Water is believed to be necessary for life, but it's not the only factor. For example, in our solar system Venus and Mars, though situated in the sun's habitable zone, presently do not have life-friendly atmospheres.
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Increased solar activity from a very young sun may be partly or wholly to blame for why Venus and Mars evolved so differently from Earth. The new research suggests that if the sun had a suitably positioned partner star, its early, violent outbursts may have been tempered.
"Habitability is not just a matter of being inside a star's habitable zone. You need to consider other factors. For example, the magnetic activity of the star, the magnetic protection of the planets and the emission of a higher level of radiation at the beginning of the stellar life," Zuluaga said.
To illustrate, scientists took a closer look at six binary systems with orbiting planets found by NASA's Kepler space telescope. Taking into account the stars' gravitational interactions, the researchers found three of the systems were well-suited to have planets in habitable zones. The study showed the habitable zone of one system, known as Kepler 34, was nearly free of damaging X-ray and extreme ultraviolet radiation.