Good news for alien hunters: more than 300 million worlds scattered throughout the Milky Way have the conditions necessary for the potential development of a life form. In fact, according to a new study published in the Astronomical Journal on October 28th, about half of the solar systems in our galaxy are home to Earth-like rocky planets on which liquid water could accumulate or flow—a sine qua non of life.Worlds similar to oursResearchers came up with this estimate thanks to data from NASA's Kepler spacecraft, which hunted exoplanets for nine years. To do this, the space telescope fixed stars in search of brief twinkles, which would signify a celestial body orbiting around and passing in front of their rays. When it retired in 2018, the satellite had spotted some 2,800 planets revolving around a star that wasn't the Sun. But most of these worlds had nothing to do with our Solar System.So to determine how much certain planets have in common with ours, we had to cross Kepler's observations with those of the European Space Agency's (ESA) Gaia spacecraft, which made it possible to sort the stars based on energy flow criteria (temperature, age) comparable to those of the Sun. As a result, red dwarf-type stars were discarded, because their repeated storms make their environment very inhospitable… and likely sterile.Another step forward in extraterrestrial researchIn the end, they determined that there are approximately 300 million rocky Earth-like planets orbiting around stable stars like the Sun that are at a moderate distance from them. These planets could support life, granted, of course, that all the ingredients of the 'miracle recipe' (which remains a mystery) are there. Scientists say the closest ones are 'just' about 20 light-years away from us. Close enough to raise our hopes of meeting a distant galactic neighbour someday.While the estimate may not be accurate, it narrows down what researchers should be looking for in their search for alien lifeforms in the Milky Way. Astronomer Steve Bryson of NASA's Ames Research Center said:Though this result is far from a final value, and water on a planet's surface is only one of many factors to support life, it's extremely exciting that we calculated these worlds are this common with such high confidence and precision.Massachusetts Institute of Technology (MIT) astronomer Michelle Kunimoto added in a statement:Knowing how common different kinds of planets are is extremely valuable for the design of upcoming exoplanet-finding missions. Surveys aimed at small, potentially habitable planets around Sun-like stars will depend on results like these to maximize their chance of success.