According to new estimates, for every five Sun-like (G-type) stars in the Milky Way there may be as many as one Earth-like planet. When crunching our galaxy’s numbers, this could mean that there are up to 6 billion of these rocky worlds in our pocket of the universe.
“My calculations place an upper limit of 0.18 Earth-like planets per G-type star,” Michelle Kunimoto of the University of British Columbia (UBC), Canada, said in a statement. “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.”
For a planet to be classified as Earth-like, it must be rocky, roughly Earth-sized, and orbit in the habitable zone of a Sun-like star. Such planets are harder to detect than their larger gas giant companions, as they have a much smaller impact on their host stars spectra – commonly used to discover orbiting planets. Indeed the majority of the 4,164 confirmed exoplanets are ice and gas giants.
However, Kunimoto and fellow UBC astronomer Jaymie Matthews’ new study, published in The Astrophysical Journal, highlights the possibility that there are many more Earth-like worlds out there.
“Our Milky Way has as many as 400 billion stars, with 7 percent of them being G-type,” Matthews said in the statement. “That means less than 6 billion stars may have Earth-like planets in our galaxy.”
Kunimoto and Matthews combined data from NASA’s retired planet-hunting Kepler mission with a technique called “forward modeling”, to estimate the proportion of Earth-like planets that Kepler may have “missed” in its roughly 200,000-star search.
“I started by simulating the full population of exoplanets around the stars Kepler searched,” Kunimoto 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.”
Using this method, the number of planets with a radius between 0.75 to 1.5 times that of Earth, orbiting its Sun-like star at a distance between 0.99 to 1.7 astronomical units, or AU (the distance between Earth and the Sun), and assuming the upper-limit of G-type stars in the galaxy, sat at almost 6 billion.
Kunimoto is no stranger to Kepler’s wealth of data. Earlier this year, she discovered 17 new planetary candidates amongst the first four years of Kepler data, one of which lies in the habitable zone of its star. On top of this, when she was only an undergraduate student Kunimoto found four new planets, including one thought to have large moons. Some people are just destined for the stars.