Andrew Siemion, director of the University of California Berkeley SETI Research Center, recently wrote in an email to The Daily Galaxy that “I suspect there are other intelligent civilizations in the Milky Way Galaxy– we just need to look!” Now, new calculations place an upper limit of 0.18 Earth-like planets per G-type star in our Galaxy, according to University of British Columbia researcher Michelle Kunimoto about six billion rocky, Earth-sized planets orbiting in the habitable zones of Sun-like (G) stars in the Milky Way.
“Estimating how common different kinds of planets are around different stars,” Kunimoto said, “can provide important constraints on planet formation and evolution theories, and help optimize future missions dedicated to finding exoplanets.”
One Per Sun-like Star
“Our Milky Way has as many as 400 billion stars, with seven percent of them being G-type,” says UBC astrophysicist Jaymie Matthews. “That means less than six billion stars may have Earth-like planets in our Galaxy.” Previous estimates of the frequency of Earth-like planets range from roughly 0.02 potentially habitable planets per Sun-like star, to more than one per Sun-like star.
Typically, planets like Earth are more likely to be missed by a planet search than other types, as they are so small and orbit so far from their stars. That means that a planet catalog represents only a small subset of the planets that are actually in orbit around the stars searched. Kunimoto used a technique known as ‘forward modeling’ to overcome these challenges.
‘Forward Modeling’ –Stars Searched by the Kepler Space Telescope
“I started by simulating the full population of exoplanets around the stars Kepler searched,” she explained. “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.”
Shed’s Light on the “Radius Gap”
Kunimoto’s research also shed more light on one of the most outstanding questions in exoplanet science today: the ‘radius gap’ of planets. The radius gap demonstrates that it is uncommon for planets with orbital periods less than 100 days to have a size between 1.5 and two times that of Earth. She found that the radius gap exists over a much narrower range of orbital periods than previously thought. Her observational results can provide constraints on planet evolution models that explain the radius gap’s characteristics.
Previously, Kunimoto searched archival data from 200,000 stars of NASA’s Kepler mission. She discovered 17 new planets outside of the Solar System, or exoplanets, in addition to recovering thousands of already known planets.
Source: Michelle Kunimoto et al, Searching the Entirety of Kepler Data. II. Occurrence Rate Estimates for FGK Stars, The Astronomical Journal (2020). DOI: 10.3847/1538-3881/ab88b0
Image credit: The first accurate 3D map of our galaxy shown at top of the page reveals its true shape: warped and twisted. Astronomers from Macquarie University and the Chinese Academy of Sciences have used 1339 ‘standard’ stars to map the real shape of our home galaxy in a paper published in Nature Astronomy today. Artist’s impression above of the warped and twisted Milky Way disk. (Chen Xiaodian)