K2-18b: A Habitable Zone Exoplanet 124 Lightyears Away

Krishna Amin (St Catharine’s). February 27, 2020. 

In a paper published today (27 Feb), researchers from the Institute of Astronomy revealed findings on the interior and atmospheric composition of exoplanet K2-18b, orbiting an M-dwarf (‘low-mass’) star in the habitable zone, only 124 lightyears away from Earth.

K2-18b’s density, between those of Earth and Neptune, suggested a hydrogen-rich outer envelope surrounding a rocky interior. Previous studies of similar planets proposed temperatures of around 250-300 Kelvin (-23 to 27 °C) – similar to those found on Earth. Given these properties, the authors detected the presence of water and the absence of methane and ammonia and did not find ‘strong evidence’ for clouds in the atmosphere.

The interior of the planet was modelled with an inner iron layer, an outer silicate layer, a water layer and a hydrogen/helium layer. Notice the similarities to Earth’s own structure: iron core, silicate mantle and crust, oceans, some sort of atmosphere. Consideration of variations on the model (i.e. different compositions and masses of different layers) resulted in three ‘representative classes’ defining K2-18b that include a ‘range of possible compositions’: rocky world, mini-Neptune and water world.

Life as we know it can survive in a huge range of harsh conditions, from pressures of ~1000 atmospheres and temperatures of ~400 K (127 °C). Whether or not K2-18b is habitable depends on the extent of the hydrogen/helium atmosphere. Many solutions to the data give water at the atmosphere-ocean boundary – the surface of the water layer – to be in the ‘supercritical’ phase, but some give water in the liquid or gas phases. The ‘water world class’ has liquid water approaching normal conditions (27 °C, 1-10 atmospheres) under a thin hydrogen/helium atmosphere, a description seemingly like that of Earth. Furthermore, chemical disequilibrium – the absence of methane and ammonia – indicates the possibility of biochemical processes, although other explanations exist. The authors argue that the search for biosignatures – signs of life – should not be limited to smaller rocky worlds as larger planets such as K2-18b have the potential to host life.

Their paper:
Nikku Madhusudhan et al 2020 ApJL 891 L7.

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