While we wait to establish contact, one technique we can use back on Earth is an equation that American astronomer Frank Drake formulated in the s to calculate the number of detectable extraterrestrial civilizations may exist in the Milky Way galaxy.
It is not a rigorous equation, offering a wide range of possible answers. Instead it is more a tool used to help understand how many worlds might be out there and how those estimates change as missions like Kepler, a telescope that is currently searching for Earth-like planets, begin to discover more about our universe.
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. 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. Then from Gaia they obtained the temperatures and sizes of the stars these planets orbit. 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. 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. Astronomers are tantalizingly close to figuring out the next factor in the equation: the fraction of habitable worlds on which life evolves.
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.
All rights reserved. Inching closer to contact The Drake Equation uses seven variables to estimate the number of detectable civilizations in the Milky Way. 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. Share Tweet Email. Drake is involved in SETI. Alien illustrations by Megan Joyce. Additional images via Wikimedia Commons. Radio transmission illustration by Quastie.
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