How many alien civilizations are out there? A new galactic survey holds a clue.

Here’s a good sign for alien hunters: More than 300 million worlds with similar conditions to Earth are scattered 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.

“This is the science result we’ve all been waiting for,” says Natalie Batalha, an astronomer with the University of California, Santa Cruz, who worked on the new study.

The finding, which has been accepted for publication in the Astronomical Journal, pins down a crucial number in the Drake Equation. Devised by my father Frank Drake in 1961, the equation sets up a framework for calculating the number of detectable civilizations in the Milky Way. Now the first few variables in the formula—including the rate of sunlike star formation, the fraction of those stars with planets, and the number of habitable worlds per stellar system—are known.

The number of sunlike stars with worlds similar to Earth “could have been one in a thousand, or one in a million—nobody really knew,” says Seth Shostak, an astronomer at the Search for Extraterrestrial Intelligence (SETI) Institute who was not involved with the new study.

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 blot out a portion of their star’s light. By the end of its mission in 2018, Kepler had spotted some 2,800 exoplanets—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.

Inching closer to contact

The Drake Equation uses seven variables to estimate the number of detectable civilizations in the Milky Way. It considers factors such as the fraction of sunlike stars with planetary systems and the number of habitable planets in each of those systems. From there, it considers how often life evolves on worlds with the right conditions, and how often those lifeforms ultimately develop detectable technologies. In its original form, the equation assumes that technologically savvy aliens would evolve on planets orbiting sunlike stars.

“When astronomers talk about finding these planets, everyone’s really talking about the Drake Equation, right?” says Jason Wright, an astronomer at Pennsylvania State University who studies potentially habitable worlds but did not participate in the new study. “We all have that in mind when we’re doing this calculation.”

It took more than half a century for scientists to start pinning down how many planets could feasibly host life. In 1961, astronomers knew of no worlds orbiting

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