The exoplanet was already discredited earlier because of the detection of atmospheric water vapor. But how suitable is the planet for life?

In 2015 a planet was discovered around the red dwarf star K2-18: K2-18b. The planet is about 111 light-years away from the earth. K2-18b is located in the livable zone, has a moderate temperature and, as a blow to the mark, researchers discovered last September water vapor in the atmosphere of the super earth. That is why the question arose whether the planet could actually accommodate life. And a new study builds on this now.

More about K2-18b
K2-18b orbits around the cool dwarf star K2-18 and can be found in the constellation Leo. Although K2-18b has some similarities with the earth, there are also differences. For example, the planet has more than eight times the mass of the earth. Moreover, the planet is exposed to more radiation because it circles around a red dwarf star.

When researchers discovered that K2-18 had atmospheric water vapor, this caused quite a stir. Indeed, it is the only planet that is known to have both water and a pleasant temperature that could make life thrive here. But whether the planet actually contains life … that is the question. “Water vapor has been detected in the atmosphere of a number of exoplanets,” says research leader Nikku Madhusudhan. “But even if the planet is in the habitable zone, this does not mean that the surface conditions are suitable for life. To find out, it’s important to get a uniform understanding of the internal and atmospheric conditions on the planet – in particular, whether liquid water can exist under the atmosphere. “

Size
To reveal this, the researchers looked at the size of the exoplanet, among other things. K2-18 is more than twice the size of the earth and has 8.6 times the mass. Given the large size of K2-18, it has been suggested that it looks more like a smaller version of Neptune than a larger version of the Earth. And a so-called “mini-Neptune” is expected to have a solid core of stone and iron that is surrounded by an atmosphere consisting of hydrogen or an atmosphere that is at least rich in hydrogen. However, if this hydrogen envelope is too thick, the atmosphere and pressure on the surface of the underlying water layer would make the planet unsuitable for life.

Artistic impression of exoplanet K2-18b. image: ESA / Hubble, M. Kornmesser

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Life
Using different models and statistical methods, the researchers tried to find out how thick the hydrogen cover is likely to be. For this they used observations of the atmosphere, mass and radius. The findings are promising. Because despite the size of K2-18, the researchers have shown that the hydrogen envelope is not necessarily too thick. The shell makes up around 6 percent or less of the planet’s mass. It means that the planet may be a real ocean world where the pressure and temperature are similar to those in terrestrial oceans. In addition, the researchers discovered that the methane and ammonia concentrations were lower than expected in such an atmosphere. Whether these levels can be attributed to biological processes remains to be seen.

The findings indicate that K2-18 is potentially habitable. And that is of course a very exciting conclusion. Moreover, it also means that exoplanets that are considerably larger than the earth could be viable. Research into K2-18 continues. Because with the future James Webb space telescope it must be possible to reveal more secrets about alien atmospheres.