New estimate suggests that 18% of Sun-like (G-type) stars could have a planet the size of Earth orbiting in their conservative habitable zone.
There may be as many as one potentially habitable Earth-sized exoplanet for every five Sun-like stars in the Milky Way. Image credit: Sci-News.com.
Determining the abundance of Earth-size planets in the habitable zones of their stars, where liquid water could exist on a rocky planet’s surface, is one of the major goals of exoplanetary science. NASA’s exoplanet-finding mission Kepler was specifically designed with this goal in mind.
Aside from being the first mission capable of finding and characterizing Earth-sized planets in year-long orbits around Sun-like stars, Kepler revolutionized our perspective on the diversity of planets in the Milky Way.
Dr. Michelle Kunimoto and Dr. Jaymie Matthews, astronomers from the Department of Physics and Astronomy at the University of British Columbia, base their results on an independent planet catalog compiled from their search of nearly 200,000 stars observed over the Kepler mission.
“Our calculations place an upper limit of 0.18 Earth-like planets per G-type star,” Dr. Kunimoto said.
“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.”
“Our Milky Way has as many as 400 billion stars, with 7% of them being G-type. That means 5 billion stars may have Earth-like planets in our Galaxy,” Dr. Matthews added.
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 catalogue represents only a small subset of the planets that are actually in orbit around the stars searched.
Dr. Kunimoto and Dr. Matthews used a technique known as ‘forward modelling’ to overcome these challenges.
“We started by simulating the full population of exoplanets around the stars Kepler searched,” Dr. Kunimoto said.
“We marked each planet as detected or missed depending on how likely it was my planet search algorithm would have found them.”
“Then, we compared the detected planets to my actual catalogue 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.”
The team’s paper was published in the Astronomical Journal.
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Michelle Kunimoto & Jaymie M. Matthews. 2020. Searching the Entirety of Kepler Data. II. Occurrence Rate Estimates for FGK Stars. AJ 159, 248; doi: 10.3847/1538-3881/ab88b0
