The field of exoplanets (planets outside of our own solar system) is driven by our search for life beyond Earth. Traditionally, a planet is considered “habitable” (able to sustain life) if its surface temperature allows liquid water to exist on its surface (i.e., not too hot and not too cold). This depends, to first order, on the brightness of the host star and the distance of the planet from the star, both define the incoming stellar radiation at the top of the atmosphere (we can also consider some atmospheric effects). Therefore, we can define a “Habitable Zone” around a star at which planets can be habitable. This habitable zone will be further from the star for bright stars, and is closer to the star for faint stars.
Red-dwarfs are the most common stars in the universe. They are about one tenth to half of the Sun in size and mass, and they are the coolest and faintest of all main-sequence stars. It is common to think about Red-dwarf systems as a scaled-down version of our own solar system. Since Red-dwarfs are the faintest of all stellar types, their habitable zone is located very close to the star. This makes it easier to detect planets in the habitable zone, since close-in planets cover larger fraction of the stellar disk, and they repeat more frequently due to their short orbital period. So far we believed that the probability of detecting rocky, Earth-like planets orbiting Red-dwarf stars in the habitable zone is high, and since Red-dwarf stars are the most common in the universe, habitable planets should be very common.
The intuitive definition of habitability mentioned above was very reasonable as a starting point for studying habitable planets. However, one should keep in mind that planets are not affected only by the radiation environment, but also by the physical, space environment in which they reside. In particular, the physical parameters of the space environment of Red-dwarfs, such as plasma density, pressure, and magnetic field do not scale with the luminosity, and they are actually very high at close-in orbits. As a result, planets orbiting Red-dwarf stars in the habitable zone may reside in an extreme space environment that can strip their atmospheres over time, in a similar manner to what we believe happened to Mars (in addition to atmospheric evaporation and loss due to impacts). In order to sustain their atmospheres, these planets should have a strong internal magnetic field or a thick enough atmosphere to begin with. It is not clear that either exist for close-in planets orbiting Red-dwarf stars.
Simulations of star-planet interaction in the HD189733 system:
Coronal Mass Ejection (CME) heating an exoplanet:
The magnetosphere of HD189733:
Modulations of stellar radio emissions by an orbiting exoplanet:
