Six Billion Earth-Like Planets Around G Type Stars But Red Dwarf Systems Are Not Counted

There may be as many as one Earth-like planet for every five Sun-like stars in the Milky way Galaxy, according to new estimates by University of British Columbia astronomers using data from NASA’s Kepler mission.

To be considered Earth-like, a planet must be rocky, roughly Earth-sized and orbiting Sun-like (G-type) stars. It also has to orbit in the habitable zones of its star—the range of distances from a star in which a rocky planet could host liquid water, and potentially life, on its surface.

“My calculations place an upper limit of 0.18 Earth-like planets per G-type star,” says UBC researcher Michelle Kunimoto, co-author of the new study in The Astronomical Journal.

There are about 400 billion stars in the Milky Way.
There are 28 billion (seven percent) G-type stars.
Six billion stars may have Earth-like planets in our Galaxy.

Exoplanets 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.

This study does not consider earth-like planets around other stars. Earth-like planets could be around red dwarf stars. The major advantage that red dwarfs have is they produce light energy for a very long time. It took 4.5 billion years before humans appeared on Earth, and life as we know it will see suitable conditions for 5 billion more years or so. Red dwarfs can exist for trillions of years, because their nuclear reactions are far slower than those of larger stars. Life both would have far longer to evolve and to survive. The total amount of habitable zone around all red dwarfs combined is likely equal to the total amount around Sun-like stars given their ubiquity. Red dwarf stars are the smallest, coolest, and most common type of star. Estimates of their abundance range from 70% of stars in spiral galaxies to more than 90% of all stars in elliptical galaxies. It is estimated that 73% of solar systems in the Milky way are Red Dwarf systems.

Astronomical Journal – Searching the Entirety of Kepler Data. II. Occurrence Rate Estimates for FGK Stars

Abstract
We present exoplanet occurrence rates estimated with approximate Bayesian computation for planets with radii between 0.5 and 16 R ⊕ and orbital periods between 0.78 and 400 days orbiting FGK dwarf stars. We base our results on an independent planet catalog compiled from our search of all ~200,000 stars observed over the Kepler mission, with precise planetary radii supplemented by Gaia DR2-incorporated stellar radii. We take into account detection and vetting efficiency, planet radius uncertainty, and reliability against transit-like noise signals in the data. By analyzing our FGK occurrence rates as well as those computed after separating F-, G-, and K-type stars, we explore dependencies on stellar effective temperature, planet radius, and orbital period. We reveal new characteristics of the photoevaporation-driven “radius gap” between ~1.5 and 2 R ⊕, indicating that the bimodal distribution previously revealed for P