Stellar winds may reduce the number of habitable exoplanet candidates

Nature : Class M red dwarfs, which are the most common type of star in the galaxy, are considered strong candidates to have planets capable of sustaining life. They are small in size and mass, which makes it easier to find planets orbiting them, and they are cooler than the Sun, meaning their habitable zones are much closer in. However, if a planet is too close to its star, the star’s stellar wind could strip the planet’s atmosphere away. Ofer Cohen of the Harvard–Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and his colleagues estimated the strength of the stellar wind on three planets that are orbiting in the habitable zone of M-dwarfs and that are closer to their stars than Mercury is to the Sun. Using the properties of a well-studied M-dwarf star as a stand-in for the actual parent stars, the researchers found that the stellar wind would be anywhere from 10 to 10 000 times as strong as the wind Earth experiences from the Sun. Unless the planets had magnetic fields many times stronger than Earth’s, they would not be able to maintain atmospheres. However, the star they used is relatively young, and older stars may not have as strong a wind.