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How astronomers detected water on a potentially habitable exoplanet for the first time

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 analyze the starlight filtered by this planet using transit spectroscopy, with data provided by the Hubble Space Telescope.

This enabled us to make the first successful detection of an atmosphere with water vapor around a non-gaseous planet, which is also located within the habitable zone of its star.

In order for an exoplanet to be defined as habitable, there is a long list of requirements that need to be satisfied. One is that the planet needs to be in the habitable zone where water can exist in liquid form. It is also necessary that the planet has an atmosphere to protect the planet from any harmful radiation coming from its host star.

Another important element is the presence of water, vital for life as we know it. Although there are many other criteria for habitability, such as the presence of oxygen in the atmosphere, our research has made K2-18b the best candidate to date. It is the only exoplanet to fulfil three requirements for habitability: the right temperatures, an atmosphere and the presence of water.

However, we cannot say, with current data, exactly how likely the planet is to support life. Our data are limited to an area of the spectrum—this shows how light is broken down by wavelength—where water dominates, so other molecules can unfortunately not be confirmed.

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