithms to analyse the starlight filtered by this planet using transit spectroscopy, with data provided by the Hubble Space Telescope.