Could Life Exist in the Atmosphere of a Sub-Neptune Planet?

Planet Earth is perfectly hospitable for organic life, which means that similar planets orbiting distant stars could also be booming with life. However, proving this is a challenge. One of the best ways to find extraterrestrial life will undoubtedly be to examine the atmospheres of inhabited exoplanets; however, Earth is relatively small for a planet and has a thin atmosphere when compared to bigger planets. It will be easier to study the atmospheres of gas planets; however, could such planets hold life? A brand-new paper in Universe suggests they could.

Exobiologists have long argued that we should not think all life in the universe will happen on planets similar to Planet. The subsurface oceans of Enceladus and Ganymede can sustain terrestrial life, and Titan has abundant methane chemistry that could support exotic life. However, the majority of ideas about life need three key ingredients: energy, water, and a surface.

The first two are quite obvious. Life requires some energy source to make it through, whether solar or geothermal, and water is an excellent solution to allow complex molecules to engage. However, the requirement for a surface is a lot more refined. It is not needed permanently to endure, considering that many microorganisms can spend their entire lives in water or air. Instead, it seems to be required for life to develop. Surface chemistry is unbelievably efficient in producing large organic molecules, even in space. It likely takes a rough surface to create the foundation of life.

This brand-new paper argues that while surface chemistry could be required for life to occur in a planetary system, it is not required for life to thrive. The work focuses on warm sub-Neptune worlds. These planets are about 8 to 10 times larger than Earth and are probably tiny gas planets with a thick atmosphere with no terrestrial surface. Numerous of these planets have been located in the potentially habitable zone of their star, such as K2-18b, which very closely orbits a red dwarf star.

The group reveals that warm sub-Neptunes such as K2-18b likely have much water and organic molecules required to create a habitable zone within their atmosphere. Furthermore, being smaller gas planets, it is most likely that their habitable layer is relatively stable, allowing any life to remain aloft long enough to reproduce before sinking to the aggressive depths below. Similar arguments have been made for the possibly habitable layer of Venus’s atmosphere. However, unlike Venus, K2-18b is not likely to have a surface. So even though life might make it through in a sub-Neptune world, just how would it get there?

The team proposes that asteroids might help. If an exoplanetary system has an unstable asteroid belt, meteorite influences with the sub-Neptune world and smaller terrestrial planets would be common. This cross-pollination could carry life to a gassy world.

Because sub-Neptunes are rather common and also have a thick atmosphere, they will be amongst the initial Earth we examine for signs of life. While the odds of discovering life on these worlds could be a bit far, it is worth having a look just because we can.

Originally published on Universe Today. Read the original article.

Reference: Sara Seager et al, Possibilities for an Aerial Biosphere in Temperate Sub Neptune-Sized Exoplanet Atmospheres, Universe (2021). DOI: 10.3390/universe7060172