The Milky Way is full of habitable (宜居) places, with roughly half of all sunlike stars hosting Earth-size worlds that could be friendly to life.
Here's a good sign for alien hunters: More than 300 million worlds with similar conditions to Earth are spread throughout the Milky Way galaxy. A new analysis concludes that roughly half of the galaxy's sunlike stars host rocky worlds in habitable zones where liquid water could pool or flow over the planets' surfaces.
Astronomers estimated the number of these planets using data from NASA's planet-hunting Kepler spacecraft. For nine years, Kepler stared at the stars and watched for the brief twinkles produced when orbiting planets block a portion of their star's light. By the end of its mission in 2018, Kepler had spotted some 2,800 exoplanets (外星行星) — many of them nothing like the worlds orbiting our sun.
But Kepler's primary goal was always to determine how common planets like Earth are. The calculation required help from the European Space Agency's Gaia spacecraft, which monitors stars across the galaxy. With Gaia's observations in hand, scientists were finally able to determine that the Milky Way is populated by hundreds of millions of Earth-size planets orbiting sunlike stars — and that the nearest one is probably within 20 light-years of the solar system.
It took more than half a century for scientists to start pinning down how many planets could possibly host life. In 1961, astronomers knew of no worlds orbiting stars other than the sun — and although planetary formation theories suggested exoplanets should be common, we had no observational evidence that they existed. But over the past decade, it's become clear that planets are extremely common, outnumbering stars in the Milky Way. On average, nearly every star is home to at least one orbiting world.
Early estimates suggested that perhaps 20 percent of sunlike stars hosted a world that met those criteria. We now know that the number is closer to 50 percent, if not more.
Of course, many factors determine whether a world in the habitable zone is truly friendly for life. Planetary characteristics such as magnetic fields, atmospheres, and plate tectonics all play a role, and those are difficult to observe on small, faraway worlds.
