NASA has found that some habitable planets - those that orbit at the right distance from a host star that allows liquid to exist, might not be able to support life, as we know it, due to the frequent radiation at which they are subjected from dwarf stars. The stellar activity to which some planets are exposed can prevent the development of a planet´s atmosphere due to a loss of oxygen, which is one important component, along with hydrogen of a water molecule. This has caused some to reconsider the way in which habitable zones are defined.

Habitable zone

This is the region around a star where a planet possesses the right atmospheric pressure for the development of liquid water on its surface. Such planet must be at the right distance from its parent star, similar to that of the earth from the sun. The amount of light and heat is also considered. Since massive stars greater than the sun produce more energy, the habitable zone is much farther out than smaller stars which produce less energy and the habitable zone is much closer.

Atmospheric erosion

This is a loss of a planet’s atmospheric gasses into space. It´s believed that a star´s emission of ultraviolet light and X-rays into space, whose effect causes atmospheric erosion, could be depleting atmospheric particles, such as hydrogen and oxygen from a planet; thus preventing the formation of water, which is an important element for the development of life.

Red dwarfs - the most numerous stars in the universe

Usually, scientists search for habitable planets around red dwarfs, as they are more susceptible for planet detection. Since red dwarfs are more luminous than the sun, the habitable zone must be 10-20 times closer than that of the earth to the sun. Unfortunately, red dwarfs emit X-rays and ultraviolet radiation which break apart molecules into atoms and then ionize atmospheric gasses, knocking off electrons that escape the planet´s gravitational force, escaping into space.

Atmospheric erosion models

These models have demonstrated that lighter gases, such as hydrogen escape more rapidly from the force of a planet's gravity, leaving heavier elements such as oxygen and nitrogen in an exoatmosphere. Very powerful ejections of energized radiation-X-rays and ultraviolet radiation from a red dwarf star cause the heavier ions-oxygen and nitrogen to escape into space.

It´s believed that a red dwarf would stop the ability of an exoplanet within tens or even one hundred of millions of years. The depletion of hydrogen and oxygen would eliminate the chance for the development of life due to a lack of liquid water on its surface.