The Milky Way galaxy is enormous, and we’ve scanned only the tiniest fraction of it in search of planets. We’ve spotted a few thousand of them orbiting distant stars, and now a team of researchers from Penn State University has used that data to estimate the number of Earth-like exoplanets in the entire galaxy — they peg that number between 5 and 10 billion. That’s a lot of places to look for alien life.
Of course, we can’t know for certain how many Earth-like exoplanets exist, nor can we even say for certain what “Earth-like” means in other solar systems. For the purposes of this study, the team took Earth-like to mean a planet between three-quarters and 1.5 times the size of Earth with an orbital period between 237 and 500 days.
The researchers started their calculations with data from the Kepler Space Telescope. During its nine-year mission, Kepler identified more than 2,600 exoplanets using the transit method. It watched groups of stars for small dips in light that suggest planets passing in front of them. Kepler demonstrated that many solar systems are similar to our own, but the detection methods favored larger planets orbiting close to a star because they produce more discernible drops in light. To fill in the gaps, the team used data from the ESA’s Gaia spacecraft.
Gaia launched in 2013 to create the most accurate 3D map of the galaxy possible. The goal of the project is to determine the locations of more than 1 billion celestial objects. To estimate the number of Earth-like planets, the team used Kepler and Gaia data to create simulated universes and then “observed” those simulated stars like Kepler would have. Since they knew the true number of planets in the simulation, that gave the researchers a sense of how many exoplanets Kepler would have missed in the real world.
Naturally, there’s a great deal of uncertainty in this estimation, which is why the range is so huge. The best case is 10 billion Earth-like planets in the Milky Way, but that doesn’t mean all of them have life or even a habitable environment — they could have corrosive clouds and crushing atmospheric pressure like Venus and still count as “Earth-like” by the metrics used in the study. The team, therefore, recommends that future missions aiming to find Earth-like planets should plan on seeing them orbiting one in every 33 sun-like stars at minimum. However, it’s possible about half of those stars have at least one Earth-like planet. Instruments like the James Webb Space Telescope might be able to find exoplanets that back up this claim when it launches (we hope) in 2021.