Milky Way Could Host up to Six Billion Earth-like Planets, Scientists Say

Scientists have estimated that there could be as many as 6 billion Earth-like planets in the Milky Way.

For a study published in The Astronomical Journal, the researchers calculated that there was an upper limit of 0.18 Earth-like planets per G-type main sequence star—stars like our sun.

"Our Milky Way has as many as 400 billion stars, with seven percent of them being G-type. That means less than six billion stars may have Earth-like planets in our Galaxy," astronomer Jaymie Matthews, a co-author of the study from the University of British Columbia, Canada, said in a statement.

Earth-like planets are those that are rocky, roughly the same size as Earth and orbiting sun-like G-type stars. Furthermore, they lie in the habitable zone of their host star, a region in which liquid water, a key component for life as we know it, could exist on the planet's surface.

Determining the abundance of Earth-like planets is one of the major goals of planetary science. Knowing how common they are has important implications for our understanding of the habitability of exoplanets and the potential for life to exist outside the solar system, as well as the design of future missions focused on detecting and characterizing exoplanets.

"Estimating how common different kinds of planets are around different stars can provide important constraints on planet formation and evolution theories, and help optimize future missions dedicated to finding exoplanets," Michelle Kunimoto, another co-author of the new study from the University of British Columbia, said in the statement.

However, estimating the number is no easy task. Detecting Earth-like planets is challenging because they are relatively small and tend to orbit their stars at large distances. Furthermore, their long orbital periods means that scientists need to make several years of observations to witness enough transits to identify the planet.

Scientists often use the "transit method" to discover new planets, which involves looking out fo periodic dips in the brightness of stars caused by an orbiting planet passing in front of them.

Not surprisingly given the difficulties in estimating the abundance of Earth-like planets, scientists have previously come up with a wide range of different figures, ranging from around 0.02 Earth-like planets per sun-like star to greater than 1.

"Thus, new estimates are invaluable in bringing the exoplanet community toward consensus. This is the primary motivation behind our work," the authors wrote in their study.

Kepler telescope
Artist's illustration of NASA's Kepler space telescope observing planets transiting a distant star. NASA Ames/W Stenzel

The exoplanets we know about only represent a small proportion of the true number that are actually in orbit around their host stars. To address this issue, the authors of the latest paper used a computer simulation technique known as "forward modeling" to examine an independent planet catalog compiled using data on around 200,000 stars that were observed by NASA's now-retired Kepler space telescope.

"I started by simulating the full population of exoplanets around the stars Kepler searched," Kunimoto said. "I marked each planet as 'detected' or 'missed' depending on how likely it was my planet search algorithm would have found them. Then, I compared the detected planets to my actual catalog of planets. If the simulation produced a close match, then the initial population was likely a good representation of the actual population of planets orbiting those stars."

Kepler was the first and so far only mission capable of finding and characterizing Earth-sized planets with long orbital periods around sun-like stars. In fact, the spacecraft has revolutionized our understanding of exoplanets, having found more than half of the known total.