Earth is – for now – the only place in the universe that we know hosts life, so it makes sense that astronomers focus the search for alien life on worlds that are the most like our own. But just how common are Earth-like planets orbiting at comfortable distances from their stars? Researchers on a new study claim to have come up with the most accurate estimate yet – and they're more common than you might think.
According to simulations created for this new study, potentially habitable Earth-like exoplanets could be found around one in every four Sun-like stars. These planets would range in size from 0.75 to 1.5 times the Earth, and have "years" lasting between 237 and 500 days. That orbital period puts them in the Habitable Zone, where temperatures are just right for liquid water – and by extension, maybe life – to exist on the surface.
To come to these conclusions, researchers from Pennsylvania State and Brigham Young Universities created detailed simulations of various virtual "universes." Then they determined what percentage of the planets in these simulated universes would be detectable by an observer with our current capabilities. From this, they can then look back on our own tally of Earth-like exoplanets in the real universe and infer how many there may be out there in total.
To make sure these simulations were as accurate as possible to begin with, the researchers used data gathered from the Kepler Space Telescope's decade of exoplanet hunting, as well as Gaia's comprehensive survey of stars.
"We used the final catalog of planets identified by Kepler and improved star properties from the European Space Agency's Gaia spacecraft to build our simulations," says Danley Hsu, first author of the study. "By comparing the results to the planets cataloged by Kepler, we characterized the rate of planets per star and how that depends on planet size and orbital distance. Our novel approach allowed the team to account for several effects that have not been included in previous studies."
While one in four was the most likely figure, the team says there is quite a margin of uncertainty around it, which astronomers should take into account. This could be as low as one Earth-like planet for every 33 stars, or as high as almost one for every two stars.
The study should help future planet-hunting missions, such as the James Webb Space Telescope, which is due to launch in 2021.
The research was published in the Astronomical Journal.
Source: Penn State University