Determining the abundance of Earth-sized planets in the habitable zones of their stars, where liquid water could exist on the surface of a rocky planet, is one of the main goals of exoplanetary science. In fact, NASA’s Kepler exoplanet search mission was designed specifically with this goal in mind.
In addition to being the first mission capable of finding and characterizing Earth-size planets in year-long orbits around Sun-like stars, Kepler revolutionized our perspective on the diversity of planets in the Milky Way.
Now, a team of astronomers from the Department of Physics and Astronomy at the University of British Columbia (Canada), claim that there could be 6,000 million Earth-like planets in our galaxy. The experts base their results on a separate planet catalog compiled from their search for nearly 200,000 stars observed during the Kepler mission.
“Our calculations place an upper limit of 0.18 Earth-like planets per G-type star, ” says Michelle Kunimoto, co-author of the paper published in The Astronomical Journal.
“Calculating how common different types 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 .”
What does this data mean?
Our Milky Way has up to 400,000 million stars, of which 7% are of type G. That means that more than 5 billion stars may have Earth-like planets in our galaxy, 1 planet for every 5 sun-like stars in the Milky Way (which is much higher than previous estimates that expressed this figure at only 0 , 02 and not the current 0.18).
To be considered Earth-like, a planet must be rocky, about the size of Earth, and in orbit around Sun-like stars (type G). It also has to orbit in the habitable zones of its star, the range of distance from a star in which a rocky planet could harbor liquid water, and potentially life, on its surface.
Planets like Earth are generally more likely to be ignored by a planet hunt than other types, since they are so small and orbit far from their stars. That means that a planet catalog represents only a small subset of the planets that are actually orbiting the intended stars.
How have they arrived at this approximation?
The astronomers used a technique known as “forward modeling” to overcome these challenges.
“We started by simulating the entire population of exoplanets around the stars Kepler searched for,” Kunimoto said. “We marked each planet as detected or missing based on the probability that my planet search algorithm had found them. We then compared the detected planets to my actual catalog of planets. If the simulation produced a close match, then the initial population was probably a good representation of the actual population of planets orbiting those stars. “
Referencia: Michelle Kunimoto, Jaymie M. Matthews. Searching the Entirety of Kepler Data. II. Occurrence Rate Estimates for FGK Stars. The Astronomical Journal, 2020; 159 (6): 248 DOI: 10.3847/1538-3881/ab88b0
