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Earth-like planets around Red Dwarf stars might be dry and airless deserts

September 27, 2015


Red Dwarf stars of the spectral class M account for about 90% of the stars in our galaxy. They are thus not only much more common than all other star types, they also offer much better conditions for the discovery of planets. These stars do not outshine their planets as strongly as is the case with Sun-like stars. Also, planets within the habitable zone are closer to small stars, facilitating detection of transit events. In addition, the gravitational force of the planets acts more strongly on the Red Dwarfs, which is why they can also be detected well by measuring the radial velocity of the star. In view of their frequency, Red Dwarf stars are an interesting area for finding earth-like planets. Would such worlds be habitable?


An important point is the time a star spends on the main sequence of stellar evolution. Red Dwarf stars consume their hydrogen so slowly that they can remain on the main sequence for 50 billion years or much longer. For a biosphere this would be an important factor for long-term stability. However, because of the weaker solar radiation, the habitable zone is at the same time so close to the center star that the tidal forces would slow the rotation of a planet. Planets in this zone would each have a permanent day and night side. Such uneven heat may cause water and carbon dioxide to freeze on the planet's side and form a gigantic ice cap, while the day's side is dried out under permanent sunlight, leaving the planet with a thin atmosphere. Climate simulations have shown that a dense carbon dioxide atmosphere could distribute the heat on the entire planet's surface so efficiently that water and atmosphere might remain stable. However, there are other factors to consider.

An important difference between Sun-like stars and Red Dwarf stars is the much higher activity of the dwarf stars. Coronal mass ejections, also known as "solar torches," are much more common among Red Dwarfs, and are much stronger than in our Sun, and many of them are so violent that the luminosity of the entire star multiplies within a few minutes. An Earth-like planet's atmosphere and water would be eroded into space over time. An Earth-like reservoir of water could get lost within a few billion years. The planet would dry up, and the oxygen released from the water by photodissociation might accumulate in the atmosphere until it is removed by other geochemical processes, resulting in a dry desert planet with an oxygen-containing atmosphere.

A planet with a strong magnetic field, comparable to the terrestrial magnetic field or stronger, could escape the consequences of the particle radiation. Nevertheless, the surface and also the atmosphere would be subject to the increase in brightness and both x-ray and gamma-ray irradiation.

However, the greatest problem could already be relevant during the formation of the planets. The stellar activity of Red Dwarf stars is particularly high during the first billion years of their existence. Planets forming in orbits corresponding to the habitable zone would not accumulate sufficient water or gases in their environment during their formation phase to build up a significant water supply or atmosphere.

Even if Red Dwarf stars are the most common stars in the galaxy, most of the Earth-like planets in their environment may be dry and barren deserts.



Artistic representation of a planet orbiting a Red Dwarf star. Oxidized minerals dominate the environment and their is a lot of dust in the atmosphere. Fog accumulates near shadowy areas.

References:

Lissauer (2007): Planets formed in habitable zones of M dwarf stars probably are deficient in volatiles. The Astrophysical Journal 660: L149-L152
Lammer, Lichtenegger, Kulikov, Grießmeier, Terada, Erkaev, Biernat, Khodachenko, Ribas, Penz, Selsis (2007): Coronal Mass Ejection (CME) Activity of Low Mass M Stars as an Important Factor for the Habitability of Terrestrial Exoplanets. II. CME-Induced Ion Pick Up of Earth-like Exoplanets in Close-In Habitable Zones. Astrobiology 7(1): 185-207
Tian (2009): Thermal escape from super Earth atmospheres in the habitable zone of M stars. The Astrophysical Journal 703: 905-909
Luger, Barnes (2015): Extreme Water Loss and Abiotic O2 Buildup on Planets Throughout the Habitable Zones of M Dwarfs. Astrobiology 15(2): 119-143


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