"There may be an Earth-like planet circling one in every four stars in the night sky, according to the first census of alien worlds in our celestial neighbourhood. The survey is a yardstick for scientists searching for life elsewhere and suggests there could be 50bn planets with a similar mass to Earth in our own galaxy... Around a fifth of these distant Earths are expected to lie within their solar system's habitable or 'Goldilocks zone,' where the distance from the parent star keeps the planet's surface at just the right temperature for liquid water (widely regarded as a prerequisite for life) to form."