water on K2-18b

Image credit: Jan Mocnak from FreeImages

W is for WOW, WONDERFUL news and WATER in an exoplanet’s atmosphere!

Earlier this week astronomers announced the discovery of water in the atmosphere of a distant Super-Earth, the planet called K2-18b that orbits the red dwarf K2-18 (read the original paper here). This important discovery takes us one step further in our search for habitable worlds outside the Solar System.

Red dwarfs 

Red dwarfs (or M dwarfs) are the tiniest, coolest and the most long living of all stars. Don’t look for them on the night sky, they are too faint to be seen with the unaided eye. Red Dwarfs are very abundant in all galaxies. In the Milky Way, for example, they make up around ¾ of all stars. In old elliptical galaxies almost all stars are red dwarfs! Scientist think that 40% of all red dwarfs have small planets around them. Are these planets good places to search for alien life? Well, unfortunately, red dwarfs are not very “life-friendly”. First, they are quite active. Superflares, similar to the solar flares only much more powerful, erupt on red dwarfs while they are young, i.e. at the time when planets potentially form around them. Second, they are small and therefore their habitable zone, an area where liquid water can exist, is very close to the star itself. Finally, planets around red dwarfs are expected to be tidally locked. If they indeed have the same side always facing their star, there might be a huge temperature variation between “day side” and “night side” of these planets. For life as we know it all these factors are bad news. For a long time now astronomers thought that planets around red dwarfs are the least likely places to host life. But more recent studies suggest that these worlds might not be quite as bad after all!


Planet K2-18b was discovered back in 2015 by Kepler Space Telescope. This “Super-Earth ” is 8.6 times as massive as our home planet and 3 times its size. K2-18b’s density is lower than that of the Earth, so scientists don’t know what the planet is like and whether it has a solid surface. The exoplanet orbits its star once every 33 days at an uncomfortably close distance of 0.14AU. One astronomical unit, or AU, is equal to the Earth-Sun distance, or 149.6 million kilometers.

Although K2-18b is so close to the host star, its equilibrium temperature is similar to that of the Earth. That means it is possible that liquid water and organics are present out there. Now, equilibrium temperature is not the same as mean temperature or surface temperature. The actual temperature on any planet might be much higher than its equilibrium temperature if there is an atmosphere and if the tidal effects caused by the proximity to the host star heat the planet up from within. Both are likely to be the cases with K2-18b. 

Observations and water discovery

Hubble Space Telescope observed the planet K2-18b for 3 years between 2015 and 2017. Over that time scientists recorded 8 transits, i.e. passages of the planet in front of its star. Later, they added the data from two transits observed by Kepler Space Telescope mission and 3 transits observed by the Infra Red space telescope Spitzer. 

As a result, astronomers were able to detect water vapor in the thick hydrogen-rich atmosphere of K2-18b. What’s more, the temperature on the planet seems to be low enough to allow water condensation. That means the water vapor probably condenses into droplets in K2-18b’s middle-atmosphere, falls as rain and later evaporates in the lower thicker atmospheric layers. Whether or not there is liquid water on the actual surface of this planet, we don’t know.

What’s next?

Scientists cannot wait to learn more about this amazing planet. K2-18b is a great target candidate for the upcoming NASA James Webb Telescope mission as well as for ESA ARIEL mission. Who knows what else they will discover!

Meanwhile, stay tuned and read our space blog. And don’t forget to visit our inflatable planetarium to learn more about exoplanets and search for life in the Universe!

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