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 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.
That realization was “a really big step forward,” Wright says. “That’s what told us there were many sites for life as we know it to have potentially arisen.” But the next factor in the Drake equation—the number of habitable worlds per planetary system—was trickier to calculate, Batalha says.
Worlds like home
Kepler spots faraway worlds by looking for dips in light produced as planets cross stars’ faces and briefly blot out a fraction of starlight. Based on how much starlight is blocked, and how often, scientists can figure out how big a planet is and how long it takes to orbit its star. Using this approach, Kepler spotted thousands of exoplanets of all sizes and orbits. But scientists’ real quest was for the fraction of planets like Earth: temperate, rocky, and orbiting sunlike stars.
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.
“It’s higher than I thought. I was always saying to the public, one in four, one in five—this result is quite a pleasant surprise,” Batalha says. “Every other sunlike star is likely, on average, to have a potentially habitable planet.”
Calculating the frequency of these planets came with unanticipated challenges. The stars that Kepler observed were more active than scientists had anticipated, and they produced signals that could mimic or muddy the signatures of transiting planets. And the spacecraft itself was finicky, requiring periodic maneuvers that complicated the observations, particularly after some crucial parts failed that helped keep its gaze steady.
To reach their conclusion, Batalha and her colleagues combined data from Kepler and Gaia, which is tracking and characterizing a billion nearby stars. They identified planets from Kepler that are between 0.5 and 1.5 Earth’s radius, which are likely to be rocky rather than gassy. Then from Gaia they obtained the temperatures and sizes of the stars these planets orbit.
Instead of basing a planet’s potential habitability only on its distance from a star, the team calculated how much energy is reaching each of these worlds. From there, the team selected the worlds where temperatures would allow liquid water to survive on the surface.
Once the team had a sample size of known rocky, temperate worlds orbiting sunlike stars, they were able to estimate how many exist across the entire galaxy. They found that between 37 and 60 percent of sunlike stars in the Milky Way should host a temperate, Earth-size world—and using a more liberal calculation of the energy needed for a world to be temperate, they found that as many as 58 to 88 percent of sunlike stars could have such a world.
Of course, many factors determine whether a world in the habitable zone is truly friendly for life. Planetary characteristics such as magnetic fields, atmospheres, water content, and plate tectonics all play a role, and those are difficult to observe on small, faraway worlds.
Even so, “this paper really helps home in on exactly how many sites of life there might be,” Wright says. “And they calculate the most likely distance to the nearest such planet, and they end up with our celestial backyard.” The closest such world is probably within 20 light-years, and four should be within 33 light-years.
From habitability to civilization
Now that astronomers have a good sense of how many worlds similar to Earth are strewn across the galaxy, they can continue working through the variables in the Drake Equation. Many of the remaining factors will be tough to pin down, including the crucial questions of how often extraterrestrials develop technologies that we could detect and the length of time such civilizations are detectable.
Another outstanding question is whether scientists ought to include stars that aren’t sunlike, considering that several Earth-size worlds have been found around smaller, cooler stars. And perhaps we should be considering worlds other than planets—even though many of Kepler’s worlds are big and gassy, “they could have forest moons like Endor” in Star Wars, Wright says. “Or, I guess, Pandora, like in Avatar.”
Astronomers are tantalizingly close to figuring out the next factor in the equation: the fraction of habitable worlds on which life evolves. As we continue to explore our solar system, we’re finding that the list of habitable niches is long and diverse. Worlds such as Mars or Jupiter’s icy moon Europa could host microbial life, and even the toxic clouds above Venus could possibly harbor lifeforms.
“If it happened more than once in the solar system,” Wright says,” that gives you that number pretty quick.”
Finding just one example of life beyond Earth would demonstrate that biology is not a cosmic fluke but rather a probable outcome, given the right ingredients. And considering the amount of habitable real estate in the cosmos, many astronomers say that life is basically an inevitability.
But working out those last variables in the Drake Equation—the ones that will tell us whether Earth is home to the galaxy’s only technologically adept organisms—will be mysteries until, as my father says, we’ve heard the murmurings of alien worlds.