Our Milky Way Galaxy has as many as 400 billion stars. Seven percent of them are G-type. It means less than six billion stars may have Earth-like planets in our neighborhood, says University of British Columbia (UBC) astronomer Jaymie Matthews in a news study published in The Astronomical Journal.
What characterized Earth-like planet?
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It must be rocky, roughly Earth-sized, and orbiting Sun-like (G-type) stars which are Yellow or Orange colored dwarfs. Our Sun is in fact white. Its spectrum peaks in the blue and green light, but it can often appear yellow, orange, or red through Earth’s atmosphere.
It also has to orbit in the habitable zones of its Sun –the range of distances in which a rocky planet could hold liquid water, and potentially life, on its surface.
Michelle Kunimoto, the co-author of the study adds: “My calculations put an upper limit of 0.18 Earth-like planets per G-type star. Estimating how common different kinds of planets are around different stars can provide an important limitation on planet formation and evolution theories, to optimize future missions dedicated to finding exoplanets.”
Previous Earth-like planets estimations range from roughly 0.02 potentially habitable planets per Sun-like star to more than one per Sun-like star.
Planets like Earth typically are missed by a planet search because they are so small and orbit so far from their Sun. Meaning that a planet catalog shows only small planets that are in orbit around the stars searched.
Kunimoto’s technique known as ‘forward modeling’ to overcome these challenges.
Previously, Kunimoto had searched for data on 200,000 stars collected by NASA’s Kepler. She founds 17 new planets outside of the Solar System, or exoplanets. In addition, she was recovering thousands of already known planets.
“I started by modeling the entire exoplanets around the stars in Kepler’s search,” she explained. “I marked each planet as ‘detected’ or ‘missed’ depending on how likely it was my planet search algorithm would have found them. I then 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 these stars.”
Kunimoto’s research also sheds 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 2 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 models of planetary evolution that explain the characteristics of the radius gap.