There may be more than five billion Earth-like planets that are rocky, potentially habitable and orbit main-sequence stars like our Sun scattered across the Milky Way, according to the latest estimates.
Astronomers from The University of British Columbia (UBC), Canada, arrived at that figure by combing through old data recorded by NASA's now-defunct Kepler space telescope. They studied 200,000 stars that were observed during the 10-year mission and split them down into different star types, focusing on F and K-type stars, but particularly G-type suns like our own.
Next, they checked if any of the stars supported any exoplanets and obtained measurements of their radii from data taken by the European Space Agency's Gaia spacecraft, which is still in operation. The radius of an exoplanet and its distance from its parent star provides a good indication of whether it's a hot, gassy object or a more temperate, rocky world.
The researchers were then able to combine both data sets to estimate how likely it was main-sequence stars like our Sun supported planets like our Earth and compared their results to a computer simulation. "My calculations place an upper limit of 0.18 Earth-like planets per G-type star," said Michelle Kunimoto, co-author of the study published in The Astrophysical Journal and a graduate student at UBC.
"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," she added.
Kunimoto and her co-author Jaymie Matthews, an astrophysics professor at UBC, extrapolated their estimation to cover the total number of G-type stars in the Milky Way. "Our Milky Way has as many as 400 billion stars, with 7 per cent of them being G-type. That means less than six billion stars may have Earth-like planets in our galaxy," said Matthews.
Essentially, they believe there is about one rocky planet with a radius between 0.75 to 1.5 times to that of Earth for every five or six G-type main sequence stars in the Milky Way. The latest number is in the middle of previous calculations that estimated that there were as few as 0.02 habitable worlds to as many as more than one Earth-like exoplanet around every Sun-like star.
Kunimoto explained that the latest calculation is based on better simulations than previous studies. "The methodology was only recently introduced in the context of exoplanet demographics, and was shown to provide more robust and accurate estimates than an alternative method which is very commonly used.
"I improved on the method by taking into account the fact that some planets in the catalogue may not actually be planets, but rather false alarms due to noise or systematics in the data. This has been ignored by almost every single exoplanet occurrence rate study in the past," she told The Register.
She compared the number of exoplanets generated in the computer simulation to a real catalogue of data to test her model. "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."
G-type stars are more likely than other types to host planets capable of supporting life. They are pretty small and have a similar mass close to our Sun.
"An issue with looking at much larger, hotter stars is that any planets in the habitable zone will take far longer to orbit, making them much harder to find. Also, intelligent life likely needs billions of years to develop, and the larger the star, the shorter the lifetime.
"Meanwhile, smaller, cooler stars are much more common than G-type stars, live a longer time, and are fast becoming a popular focus of exoplanet habitability studies because it is easier to find planets in their habitable zones. But there are some complications: namely, planets in their habitable zones orbit so close to the stars that they will be bombarded by more radiation, and they may be tidally locked, where one side of the planet is always facing the star, meaning it is always hot while the other side is always cold.
"We can only speculate what that will mean for potential habitability. A nice thing about focusing on G-type stars is that we already know it is possible for them to host a habitable planet – that's us!" ®