With the recent discovery of a system with planets same size as the Earth, there are new possibilities of life existing outside of our very own planet. NASA released a statement regarding their discovery, pointing out that seven different planets exist in the ‘habitable zone’ of a star. Furthermore, this could indicate the existence of water and possibly other life forms.
But what makes a planet habitable?
So first of all, the habitable zone (HZ) is the area around a star where water can exist in its liquid form. Which we, up until now, think is necessary for supporting life. However, we look at the existence of other life forms by studying ourselves. Life on Earth was able to emerge and evolve because of the presence of water. On a planet in the HZ it is imperative that there are enough building blocks for life that are in contact with water (Lammer, et al. 2009).
Though it should be noted that many factors, in turn, influence whether water exists on a planet in its liquid form. One of those is the presence of a secondary atmosphere, this can form through internal volcanic activity on a planet. The atmosphere is also of importance in protecting (life on) Earth from harmful UV radiation.
Second, stable radiation from the central star is needed. When we look at our own star, the sun, it took 1 billion years before circumstances were right enough for life to emerge (Güdel, 2014).
Third, land planets have an advantage over aqua planets. This is due to the fact that land planets have less global freezing compared to aqua planets, because land planets simply have less liquid water to freeze (Abe, et al. 2011).
Fourth, high concentrations of, for instance, CO2 and CH4 (carbonate-silicate cycle) can stabilize the climate on a planet despite the increase in solar luminosity. This is negative feedback loop is active on Earth (Kasting & Catling, 2003).
Lastly, metallicity plays an important role in forming a habitable planet. Metallicity is the part of the mass of an astronomical object that isn’t helium or hydrogen. Gonzalez, Brownlee, and Ward (2001) estimate that half of the metallicity of the Sun is needed to create a habitable planet. They also point out that the existence of tectonic plates and the right atmospheric conditions (point 4) are imperative.
Abe, Y., Abe-Ouchi, A., Sleep, N. H., & Zahnle, K. J. (2011). Habitable zone limits for dry planets. Astrobiology, 11(5), 443-460.
Gonzalez, G., Brownlee, D., & Ward, P. (2001). The galactic habitable zone: galactic chemical evolution. Icarus, 152(1), 185-200.
Güdel, M. (2014). What Makes a Planet Habitable?. Eu-topías: revista de interculturalidad, comunicación y estudios europeos, (8), 91-100.
Kasting, J. F., & Catling, D. (2003). Evolution of a habitable planet. Annual Review of Astronomy and Astrophysics, 41(1), 429-463.
Lammer, H., Bredehöft, J. H., Coustenis, A., Khodachenko, M. L., Kaltenegger, L., Grasset, O., … & Wahlund, J. E. (2009). What makes a planet habitable?. The Astronomy and Astrophysics Review, 17(2), 181-249.